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Brake Problem

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Hi the latest chapter: so the master cylinder (MC) was rebuilt (second one), servo's refurbished by Past Parts and still had to pump pedal to get back pressure. So got a "proper" mechanic round to help, we bled the system thoroughly and he said its your MC, so brought a brand new one from BMW, fitted it and still the same. So last week I removed the servos again, sent them To Past Parts, they've double-checked them, had a long chat. Refitted the servos again tonight and a new one-way valve on the vacuum (the right way round). Bled the brakes again twice. Servos first then rears, then finally the front. And still the f****** same. Pump the pedal it goes hard, then 15 secs later push the pedal and it back to the floor. Really had enough now

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( RHD mit double vacuum booster car )   yikes!



tell us about the rear brake adjustment


if those shoes are backed off too far - that would

cause your present low pedal and pumping quickly brings them

to the drums but retract when released and driven........


are you bleeding the rear brakes ? getting good strong flow out of all bleeders ??


Edited by c.d.iesel
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Might be a little out in left field, but have you checked the flex hoses (especially the front ones)?  Old ones tend to either swell up internally, thus keeping the caliper pistons from retracting properly when the brakes are released, or they develop soft spots and bulge, allowing more pedal travel. 


Definintely strange, but with those dual servos, it's anyone's guess.  Our stateside cars aren't set up that way.\



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Try isolating the boosters one at a time using some of those 3 way joiners.

1 booster is 1 pipe, in master cyl, 2 out to front calipers

The other is 1 pipe ,in master cyl  2 out to front calipers 1 to rear

All else fails isolate both your pedal should come up hard .

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  • 3 weeks later...

I'm may not be understanding the problem, but if you have a hard pedal and then stay on the pedal for 15 or 20 seconds, does it go to the floor?


If the pedal is dropping while you have constant pressure on it, then you have a system failure issue.  That means you either have a leak or a seal problem.  No other options.  The brakes are a closed system.   Brake Booster, not part of the fluid system.


If I am mistaken and you are saying that your system works when you step on the brake and then the next time you step on the brakes it is soft, that would still seem to me that you have a seal failure or leak.   I have not read that you have a situation were you are leaking fluid, so that means your Master Cylinder seals are not sealing.


I'll watch this post for more information.

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Hi, nope. Push pedal once and it goes to the floor, pres again and it's hard and stays hard. Release and a few seconds later it's the same again. The pedal remains hard once pumped, the master cylinder is brand new ATE, there are no visible leaks. I've driven it over 200 miles this week (bravely) but no better. I have put gallons through the system, servos have been double-checked, callipers brand new, rear wheel cylinders brand new, new flexible hoses, new hoses from reservoir to MC, rear shoes adjusted correctly and followed instructions on bleeding until blue in the face. Also pushed calliper pistons back into callipers in case air was trapped there.

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Something's sucking back into itself (yes, just like... that...)


Since you can always get pedal at the second pump AND it's a solid (NOT spongy) pedal,

it's going to have to be isolated, I fear.


If it was mine, I'd block off the line going to the rears first.  You'll need some way

to plug the port going back there BUT it'll need to have some way to bleed it-

I'm thinking something like a speed bleeder that fits into the ports on the master/boosters...


Then work your way forwards.  It's going to be a real pain, I'm afraid.


all I got,



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I also had major problems after I rebuilt my MC rebuilt on my RHD car with twin boosters. My pedal would only firm up very close to the carpet, but on a quick release and brake I could get normal feel.


It turned out that one of the rubber seals on the second piston on the MC was mounted in the wrong direction. The problem is that the new replacement parts from BMW look different(one less seal) to the older parts and the diagram indicating the assembly if very vague.


I think you might have a similar problem. If so, one of the bleed circuits on the front calipers will purge considerably less fluid with one stroke of the brake pedal than the other circuits. You can't use a pressure bleeder to test this, you need to push the actual MC to see the effect.


Hope this helps. 




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