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Losing Brake Pedal When Engine Running

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Re: Losing Brake Pedal When Engine Running

Post by Jono B good » Wed Sep 05, 2018 1:06 pm

Hmmmm the plot thickens...
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Re: Losing Brake Pedal When Engine Running

Post by NigelC » Wed Sep 05, 2018 9:46 pm

re: my disappearing article sent to forum.
Thank you Brucey for some very useful tips, I like the idea of preparing it first in word then cutting and pasting.

I will try again in the next few days when I have some more time as I think some members may find it interesting.

cheers, Nigel.

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Re: Losing Brake Pedal When Engine Running

Post by Boggie » Thu Sep 27, 2018 8:41 am

Hi Nigel,

Any news on this please? I started a new job earlier this month so the brakes have been on hold (no pun intended). However, this weekend I plan to try Brucey's suggestion to connect the 2 outputs directly to try and eliminate any other potential factors. Before this I would like to cycle the ABS pump, as a last hope before the Brucey test.

Thanks,
Ian
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Re: Losing Brake Pedal When Engine Running

Post by NigelC » Thu Sep 27, 2018 11:45 am

Hi Ian, I am away until 2nd Oct, will send details when back home, regards, Nigel.

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Re: Losing Brake Pedal When Engine Running

Post by Jono B good » Fri Sep 28, 2018 6:44 pm

Keep us updated please.
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Re: Losing Brake Pedal When Engine Running

Post by Boggie » Fri Sep 28, 2018 9:04 pm

Will do!
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Re: Losing Brake Pedal When Engine Running

Post by NigelC » Wed Oct 03, 2018 10:34 pm

Here are the details of my ABS module switchbox ( article in 2 parts due to cut and paste issues!)
It gives control of all the solenoid valves and pump to help dislodge any trapped air in the module.
First, disconnect battery positive terminal, remove plastic cover on the abs module to expose the loom and plug,
remove both screws holding the loom clamp to base and remove multi-plug and small square relay.
With the large terminal(13) on the left, the upper pins are marked : 11, 9, 7, 5, 3, 1.
the lower pins are marked : 12, 10, 8, 6, 4, 2.
Make up power lead using 25amp cable, crocodile clip onto positive battery terminal, other end to large crimp
terminal pushed onto(13), also attach 2 x 5 inch 10amp wires to same large crimp terminal. Fit std. female crimp
to one wire and attach to pin(2), fit male crimp to second wire and plug into relay socket(30). Use about 10 feet of
7 core trailer cable to connect switches in box to module pins, (this allows control of the abs valves from the side of
the car)
Switch for L/H Front solenoid wired to female crimp pushed onto pin (1)
Switch for R/H Front solenoid wired to female crimp pushed onto pin (3)
DSCF1219.JPG
DSCF1219.JPG (1.24 MiB) Viewed 712 times
Switch for L/H Rear solenoid wired to female crimp pushed onto pin (5)
Switch for R/H Rear solenoid wired to female crimp pushed onto pin (7)
Switch for ABS pump wired to female crimp pushed onto pin (11)
Link second terminal of all switches together and wire to battery negative using 10amp cable and crocodile clip.
Leave car positive cable disconnected and connect both abs jump leads to the battery, you can now run abs pump
and control the individual solenoid valves ( remember that powering solenoid shuts off valve)
(continues in part 2 )
DSCF1221.JPG
DSCF1221.JPG (1.33 MiB) Viewed 712 times

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Re: Losing Brake Pedal When Engine Running

Post by NigelC » Thu Oct 04, 2018 12:20 am

ABS switchbox -continued.
My method to clear trapped air in abs module :
Connect up pressure bleeder at 10psi to move fluid through the pipes and keep reservoir topped up.
Have bleeder jar connected to caliper nipple -still closed.
Switch on abs pump, toggle solenoid valve switch on and off repeatedly - then leave switched off.
(obviously valve must match caliper position but order of bleeding did not seem to matter)
Remember that brake fluid will not flow through valve when powered up.
With abs pump still running, open caliper bleed screw and pump brake pedal by hand to give some extra pressure,
( servo exhausted beforehand) After bleed jar is full, tighten bleed screw. Turn off abs pump. Repeat for all corners.
Most of the trapped air came out immediately on each corner from just using the pressure bleeder, I continued pumping
the master cylinder as well but no more air came out.
My initial bleeding of the new braking system ( after using many different methods) gave a reasonable pedal - albeit
about halfway down , with no feeling of sponginess, but after purging the abs module the brake pedal was almost right
to the top of the stroke - with nothing else done.
After bleeding the brakes so many times with no more air coming out I felt sure that it was air trapped in the abs module.
Researching the internet I found dealer equipment for modern abs car faults costing between £2,500 and £4,000 which
spurred me on to making this simplified switchbox to suit our cars. Hope this helps! cheers, Nigel

