Weird braking symptom

[Dandydog]

My treasured '88 ic is displaying a disconcerting braking problem: infrequently, the brake pedal offers only limited resistence (and very little braking effect) when depressed. Allowing the pedal to return to the top of its travel and depressing it again restores brake function but not my pulse rate.
There's no sign of leakage either at the master or wheel cylinders, and the problem occurs only occasionally (once a week or so); still, I'd kinda like to fix it.
Brake booster leak? Water in the brake lines? (This problem didn't start until it got below freezing outside.) I really can't think of anything else that would cause such erratic behavior, and I'd welcome some other suggestions/opinions.
Thanks for your help -
Neil

[superj]

no idea here. maybe start with a fluid change and see what happens. it can't hurt because it needs to be done anyways

[aaverhulst]

It would not be your booster as that would make the pedal harder to push. If no leaks, seems like it could be your master cylinder. And you are right, fix as soon as possible.

[Geoffzie]

Has your car been sitting for any length of time?

Actually, I've noticed the same behavior on my newly acquired 1988 325i convertible. I bought this car with a broken timing belt, so the car had been sitting for an unknown period of time. After I got the engine back together and began driving the car, the brake peddle would do it somewhat often ... but since I've driven it a while, it only rarely does it ... but it's still there.

I've been fixing other things and haven't really been driving the car much ... so I need to get into it.

I believe there are two things that could cause it ... internally leaking master cylinder ... or brake pads that are hanging or sticking because of rusty caliper slides (usually because the car has been sitting for a while).

When a master cylinder is leaking internally, there will be no external signs of leakage ... it's leaking from the pressure side ... to the reservoir side of the internal seals. And it's usually caused by water getting into the brake fluid (usually from the car sitting) and corroding the bore of the master cylinder.

The best way to check for that is, with the engine running ... push down on the brake peddle hard, then slightly release pressure WITHOUT really lifting your foot off the brake ... then pushing down hard on the brake peddle again. If the peddle goes down noticeably further ... you've got internal seal leakage in the master cylinder ... time to replace it.

I would start by flushing and replacing the brake fluid as superj suggested ... especially important since older cars (and older brake fluids) are more prone to collecting water. I would also check all four calipers to make sure your pads are sliding freely ... make sure the slides are free from rust ... and I even SLIGHTLY grease my brake pad ears to allow them to slide better. (obviously, don't get any grease on the discs or pad surfaces)

[superj]

that is a great thing to look for, the rusty calipers. I slightly grease slide areas on brakes also. I always have, I don't know why, even on drum brakes back in my old mustang and karmann ghia days

[Dandydog]

Great analysis, Geoffzie! I was puzzled because i could't think of any condition that would produce those symptoms intermittently; both of your suggestions fit the bill perfectly.
And your test for master-cylinder leakage makes good sense, also: I'll run that check in the morning, and follow through on your maintenance suggestions over the weekend.
Thanks a bunch for your help!

[Dandydog]

Anybody got a source for E30 master cylinder rebuild kits? (I've struck out on BavAuto and Pelican Parts.)

[Jeffnhiscars]

Seems to me I couldn't find a rebuild kit and not really knowing how rebuildable the old master was, I grabbed a new Ate for $168. Seemed reasonable given that lives depend on it

[superj]

sounds good

[Geoffzie]

It's generally better to replace the master cylinder rather than rebuilding the old one ... especially on older cars. If in fact water DID get into the brake fluid ... it would have corroded the inner bore, making rebuilding unprofitable.

The only way to tell whether it's a corroded bore ... or just worn-out seals ... is take it apart and inspect the bore.

[Dandydog]

A couple of notes:
- BMW strongly recommends replacing rather than rebuilding the master cylinder. (They enforce this preference by using a circlip on the cylinder bore that doesn’t have any holes by which it can be compressed.)
- After replacing the cylinder and bleeding the system, I’ve still got the same problem. Bummer.
- The little bottle-and-tubing device available online and in auto-parts stores is the best $5 plastic tool on the market. You need to replace the male bleeder-nipple fittings in the kit with a female one to make it work well, but with that mod the tool takes all the work out of bleeding the brakes.
- Next step’ll be an inspection of the calipers, pins and pads. I shoulda done that first.

[superj]

dang, that sucks that the problem is still there.

- - - Updated - - -

maybe its the rubber brake lines are expanding when you press the brake, like they need replacing?

- - - Updated - - -

I have no idea, I am just guessing

[doctorx69]

a couple of thoughts after a complete rebuild of mine.... i would get a pressure bleeder and redo the bleeding from the start. At least when i was doing mine, the abs pump was a pita to get purged. My guess what you are seeing is air trapped in abs pump. Also, it could be a caliper sticking. I ended up replacing all 4 of my calipers because when i took one off, it was rusted solid. In my case, the brake booster was bad. It would bind and the brake would stay on floor with no brakes. I drove it 40 miles like that from my parents. Yes, replace the hoses too.