The Stovebolt Site Forums The Shop Area General Truck Talk Rear brakes locking up

Hi , just did a front brake disc brake conversion on my 57-3100. The rear brakes lock up easily and the brakes engage at the very top of the brake, so there is no ability to slow down just lock up. It is almost unable to be driven, actually it was doing this before the change over but not as bad. Any thoughts? Thanks kevin

You did not say if you have disc/drum or disc/disc.
Since your MC is below the wheel cylinders or close to it, you probably need residual valves. Most kits will come with 2 lb for front and 10 lb for rear. (disc/drum). It could have come with a built in proportioning and porting for what you have. Adjustable valves are sometimes recommended. Refer to your instructions, double check your plumbing and contact vendor. If it was doing it before change over I would suspect weak return springs or other malfunction inside brake drum. Should replace all parts in there. Shoes need to contact stop boss when brake released. If you had too much pressure in rear it would do that but you can't have too much with a 10 lb residual or no residual. If you have a adjustable maybe it's set wrong. Otherwise it has to be bad/missing parts inside drum..... If you have disc/disc, different story involved.

Hi should have clarified. The change involved drum to disc on the front with a cpp kit. Drums still in back. New set up is a fire wall mc and booster. Ran new lines throughout front and back. Wondering is brake cylinders on rear drum could be frozen or something g like that

Hard to say without inspection. Dig in and find out. Take drums off and let us know what you see...or post pix.

Be sure the shoes are in the right location, longer one to the back shorter on the front. Get them swapped, and it will grab every time.
Also be sure you plumbed the master cylinder right. The first one I used was backwards from what I through. I had the rear brakes plumbed to the front portion of the cylinder and the front to the rear, it made for all kinds of funny stopping. The front brakes apply first, and having them plumbed wrong, made the rears lock right up with little to no front brake.

I had to add a adjustable proportioning valve and limit the fluid going to the rear shoes. Normal driving was OK, under a hard stop it would just lock the rears. Now with some limitation to the rear, its a stiffer pedal, but I live with it knowing under a panic stop its not going to lock and slide.

I used a disk setup on the front, and stock '79 GM rear drums, non-power master cylinder mounted under the truck with out residual valves. The adjustable proportioning valve came from Speedway Motors
Joe

Howdy Kevin,.... At the risk of being presumptuous, I hope it's not thought as that, but when you have dual brake system with disk on the front and drum on the back the master cylinder will have a pair of chambers. The big reservoir goes to the disc brake, the little one goes to the back brake, it has a residual check valve to maintain about 5 pounds pressure, the disc brake chamber has no residual check valve, if it did it would wear the brake linings off. If all that is good , then as previously mentioned, by Joe, check your rear linings. If the front brake shoe lining goes to the top of the shoe your brake application will either be absolutely on, or absolutely off ,it won't be gradual. You either have them on backwards or the wrong linings. Good luck.....Terry