Right rear brake premature lock up

samotn · Mar 12, 2008

I am having a problem with my '69 Chevelle SS-396. Car specifics - power disc/drum brakes, 396 CI /325 HP, 4 speed, 4 year old restoration, mostly stock. The problem - any time I have to apply anything more than light brake pressure, the right rear wheel locks up. The remaining three wheels are fine, apply the brakes normally. I have inspected the front calipers, rear wheel cylinders, they seem to be working fine. I replaced the master cylinder, prop valve and vacuum booster 6 months ago, with no improvement. In reality it seems to be getting worse. Any ideas or suggestions on possible causes. This weekend I am going to re-bleed the brake system starting with the furthest wheel from the master cylinder, working towards the closest. Has anyone every had this, if so, is there a cure. I an pondering addition of an in line brake proportioning valve to the rear wheels if this continues.

Regards,

Nelson

Gary S · Mar 12, 2008

If only one rear wheel locks up, it isn't a proportioning problem because the rear wheels share a single line.
The problem is something in the right rear wheel brake. Do some simple detective work. Does that drum run warmer than the other side? If so, it probably needs new brake springs to retract the shoes properly.
Even more likely is that you have a slow leak in the axle seal on that wheel and you are getting an occasional drop of rear end lube on the brake shoes. Inspect the shoes carefully for dark spots that indicate possible contamination. It takes only one drop of rear end lube on a brake shoe to cause problems like you are having.

vrooom3440 · Mar 12, 2008

In my experience leaking rear end oil will cause the brakes to NOT lock under any circumstances. I have that issue where my left always locks and my right never locks. But then the right got a good dose of oil all over it. Kinda a poor man's ABS.

samotn · Mar 17, 2008

Thanks for the feedback. I ordered a rebuild kit for the rears, new wheel cylinders. I had the wheel and drum off this past weekend, did not spot any leaking fluid. Could it be trash in the brake lines or weak return springs? I am going to flush and bleed the brake system again once the components are replaced. Will the prop valve interfere with bleeding?

Regards,

Nelson

Dean · Mar 17, 2008

My dad taught me to use regular rubbing alcohol to wash the brake shoes with when they grabbed due to having grease or brake fluid on them and it worked great.
Back then rubbing alcohol was only 10 cents for a big bottle so the fix was cheap.

It was also good to use in the gas tank in the winter time.

samotn · Mar 19, 2008

I will give it a try. I usually cleaned the pads or shoes with brake parts cleaner prior to installing. I worked on it this weekend with no success. I am now contemplating taking the guts out of the proportioning valve and installing an in line prop valve underneath along the frame rail (hidden from view)

65cayne · Mar 20, 2008

I had the exact same problem on my '97 cav (rear drums). I replaced the brake cylinder and also loosened the adjuster screw at the bottom (which I had previously over adjusted).

One thing mine also did though, which you did not mention in your case, was that the brake would remain engaged when you released the pedal, then it would disengage a moment or two later (clunk). Anyway, $7 later and it's all good now.

samotn · Mar 26, 2008

Thanks for the insight. I bled the brakes, got all kinds of krud, not to mention there was some debris in the wheel cylinder. I have ordered new wheel cylinders, all new brake lines. Once I get everything in, it should be like new.

Olle · Mar 26, 2008

samotn said:
Thanks for the insight. I bled the brakes, got all kinds of krud, not to mention there was some debris in the wheel cylinder. I have ordered new wheel cylinders, all new brake lines. Once I get everything in, it should be like new.

That's a very good start, it could be a stuck wheel cylinder and it sounds like it's time to change it anyway. Also check the self-adjusters, they can cause the problem you're seeing if they don't work right. They can be a bit finnicky, but they usually work well when they are clean, lubed and set up right.

Like Gary said, it could also be something that prevents the shoes from retracting. The prime suspects would be the springs or something that's stuck in the parking brake assembly.

samotn · Apr 2, 2008

I could not get the passenger rear drum off it was so tight. No wonder it would lock and hold. I pulled everything off and did a complete rebuild including wheel cylinders, springs, shoes, pad. As I do not have self adjusters, I used micrometers to compare drum to shoe dimensions, adjusted to obtain the proper clearance. I still have to finish bleeding, hopefully will take it out for it's first Spring cruise this weekend. Thanks to all for the advise and assistance.

Right rear brake premature lock up

samotn

Gary S

vrooom3440

samotn

Dean

samotn

65cayne

samotn

Olle

samotn

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