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Re: Losing Brake Pedal When Engine Running

Post by Brucey » Fri Oct 05, 2018 10:44 pm

nice work! A lot more scientific than my 'use the brakes on wet grass' approach.... :wink:

I wonder however why it is that after a few excursions of fluid going in to the ABS pump (causing a spongy pedal), why it is that the air doesn't come out of the ABS pump into the lines? If it does (but then goes right back in again the next time the pedal is pressed), then maybe the ABS pump can be bled out at the unions right by the pump.

Has anyone tried this?

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Re: Losing Brake Pedal When Engine Running

Post by Boggie » Sat Oct 13, 2018 4:37 pm

Thanks Nigel, that is a great write-up and nice piece of work. Thanks!

So,I finally got to run Brucey's test today. I made up a pair of male to male short brake pipes, screwed them into the two outputs on the master cylinder and joined them together using a female to female connector. I bled the system at the female connector and the brake pedal was solid but I mean rock-solid with almost no movement at the pedal even with significant foot pressure. I guess this was because there was such a short set of pipes with minimal fluid to compress.

I set about seeing if I could replicate the problems I had with the pedal sinking once the brake booster was significantly increasing the pressure on the master cylinder seals. I started the engine to see if I had the same slow sinking of the pedal with the hydraulic brake booster applying more pressure. The pedal was the same; rock solid with no sinking at all. Now that initially made me think the master cylinder is OK so I reconnected and bled the car brake pipes, got a really good pedal with no sinking (no matter how hard I pushed the pedal) but when I started the engine the pedal under heavy pressure sank slowly once more.

Have a look at this diagram. I suspect the test I had done did not actually achieve what was intended. I.E. to apply full booster pressure on the master cylinder with the car circuits, pipes, connectors, ABS, calipers and hoses taken out of the equasion. Because the brake pedal was so solid, I suspect it did not move enough to actuate the booster. What do you guys think?

Cheers,
Ian
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Re: Losing Brake Pedal When Engine Running

Post by Brucey » Sat Oct 13, 2018 9:17 pm

there should (initially) always be some movement of the MC piston as the seals slide over the compensation ports. However thinking about it, without a slave piston on the MC, if the MC seals are good, then the MC may not return fully. Provided the pedal isn't causing the MC to bottom, then the solid pedal means the MC pistons are good; if they were leaking then the pedal would slowly return to full height.

cheers
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Re: Losing Brake Pedal When Engine Running

Post by Boggie » Sat Oct 13, 2018 10:12 pm

Thanks Brucey.
Yes, that aligns with what I was thinking too. However, I was wondering if it was worth repeating the exercise but leaving a little air in the system so my pedal (i.e. MC) moves enough for the hydraulic boost to fully kick in and I can check this is not an issue of MC seals leaking under boosted pressure...

The question remains; If we assume that the MC seals are OK, why is my pedal going to the deck when I have no leaks...?
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Re: Losing Brake Pedal When Engine Running

Post by Brucey » Sun Oct 14, 2018 12:25 am

I think the servo works without there being any movement of the MC itself. This is because there is a tiny movement between the parts inside the servo which opens the valves so that the full force is obtained. Were it not so, it mightn't be possible to modulate the brake pressure once the brakes were 'on'.

I think the original choice was between bad MC and air in the system somewhere (ABS unit being the prime suspect). We know that running the ABS unit clears air out of it ( as per above, or expensive BMW tool, or simply braking on wet grass etc) but the thing that surprises me is that the air doesn't burp out of the ABS unit into the lines and thence is bled out (eventually) after repeat applications of full pressure in the brakes.

