Hard pedal, intermittent problem.

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Brucey
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Hard pedal, intermittent problem.

Post by Brucey »

Over the last few weeks I've had a few instances of the 'hard brake pedal, is my brake bomb going bad?' syndrome reported by others. Well yesterday it got a whole lot different.

I was driving in heavy traffic. The brake pedal had gone hard (short travel, still working OK) a few miles earlier. After a little while it seemed like I was was driving uphill; the car wasn't at all rolling well. I reasoned the brakes must be dragging a little. When I was about a mile from home, it was noticable that the brakes were dragging a lot, and were now getting pretty hot (track-day smells, all discs hot).

Well, I did a little diagnosis;

1) Stopping the engine and restarting made no difference.

2) I got no warning lights (although this isn't a good test; at least one of my hydraulic pressure regulator switches is faulty and isn't presently connected).

3) Stopping the engine and testing the brake bomb (by applying the brakes several times) the system felt more normal, and the fluid level rose in the hydraulic reservoir exactly as it is meant to.

4) The hydraulic fluid (ATF) in my system is clean looking.

5) The brake fluid in the master cylinder is bone dry (checked with a moisture meter).

6) After 3) the pedal felt different; it was now longer than normal.

7) A short test drive showed that at low speeds the car pulled up straight, albeit with a longer than normal pedal.

8) Today I drove the car again (with some trepidation) and the pedal was fine, back to normal (about 1-1.5" movement).

9) The car is rolling well again, no dragging at all.

10) I note that the pedal feels longer (about 2") when stationary with the engine running than on the move. I have noted this before on ABS equipped cars.

11) When the fault was present, at no stage was the steering affected in any way.

12) I repeated 3) and it was the same result each time.

From the above I would conclude the following;

a) My calipers are OK (they wouldn't all drag at once).
b) My bomb is fine, too.
c) It is possible that there is a fault in the hydraulic pressure regulation, but unlikely ( the brakes modulated normally with pedal pressure, if not feel, when the pedal went 'hard').
d) My master cylinder brake fluid might be fine, but this says nothing about the stuff elsewhere in the system; it is overdue for a change.
e) The only fault with the master cylinder that might cause the symptoms would be if the pistons couldn't return past the point where the reservoir is open to the circuit again. I suppose this is possible, but I don't think it is very likely.
f) I think the PAS pump continued to work throughout just fine.
g) I note the brake lines all route through the ABS modulator.

So my attention is now focused on the ABS unit and the fact that I haven't bled the brakes out for too long; could it be that I had some air in the system? Could this cause the ABs unit to go faulty temporarily? Why did cycling the power to the ABS unit make no difference, but cycling the pressure in the hydraulic system apparently 'cure' the problem?

If anyone has any comments or suggestions, please let me know. I don't have the bentley manual, so if anyone has a functional diagram for this system I'd be grateful- the haynes manual is very light in this area.

I know others have had similar troubles, too. Maybe their problem was the same as mine.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
TimS

Post by TimS »

Brucey
You have just described exactly what problem a friend of mine is having with his E28's brakes. He has replaced the pressure regulator and bomb, dismantled the calipers and replaced the brake fluid. He thought he had cured the problem as it disapeared for 6 weeks until last week.
He dismantled the hydraulic booster located in front of the master cylinder and found no obvious fault and is enquiring to see if the seals are available. We are convinced this is cause of the problem. I am assuming that you do not have the brake link bar which crosses the bulkhead on early cars as these have been known to stiffen up. The fact that the pedal is hard convinces me the hydraulic booster is acting when it shoudn't. Will follow up. He has just told me that someone on another forum replaced the master cylinder and cured it. The brake booster is ?400. BMW say thet don't go wrong so they don't sell many. No seals available.Good Luck.
Tim
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Brucey
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Post by Brucey »

-yes, mine is an '87 model, so has the later right hand master cylinder, with no torsion bar.

When you say 'the brake booster' which bit do you mean?

I'll be interested to hear how your friend gets on.

