Rear brakes locked?

[deronmoped]

Today my rear brakes locked up on me.

I had just gotten off the freeway and was stopped at a light. As I pulled away from the light I noticed the rear brakes dragging. They kept getting worse, I had to pull over. At this point I could smell the brake linings and a very small amount of smoke was coming off the rear brake drums. I let them cool for a few minutes and then climbed under the Bus and took a wrench to the proportion valve side for the rear brakes. A little fluid came out and I could hear the brakes release fairly loudly. At this point I was able to drive away and the brakes were working correctly all the way home.

What the heck is going on? I know the proportion valve is supposed to hold like ten pounds of pressure on the rear brakes so as to make them react quicker. The only thing I can think of is this valve was holding too much pressure this time. I guess I should tear it apart and see what is going on.

Anybody have similar problems.

Thanks, Deron.

[Wildthings]

The proportioning valve decreases the pressure to the rear brakes in proportion to the front during hard braking to keep the rear wheels from locking. I don't know that it hold any pressure against the rear brakes at all. That is certainly not its job.

If your brake pedal doesn't return all the way or something blocks one of the ports in the master cylinder you will get brake dragging.

[SGKent]

[skills@eurocarsplus]

i would check the rubber hoses too. had one go bad on my single cab. but hell it was 40 years old. did the same thing your talking about
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[1975 Kombi]

[enjoispammy8]

start with the hoses and work your way back from there.
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1969 Westy, 1690cc. montana red and cloud white.
1969 Standard Beetle [sold]
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[SGKent]

you wouldn't think both read hoses would fail at the exact same moment. On principal all the rubber brake hoses should be replaced on a 30+ year old car.
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"Most people don't know what they're doing, and a lot of them are really good at it." - George Carlin

[1975 Kombi]

Small piece of rubber blocking the return at the rear union.
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Brett

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[deronmoped]

I doubt it's the hoses, could be the pedal is not coming all the way up. If you read my post again, I released pressure at the rear brake line attachment to the proportion valve. If the pressure was blocked at the hose, what I did would have not released the pressure. Plus it would have to be both hoses failing at the same time.

I'm going to check the free play and make sure the pedal returns all the way.

Deron.

[SGKent]

as the master cyl is pressed, the cups move forward and cover a small port that leads up to the reservoir. When the cups return to rest that port is unblocked so excess fluid can return to the reservoir and the rear pistons fully retract. There is fluid that gets behind the cups as well. When debris blocks that port (one for front and one for back) the fluid behind the cups can't return to the master so it moves past the cups because of the way they are built. It collects on the front side where pressure is less and holds the brakes engaged.

Those small ports can be blocked by the MC not returning as you suspect, or by debris - such as s small piece of rubber torn or worn from the cup.
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"Most people don't know what they're doing, and a lot of them are really good at it." - George Carlin

[Eric&Barb]

[wampe]

Maybe the brake shoes are adjusted too tight.

[Desertbusman]

Pull the rear drums and give everything a good inspection. Might be time for a complete rebuild of your brake system.
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71 Superbug
71 Westy

[Tom Powell]

the parking brake handle/cable was not fully releasing, causing the right rear to drag and .. new ..

[SGKent]

looks like time for new shoes and maybe drums too.
_________________
"Most people don't know what they're doing, and a lot of them are really good at it." - George Carlin

[wampe]

Glad you found the problem. It looks like you need a total brake rebuild back there. Did the drums get scored? I did the brakes on mine recently. Replaced shoes, wheel cylinders and hardware. My drums were good. It was a pretty standard brake job, nothing really strange. I would check the front brakes also, they may be as old as the rear.

[deronmoped]

I picked up a spring kit for the rear drums. I know the shoes are in good shape as I installed them not too long ago. Weak springs could cause the shoes to not return all the way. Well take a look see and if there is anything else wrong, I will take care of that.

When it rains it pours. Turns out, on the way home my clutch started going, was stuck in bumper to bumper traffic, did make it home though. It was the cable that was frayed and stretching, easy fix.

Deron.

[wampe]

Maybe it's just the photo but it doesn't look like there is any lining left on the shoes. What is left is saturated with fluid which means they won't stop very well. I would recommend a total rebuild. It's best to be safe.

[Wildthings]

The return of the parking brake cable is partially done by the spring on the cable itself. If the cable is not returning then replacement of the cable itself is probably in order.

A sticking parking brake cable is not going to cause any residual pressure in your lines, so fixing the parking brake will likely not correct your original problem.

I agree with others, from the pictures it doesn't look like there is any lining left at all, maybe its just the angle and shadows though. It does look like there is something leaking, maybe brake fluid or maybe wheel bearing grease. Slave cylinders and wheel seals may be in order.

[germansupplyscott]