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Losing Brake Pedal When Engine Running

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Jono B good
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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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NigelC
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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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Boggie
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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
1985 M635CSI
Classic Car Collector & Restorer.

NigelC
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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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Jono B good
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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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Boggie
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Re: Losing Brake Pedal When Engine Running

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

Will do!
1985 M635CSI
Classic Car Collector & Restorer.

NigelC
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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 289 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 289 times

NigelC
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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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Brucey
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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?

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

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Boggie
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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
1985 M635CSI
Classic Car Collector & Restorer.

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Brucey
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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
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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Boggie
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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...?
1985 M635CSI
Classic Car Collector & Restorer.

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Brucey
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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.

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

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Boggie
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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.
1985 M635CSI
Classic Car Collector & Restorer.

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