After replacing the rear calipers and pads my brakes were locking up. I'm a Quality Engineer so I was determined to understand. After a lot of experimenting I ended up with the booster and master cylinder.
The master cylinder was opened up and it's aluminum with a sliding plunger AND check valve. The check valve needs to touch a pin and only has about 3/16" travel. (You can actually remove the reservoir, small disk and fluid to see the check valve touch the pin. But I did this only after removal and reinstall onto the car.)
Why was my check valve not touching the pin? Since my car has about 120K the springs in the master and /or booster had gotten weak and the booster was barely keeping the check valve from actuating. Vacuum moves the check valve away from the trigger pin when the car runs and if you don't spring back the check valve stays shut thus locking the brakes.
You are in luck. The booster plunger can be adjusted by pressing the brake to extend it while the master is removed. (A 24" board between the seat and pedal help!)I used vice grips to hold on part of it and a wrench to adjust it. They appear to be glued together but I won. Perhaps my small torch helped. After adjusting as far as possible everything was back to normal -- almost. Once the brakes worked again the switch on the brake pedal was not contacting. This was fixed McGiver style with some stick on spacer(s) to trigger the brake pedal switch.
Why did changing pads cause a new problem of wheel locking up? My guess is there was some air in the lines that was eliminated during caliper bleeding.
Good luck to ya.
Wednesday, August 10th, 2016 AT 5:55 PM