TheSamba.com :: View topic - Brakes needing to be pumped after new master cylinder

Wed May 21, 2025 9:24 am

A possible clue here is the report that installing the pistons into the calipers was difficult. The square profile O ring caliper/piston seal may have been distorted. In normal operation the seal deflects when the brakes are applied and when the pedal is released the seal returns the piston into the caliper. That's so the pads do not drag excessively on the rotor. If the seal is distorted the piston may not extend correctly.
Try this: With wheels off the ground have someone push on the brake pedal while you spin the front wheels. See how long it takes (pedal travel or pumping) to stop the wheel. Then let off the pedal and watch the pads and see how far they retract. Should be barely visible, but the wheel should turn freely, there will likely be some rubbing of the pads on the rotor, as long as there is no excess drag its OK.
If the pistons retract a lot the calipers may need repairing. May need new O rings.

Thu May 22, 2025 10:20 pm

Just wanted to say that I was having a similar issue in my 90 Multivan and a new brake master cylinder solved the problem.

Fri May 23, 2025 7:16 pm

Bone head move of the year. I installed the calipers on upside down… bleeder screw was lower than the trapped air in the cylinder. Brakes are working great now. Thank you all for the help!

Sat May 24, 2025 4:00 am

Thanks for the update. The experience curve can bite at times. Probably should thank Go Westy for going above and beyond with you. That has been my experience.

Sun May 25, 2025 8:38 am

Turns out it was a problem with the calipers, but not the way I had theorized. This type of situation, calipers on opposite sides, is why in aircraft maintenance a second or even third set of eyes is on the procedure before things go back into service.