Brake issue

Peter Coomaraswamy

Peter Coomaraswamy

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Hoping I can get some suggestions,

We had a little get-together for BMW owners here in Austin over the weekend down by Lake Travis, It was about a 40 mile round trip- nice day and all that. On my way home my brakes started "dragging" I mean it was noticeable at about 30 MPH coming up to a light so I pulled over into a parking lot and it seemed that all 4 brakes were locking and the pedal got very hard. the wheels were not overly warm and this went on for about 15 minutes of driving slow, stopping, checking, etc. The brake fluid is new and there was no excessive heat anywhere- then things went back to normal. But, not the kind of thing I want to leave unattended. Any ideas what could cause this? Thanks!
 
Could be the rubber brake lines are old and constricted and allowing fluid to go to the calipers under pressure but not allow the calipers to release, when's the last time they were changed?
 
Strange to have it happen on more than one, but corrosion or dirt causing the pistons to hang up in the caliper cylinders, not releasing well when the pressure is removed???

I removed all four of my calipers this winter, as one of the rear calipers had a piston that was not releasing well. Removed all the pistons, cleaned them and the bores with brake cleaner spray, installed new sealing rings and dust boots. They now release wonderfully, and when I 'spin' a front wheel with the car on the lift, it spins several rotations before air/bearing friction (newly refurbed wheel bearings as well) cause it to stop. NO dragging of the pistons (should reduce my fuel consumption - ha).
 
with all that in mind

Thanks folks,
Andrew, all lines new as of 2013, 2500 miles on the car since then.

Gary, I'll throw it on the lift and try the spin (milage) test, they seem free now?

VIP, master cyl. new from Carl & Ben, same as brake lines, rear valve was not jacked with- do you think that could be the problem? Also, replaced all rubber lines, and only a couple of the steel ones :-(
 
Hello Peter,
I'm thinking power brake booster. Seals fail, brakes lock. Lucky you got home. I have seen, all be it on other power assisted vehicles, brake servo fail and all four wheels lock up. Perhaps just check valve?
Mike
 
OK, Mike- that and the rear valve- whatever you call it- are the only 2 things I did not rebuild on the system... painted them pretty though, anyhow can you or anyone else chime in on the mechanics of a booster failing causing a brake lock up- I thought it would be the opposite, losing the boost causing the driver to revert to non-power brakes as the only effect.

Thanks so much!

Peter
 
Brake issue

Hey Peter,
In my experience, when a brake servo has failed the seals have allowed brake fluid to leak internally where it is not supposed to be. Confirmation of this is when they start to lock on, you disconnect the vacuum hose, and brakes work again, even though you need much more pedal effort to stop the car. So, if this happens again, remove vacuum hose, plug it with a bolt and hose clamp, and try brakes. They will probably work, but be prepared to really increase effort on pedal.Good luck! Mike
 
When your master cylinder piston does not return enough the pressure is never released
 
Peter,

A suggestion: Before you push the button on the lift to raise the car, open the vacuum system to eliminate the servo, step on the brake pedal and push it as hard as you can. Release, get out and raise the car. If only one or two of the wheels are hard to turn, most likely problem is those individual calipers. IF all are hard to turn, most likely the master cylinder isn't satisfactorily releasing.

Then, if all released, you can lower the car, re-connect the vacuum system, start the car and depress the brake pedal 2 or 3 times hard with the engine running. Shut it off, raise again and check the free spinning. IF none of the four are free spinning this time, you've isolated the problem, and it is most likely the servo.

Good luck.
 
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Peter, I remembered a key symptom I experienced with a failing brake servo. Fluid was disappearing into the reservoir, but not seen leaking out of any wheel cylinder, brake hose or anywhere. Just kept topping up, until brakes locked on. Turned out servo itself was filling with fluid as the seals had gone. Let us know what you find. Mike
 
Masters have fluid that can leak into servos (booster).
It is the master seals that leak.
Boosters leak air or vacuum
 
Thanks guys, I will try Gary's experiment and if I come up with no definitive answer I will check the booster for fluid by pulling the master but there is no drop in fluid and the master was new with only 2500 miles on it so this should be fun :-(

I'll post whatever I find-
 
master

cylinder clearance at the actuating rod or lack thereof preventing the piston retracting. .5 mm IIRC. Gets worse as things heat up and expands.

Check the pedal dimensions in the work book. Loose or weak return spring or the bush is worn, got moved a bit, and is now hanging up sometimes.

Last but not least, crack the brake lines at the master to bleed a bit off. If the brakes work normal, then you've sourced the issue.

Don gets my vote. The master is applying the brakes on for a reason. Weak spring, bore, etc.
 
When the master cylinder on my coupe leaked there were no symptoms other than disappearing fluid. The fluid eventually leaked inside the car, ran down the floor, and made a puddle under the carpet.
 
I had a master cylinder start leaking on my '73, back in about '78. I was building a house and had no place to do any work on my car, as our family was living in a friends home while they were away for the summer. So, took the car to a local shop, they changed the oil/filter, and 'topped off' the brake fluid reservoir with whatever US brake fluid they had at the time (this may have been the only BMW they had ever seen in their shop - associated with a filling station). Within about a week, the MC started leaking into the servo (it may have already been starting to leak, since it was about 1/3 low). BUT - I wasn't happy with their service, so never took it there again. Not a fun week!
 
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