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.

 

[xylorex]

You get the lines swapped around at the master cylinder?

 

[1KWIKSIX]

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.

Perhaps you’ve got the larger 7/8”bore rear brake cylinders installed (off a manual S-10 pick up truck)? Vs the originally supplied 3/4” bore cylinders?

 

[Tomeal]

If you did any work to the rear brakes, make sure they are adjusted properly. Drum brakes have a star adjuster that sets the rear pads.

 

[bracketchev1221]

Is there anything that’s been replaced recently or disassembled? Incorrect hole on pedal to booster? Brake lines reversed? Are the hoses good? Could be proportioning valve is sticking.

 

[383Regal]

The rear is a 72 gm rear, so I don’t think it will have s10 cylinders. Other than adjusting the rear brakes I have never messed with it. I know the lines from the master cylinder are not swapped because the car has line lock. And that works properly. I have visually checked the brake lines and they appear to be ok. That doesn’t mean they are. I can stop the car, but once I get the pedal to a certain point the rear brakes lock up.

 

[-dw-]

The rear is a 72 gm rear, so I don’t think it will have s10 cylinders.

Thatll do it. The 72 rear axle has 7/8" cylinders.

 

[383Regal]

Thatll do it. The 72 rear axle has 7/8" cylinders.

Thank you for your input. What’s my fix?

 

[scoti]

An inline adjustable bias valve for the rears would allow dialing things in.

 

[-dw-]

As mentioned adjustable rear brake proportioning valve. I tried four short shoes once. It helped alittle. Actually it cost nothing to cut a bit of friction material off to make long shoes short. I used to drove around alot on drag radials and rear brake lockup wasnt an issue.