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Re: Losing Brake Pedal When Engine Running

Post by Boggie » Mon Oct 22, 2018 9:20 pm

Cheers Brucey,

I was wondering if it was worth introducing a little air into the closed circuit pipes I made to join the 2 outputs and repeating first. I.E to ensure there is a similar initial pedal movement to the point where the brakes are fully applied

However, can any of you do me a favour please; start your engine and press the brake pedal down hard and hold to see if it stays or slowly sinks? I spoke to a tech today and he suggested that this might be a feature, not a fault and that the time it takes to sink fully is more than enough time to perform an emergency stop from maximum speed....

I am not convinced.
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Re: Losing Brake Pedal When Engine Running

Post by Boggie » Sun Nov 04, 2018 8:46 pm

Finally got a chance to cycle the ABS system today, although it was not as elegant as Nigel's solution (a bit more Heath Robinson TBH) it worked well. All four corners cycled and bled, as per Nigel's excellent write-up.

Whilst I was sceptical that this would fix my issues it was something I wanted to do to ensure the brake system was purged of any old fluid and crap. However, when I initially started the engine and applied the brakes it appeared to have resolved my sinking pedal issues! The pedal felt great and stayed where it should. So, feeling rather pleased with myself (and very grateful to Nigel) I cleaned up and packed away my tools. I then started her up again, before closing the workshop and retiring for a celebratory beer, and pressed the brake pedal hard once more but this time the brake pedal sank slowly to the floor again !!

What am I missing? I have no leaks visible anywhere, and after repeating my earlier 'Brucey test' (this time with a little air in the link pipe to ensure the brake pedal had some travel before going hard), I know the MC is ok. It doesn't make any sense; the pressure must be dropping to let the peday sink but where and how....?
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Re: Losing Brake Pedal When Engine Running

Post by plip1953 » Mon Nov 05, 2018 6:47 pm

Howsoever it happens, could it be the same kind of thing that I've experienced on my Merc ML - and yet there is no discernible impact on day to day braking.

But even if that is true, it would be very interesting to understand why.
Phil
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Re: Losing Brake Pedal When Engine Running

Post by Boggie » Sun Nov 11, 2018 11:48 am

Based upon the fact that after following Nigel's procedure I appeared initially to have fixed the issue but it then reappeared I started researching ABS linked sinking pedal issues and found THIS LINK. As I have replaced everything in the whole braking system, tested and retested the MC, bled and re-bled the system, checked for leaks and came up with nothing, all I have left is the ABS unit.

However, the ABS unit is no longer available so I am on the hunt for a replacement. Anyone got a good second hand unit I can buy please?
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Re: Losing Brake Pedal When Engine Running

Post by NigelC » Sun Nov 11, 2018 1:17 pm

Hi Ian,
I followed your link to the video -this fits the symptoms exactly, so trying a secondhand pump is the next step.
( I just checked E-Bay again this morning as there has been one listed for a while for £45 but it has gone ugh!)
I also came across a short bimmerforums thread where someone found disconnecting the power supply to the unit
( I would pull the multi-plug on the unit) stopped the pedal sinking and gave perfect braking. I am not sure about that
but it might be a try! It all points to the pump being faulty.
It seems to be a very rare fault as no one else is writing in to the forum with the same problem. I am lucky that my
sinking pedal is very slow at the moment so I can still drive it but I will have to replace mine in the near future.
( I am quite keen to strip it down and see what makes it tick!) cheers Nigel.

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Re: Losing Brake Pedal When Engine Running

Post by plip1953 » Sun Nov 11, 2018 3:06 pm

This has now got me worried that I might have a faulty ABS unit in my Merc lol!!

Assuming an ABS unit replacement does do the trick, what I really find difficult to understand is where the pressure can possibly be going to? Can anyone offer an explanation?
Phil
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Re: Losing Brake Pedal When Engine Running

Post by Boggie » Thu Nov 29, 2018 6:09 pm

I managed to find a breaker with a (relatively) low mileage E23 which uses the same 008 Bosch unit as the E24. £60 quid later a very good looking ABS unit was in my workshop. I gave it a thorough clean-up, flushed it through with fresh DOT4 and cleaned all the electrical connectors and it looks as good as new! An hour after that it was sitting in the car, all plumbed up, ready for system bleeding and ABS pump run / valve cycling but I had to stop as it was getting late.

