Passenger rear drum locks up

[adamm]

Something that affects my car occasionally is when I hit a bump or a pothole the passenger side rear drum locks up which can be a bit hairy at speed.

I've been having no end of trouble with my brakes lately but I wont go into that here, just wondered if the rear drum locking was indicative of anything in particular that I might be able to look at.

In a nutshell the whole brake system was new about 8-10k miles ago, I've recently had problems with the twin servos (RHD) and master cylinder packing up. Both of these have been reconditioned by a specialist. I'm pretty certain that I have a load of air in the system still as the servos are a pig to bleed properly - would this affect the rear brakes in the way I'm experiencing?

Any help appreciated

PS Unfortunately nothing seemed to be amiss when I followed the suggestions here: http://www.bmw2002faq.com/component/option,com_forum/Itemid,57/page,viewtopic/t,310788/highlight,drum/

[jgerry2002]

Did you replace all rubber brake lines? I've seen people replace "everything" and skip the two hoses in the back.

Also if you have a drum tightened down too much they can start to lock up.. though it would be more a constant thing, basically as it heats up.

Not too familiar with the dual servo system at all though. But mechanically you may want to check adjustment/linkage to see if its binding somewhere.

[Paul7002]

(nt)

[JerryC]

Your's is the first I've ever heard of doing this, something's put together wrong or something is failing. If you're not real familiar with the '02 rear brake system, you might take it to a knowledgable shop and see if they can find any missing/broken return spring(s), misadjustment, leaking cylinder, improperly mounted brake cable, swollen rear hoses, rusty stuck parts, etc.

Justin's right on the money about the rear hoses - my tii's PO installed stainless-jacketed brake lines on the FRONT ONLY, the rears were still original as far as I could tell. When I went to bleed the rear brakes a few years ago, I could barely get any fluid to come out using a pump-style pressure bleeder. Replacing those flex lines solved the problem, reduced pedal effort, and improved overall braking.

[adamm]

Hi all, yeah it's a strange one for sure and I thought I'd cured it/it had cured itself but hey!

I did mean 'everything' - I laid under her (I don't have the luxury of ramps!) and did it when I was restoring/rebuilding her. She's just coming to the end of her second year on the road. Is it likely/possible that the rubber lines could have gone bad in this short time?

When this first happened towards the end of last year, I checked the cylinders and both the springs, the pads, the drum, replaced the back plate as although I didn't think it was the culprit it had a ding in it. I visually inspected the rubber lines as well as the hard lines and drew a blank. Finally I readjusted the drums as I'd had the back plate off, and as I recall it had happened since.

Is it maybe because I've had the servos off a couple of times since and it's cause it to go out of whack?!

[Guest Anonymous]

Shortly after replacing the brake shoes. The right rear locked up at speed and flat-spotted the tire. I attributed it to improperly adjusted shoes. I readjusted the rear brakes and it never happened again.

Your problem doesn't seem this simple. I've read post here describing problems w/hydraulics that causes pressure to be applied to the brakes without the engaging the brake pedal. You may have a similar gremlin.

[Guest Anonymous]

I am not that quick to fault the flex hoses for a brake lockup. Three things come to mind though.

1). If the brakes are too tight, the drag will cause things to heat up and expand. It does not take much for a half applied drum brake to become fully applied.

2). A sticky/worn rear cylinder can cause erratic brake behavior. All you need is just one piston moving with the other stuck and it is easy to understand how both shoes are unevenly applied. Even with all the adjustment in the world, one bad wheel cylinder will affect the drum brake operation so that it won't brake like the opposing rear wheel.

3). Weak "W" spring. If this has been stretched over the years, it may not offer enough tension to keep the bottom of both shoes next to the bottom center post. The W spring can be bent back into place. I have done it myself and I have seen it in the archives.