3.0 CSi brake master cylinder

owson

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I believe that what I am seeing indicates a new brake master cylinder is needed, but my mechanic says he hasn't seen this precise effect before in his (fairly long) experience with Coupes :

Easiest to see on gentle downhill - let car roll, apply brakes, ease up but do not release brake pedal pressure, roll a bit, re-apply brakes - repeat. When I do this, the brake pedal gets successively closer and closer to the floorboards. Eventually, it seems like I may be pushing the primary piston up against the secondary piston, but I'm not sure. I do not feel the brake pedal sink when applying steady pressure - but only if I do the "rachet" procedure.

Is fluid simply leaking past a seal (internally) ? I can't think of another explanation. Anyone seen this effect ?
 
It's certainly different. I haven't see that before.
Couple,or more, things

1.. after a run turn the engine off and pump the pedal, does it get harder as the servo vacuum is used up?
2.. After the above keep pressure on the pedal, does is sink to the floor?
3.. with your foot on the pedal start the car does the pedal appear to sink as the vacuum in the servo builds?
4... If you really push hard and fast does the pedal sink or is it "solid"?

From the symptoms you describe it would seem that the "floating" piston in the master cylinder is sticking and not returning to the rest position. Check that the rod from the pedal is not jamming and has not been adjusted incorrectly. Also some master cylinders have a dust cover/cap on the inside of the car or servo, make sure it's clean and free of debris. The piston should come all the way back to the retaining circlip.
A change of brake fluid might help, and worth doing if you haven't changed it in the last couple of years, otherwise you will have to remove it and take it apart to see what's happening. Maybe a good clean and/or a change of seals will do the trick. Make sure the internal spring is not binding or broken.
If you do remove the master cylinder this is a chance to look inside the servo for brake fluid and other crud (I assume your coupe is LHD)
HTH
Malc
 
Thanks for your input - Good to hear from far away Scotland ...

The answer to your questions is : yes, no, yes and "solid". And in fact, I just had all the fluid replaced (and system bled three times) as part of brake service (rear rotors/pads and rear caliper piston rebuild, both sides). And yes, the car is LHD.

I'll check about the dust cover - and about removing and checking the master cylinder. Not sure what is generally done for this vintage ('72) - a rebuild or complete part replacement (if they are available). I've heard that there are two sizes for master cylinders used for these cars (differ by 1.5 mm I think in ID), and it often isn't clear which has been installed w/o removing the entire thing. I'll see what my shop has to say.
 
I think replacements for LHD car are available from the usual suspects! However you will probably find one for an E28 might fit if you get really stuck.
BMW's usually have a nightmare of a system to bleed, a pressure bleeder always helps so for yet another tin of fluid and a pressure bleeder it might be worth another go before stripping out the MC
Good luck
Malc
 
mmmmm

Suggest remove and clean the M/C.

And, though you've not mentioned symptoms of a brake hose collapsing internally, suggest you verify that the calipers release when you let off the brake pedal. Not that I think this is your problem (I don't) but with the level of involvement, and tendency of older bimmers to have internal collapse problems with the flex lines, it's time to verify these. There are six. Checking is easiest to do on a very slight incline, such that the car barely rolls when the brake is released. If the car doesn't start rolling again when the pedal is released, it's time to change hoses. BTDT when one of mine wouldn't release.
 
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