Last night I built an ABS Valve Cycling switch box so that I can run the ABS pump and automatically fast-cycle each valve in turn with a make/break relay. I needed to do this as my helper won't be around on Sunday (and my previous version was a bit of a lash-up) so I fitted it with 5m of wire to be able to operate it from each corner of the car. Between that and my pressurised bleeder I should be able to do it all on my own. Just got to put the car on a stand at each corner and remove the wheels and I am good to go.

Apart from the ABS unit, absolutely everything on my car's braking system is new and by process of elimination I have discounted everything else, so I have high hopes. I suspect one of the valves was not sealing perfectly and allowing fluid to get through under the increased pressure from the hydraulic assist. Perhaps a valve is worn or more likely some crap in the system (You should have seen the state of the fluid I drained out) is stopping the valve making a perfect seal... who knows. Either way I will know on Sunday.

I will report back when done!
Ian
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Re: Losing Brake Pedal When Engine Running

Post by Boggie » Sun Dec 02, 2018 9:29 pm

So, another afternoon / evening fettling with E24 brakes in the workshop and I have the whole system, including new ABS unit bled with premium synthetic Dot4. The good news is that new switch box I put together worked a treat and when combined with my pressurised brake bleeding system, allowed me to sit at each corner of the car and run the ABS pump / cycle the valves whilst I bled each caliper. I again had a great pedal that (without the engine running) would not sink, no matter how hard I pushed. Foolishly I had left the battery connected on the car for the last few weeks so there wasn't enough power to start the engine and see if all my extra work had resolved the issue. So, I had to wait a couple of hours whilst the battery charged to find out. The bad news is that I just started the car and the issue remains; once the engine is running and the hydro-assist is in play, a couple of seconds after I push the pedal down hard, it starts to sink again....

I am at a loss. Everything, apart from the ABS unit, is new and double checked / tested and as I have the same issue with 2 different ABS units I think we can discount that being the issue. Where is this pressure leak? It must be on both circuits too as the pedal eventually gets to the floor....

Feeling rather despondent, right now.
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Re: Losing Brake Pedal When Engine Running

Post by plip1953 » Mon Dec 03, 2018 10:42 am

I share your frustration!

Even if it's bread and butter basic stuff to most of you guys, please could someone explain in detail what happens inside the MC when the pedal gets pressed? Ok, so a piston(s) moves inside the casing and fluid is then forced through a pair of small bore brake pipes in the direction of the ABS unit and then on to each wheel caliper. But in this instance the fluid should very quickly encountesr strong resistance because
a) there are no leaks in the overall system; and
b) the pads have by now been brought fully into contact with the discs and will move no further.

But the MC piston is not encountering significant resistance (engine running) and therefore fluid MUST be being displaced from within the MC to somewhere else. Where can it possibly go? Could it:-
a) be displaced into the MC reservoir? Is that possible? And if so, what would it take to make that happen? Has the level of fluid in the reservoir been observed while the pedal is sinking?
b) be recycling itself within the confines of MC? This is where my knowledge is scant, but when the MC piston moves does the area within the cylinder behind the piston simply become empty? Or, if not, what does happen? Is it conceivable that the displaced fluid from the piston when under pressure is simply leaking past the piston seal and filling up the space behind (sorry if this seems such a basic point and I'm pretty sure it's been covered previously and explained/dismissed)? And if that is possible, even if multiple MC swap has already been made, how would you test for it to provide 100% certainty?

And related to the above, why does the pedal remain firm when the engine isn't running? Is the only difference that with the engine running the force being applied to the MC piston is massively higher than with the engine off? And, if so, how can that impact differently on the various system elements?
Phil
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Re: Losing Brake Pedal When Engine Running

Post by Brucey » Mon Dec 03, 2018 2:00 pm

the workings of the MC are fairly simple; only the tandem operation and the workings of the compensation port make it any more complicated than a bicycle pump.

That Boggie has encountered the same problem with two ABS units and with a MC that has 'tested good' on a leak test means that there is fluid going (slowly) into the ABS unit. It also makes me wonder if the problem is

a) in the ABS unit itself and
b) is fairly 'normal' (or at least inconsequential).

