Rear brakes locking up

[383Regal]

I have a 1981 Buick Regal, and I installed a Chevy 383 stroker motor. The car is 90% complete. But….. when I try to stop the car quickly, the rear brakes lock up. The fronts are fine. I replaced the power brake booster check valve, and every test shows that the booster is fine. Please help. I am tossed between the proportioning valve, or a complete new rear brake setup. I am no A tech mechanic, but I am handy. I am pulling my hair out trying to figure this out. I need to be able to stop this car.

 

[CopperNick]

On the front of the stock factory proportioning valve there is a stubby button that needs to get pushed in before the brakes are bled. it controls the residual line pressure to the rear brake shoes. Basically the rear shoes need pressure to already be present in their lines to help them actuate. Although braking is mostly accomplished by the front drums or discs, it actually starts at the back. The wheel cylinders don't self-actuate as such. They need that little bit of help to nudge them into actually pushing on the shoes.

Once the lines have been bled and all the air evacuated using the brakes will reset the valve and all should be well. if you do go to the manually adjustable valve then I would suggest deleting the factory one out of the system. My thinking here is that they might fight each other for whcih one is doing the work and which is along for the ride and having them both might mean you end up with too much line pressure being fed to the rear shoes which could make them twitchy and prone to maybe lock up????



Nick