BTW everything has been fine for the last two days now.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
TimS

Post by TimS »

Saw this on a forum!





Like you, I did not drive her much and I had my front calipers start dragging.........got really bad too! Here is a note I sent a friend about my final resolve:





I finally broke down and pulled the booster off of my '87 E24 and replaced

the "O" ring in the end of the booster. A total cost of about forty cents

for the "O" ring at PEP Boys. After the high cost of the part and about

eight hours of labor (six to do the work, one to clean up the blood stains

from the leather from the under dash work and another hour to clean the

garage floor of antifreeze/brake fluid/ATF) I now think I have the leaking

booster fixed. Now if I can get all of the ATF off the underneath side of

the car it should be over.



Here is the lesson learned: It all started with my front calipers sticking

and causing the rotors to heat up. I figured I had bad calipers and

hoses.......(Wrong!). Then I figured it was a bleeding problem, must have

run a couple of gallons of brake fluid through it! (Wrong!). I then

installed a master cylinder which corrected the problem (Right!), but I

should have changed the "O" ring in the booster while I had it apart.

Apparently the old "O" ring could not handle the disturbance of having the

new Master cyl. installed and decided it would start leaking. By the way,

there is only one "3mm x 38mm "O" ring in the booster and its on the plastic

cup in the end of the booster. With a little ingenuity, I think the ring

could be replaced without taking the booster off, you'd just have to be

careful with the infernal spring inside the booster.



I guess the bottom line is to consider replacing the master cyl and "O"

ring in the booster to cure the dragging caliper problem, if you have done

nothing else to your brake system and all of a sudden the calipers start

dragging.





Jon
This is a post that Jon has just found. The brake booster is the cylinder that the power steering hydraulic pipes connect to in between the bulkhead and the master cylinder. Inside is a piston which uses hydaulic pressure to boost the braking action from the brake pedal. In the above post it seems that the master cylinder was the cause of the problem but leaked hydraulic fluid when replaced without changing the O ring.
Drew

Post by Drew »

just FYI really as I have a very different brake system.

My car has the older twin brake servo system rather than the bomb, I get dragging rear brakes when in traffic but not on the open road. I have rebuilt the calipers (all 4) to no avail and have replaced all the fluid at the same time.

on my car the brake master cylinder is about 1/2 an inch from the exhaust and is protected by a v small heat shield. So my thinking was that the brake fluid is getting very hot in the master cylinder and somehow applying slight pressure to the brakes. And when I'm moving in traffic less heat is building up near the exhaust manifold and so the brake fluid cools down again.

I am trying to make myself a bigger heat shield which will provide better protection for the master cylinder (and also the airbox).

my car has no ABS so I can rule that out :lol:
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Brucey
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Post by Brucey »

BTW Drew I have spare brake servos for E12 brakes (two sets!) so get in touch if you need them.

Thanks to Tim S for the info; I guess I should be thinking of the master cylinder....

Oh, and there was some fall-out from the dragging brakes; I now have quite bad DTV where it was slight before. Something to do with the brakes clamping hard on a static hot disc- something that happens a fair biot on auto cars anyway....

fingers crossed it does not happen again....

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Chris Wright

Re: Hard pedal, intermittent problem.

Post by Chris Wright »

Brucey wrote: ............ I don't have the bentley manual, so if anyone has a functional diagram for this system I'd be grateful- the haynes manual is very light in this area.
cheers
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tonomontt
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Re: Hard pedal, intermittent problem.

Post by tonomontt »

Brucey,

I have exactly the same issue with my brakes. Can you tell me how did you fix it?
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hornhospital
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Re: Hard pedal, intermittent problem.

Post by hornhospital »

Wow, that's a heck of a thread revival! Thanks for searching! Too few that come here for help with a specific problem search the forum first.
Ken Kanne
'84 633CSi "Sylvia"; '85 635CSi "Katja";'85 325e "Hazel Ann"; '95 M3 "Ashlyn"
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