This picture

Image

is of a two channel ABS system as fitted to a motorbike. You can see that the valves open when the ABS ECU instructs them to, and modulates them at high speed to inhibit a full wheel lock-up. Car ABS presumably works in a similar way, however I can't find a functional diagram of a four-channel ABS system.
[Edit; there is this picture of a four-channel system

Image

which shows the same kind of layout but the labelling is slightly different. You can tell which valves are the inlet valves for each brake caliper because they have a check valve in parallel with them, wheras the outlets don't. The MC cross section is an absolute dead ringer for ATE one used on most E24s too. Also NB the system split shown is a simple front/rear split, not a diagonal split as per E24.]

You will note that the system contains parts that are labelled 'accumulator' and 'reservoir'. These look to me to have the facility to accommodate extra fluid if the valves are opened in a particular way and the MC pressure is high enough. In addition there is the possibility of air being trapped in the ABS plumbing.

So the possibilities for a sinking pedal are

1) duff MC
2) air in system
3) air in ABS module
4) normal workings of the reservoir/accumulator parts of the system

note that, as configured, just running the ABS pump won't purge all the air out of the system; that requires that the valves themselves are cycled (so that fluid runs through the outlet valves) and may require multiple MC pressure on/pressure off cycles in order that the reservoir parts are entirely purged of air.

Note that if the outlet valves are left open when the car is stationary, or they leak slightly, pushing down on the foot pedal will force fluid into the reservoirs and this will cause the pedal to descend slowly, but presumably only by a finite amount.

Food for thought?

hth

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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Re: Losing Brake Pedal When Engine Running

Post by plip1953 » Wed Dec 05, 2018 9:18 am

Thank you Brucey for the explanation.

I found this which I thought was quite useful/interesting...

https://mechanics.stackexchange.com/que ... -activated

Essentially, what I take from this is that the the sinking pedal could be attributed to the ABS system, but only if the sinking happens while the ABS is actually operating. That seems to make sense to me.

To satisfy my curiosity, please could someone explain how can we be certain that fluid isn't somehow getting back from the MC into the main brake fluid reservoir? Presumably there is some kind of valving system. And if so, couldn't that be faulty in some way?

Ruling the ABS out of the equation, ans also my thought about fluid leaking back into the reservoir, logic suggests that there must still be some air in the system. I know this has been considered multiple times and every possible step taken to eliminate it from the equation, and yet the way the pedal sink still seems consistent with that possibility.

One other thought - with the engine and and brake pedal pressed, and sinking, does it make any difference if the pedal is pumped? What actually happens in those circumstance?
Phil
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Re: Losing Brake Pedal When Engine Running

Post by olympia57 » Wed Dec 05, 2018 11:56 am

Well , I sympathise with you re your problems on the E24 , brakes can be a real PITA on these cars ,and I speak from experience .
I've read through the thread and have to say that I experienced a very similar problem on my DD recently.
Pedal creep when engine was started.
About 15 months ago I had rebuilt the rear calipers on the car and flushed the system through with fresh fluid. Evere since then the pedal would slowly drop when pressure applied when the engine was running , it never reached the floor but went much lower than it had done in the past . I hasten to add that the car passed it's annual roadworthy test but the efficiency was lower than noted on previous results.
It really annoyed me and I found it very unpleasent to drive .
I bled and bled the system thinking that there must be trapped air in it but it never got bettter . The fact that when the engine was off the pedal was rock hard told me that the MC was OK but there was always doubt there . The ABS system worked but I was actutly aware that there could be an issue there too.
My thoughts kept coming back to the fact that with servo assistence there was some fluid movement that could not be replicated with human foot pressure alone .
I too , over a period of time , had steadily replaced and or renewed most of the serviceable components in the cars braking system , more down to good maintenance than failure and had renewed the four flexible hoses some three years ago. I therefore believed that these would be OK.
However before I bit the bullet and purchased a new MC at considerable cost ( no service kit available ) I decided to double check the hoses.
I simply clamped off both front flexible hoses ,started the engine and applied the brakes and ....... solid pedal , no creep.
There was just enough expansion in the flexibles to allow enough fluid movement under servo assitance to cause the pedal to drop and it continued to creep as the hose(s) slowly expanded until it solidified midway down. Very little fluid movement is required to drop the pedal 2-3 inches .
As I said these hoses were not old and visually looked as new but after changing all four hoses again the problem was gone .
Now this may be a red herring but even with your new Goodrich hoses , hopefully genuine :roll: , it will only need a slight expansion to give you the symptoms you have .
Clamp up each hose and see if it makes any difference .
Good luck
Don
1981 635 series 1

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