Hi, When I'm stopped at a light the brake pedal slowly sinks. If I release and reapply the brakes it pumps right up to where I started and begins sinking again. I'm not losing any fluid. Is the Master cylinder bypassing fluid internally? Is there anything to be done short of replacing the master cylinder? I've heard some problems with rebuilt master cylinders, where would you recommend I go to find a good one? Thanks for all the help, John
John, It sounds like the fluid is bypassing internally inside the master cylinder. You can either buy a rebuilt MC or a rebuild kit, both of which are still available at NAPA. I have had problems in the past with rebuilt MC's but usually only from the discount chains i.e. Pep Boys/Kragen. Whichever route you choose, be sure to follow the bench bleeding procedures exactly. Pushing the plunger all the way in on a new, dry MC can sometimes cause the orings to tear on the internal passages. It may be a good time to replace any rusted or dry rotted brake lines. Hope this helps, Rob
You have the exact same symtons my '78 Impala had and it was the master cylinder. Bought a rebuilt from Advanced Auto for I think about $30 and it solved the problem.
Assuming the car is a 4 speed, sounds like it is still car-guy safe. I would know - just don't drive in front of me. :laugh: :laugh:
When you say "bypassing fluid internally" do you just mean there's a leak inside the master cylider? How much does a rebuild involve, just replacing pistons/springs/seals? Is it more advantageous to rebuild or buy a new one? Just noticed my brake pedal doing the same thing at a stop light the other day and I couldn't find any fluid leakage on the outstide, reservoir was still full too. Larry Long Live the A-body!!
Larry, The fluid bypasses internally by going around the piston seals instead of staying inside the chamber. Assuming your MC is not in bad shape, (rusted or scored badly), A rebuild is pretty easy. Remove the snap ring that holds the piston and ot should slide right out. When you install the rebuilt kit, make sure everyhing is super clean and coat the parts with fresh brake fluid. Hope this helps, Rob
For the price of good-quality reman units, you may not want to spend the time rebuilding one. Check with your parts store on reman m/c vs. kit. Just a thought.
i prefer new m/cs. the only way i would consider a rebuilt m/c would be if the mc was bored out & a new sleeve installed.
Ive got the same problem in my Skylark and its the master cylinder. When the guy I had bend up my brake lines took apart my MC (I asked him to check out the rest of the braking system while he was at it), he said that mine was starting to let a bit of fluid around the seals and it was full of sludge. He cleaned the sludge out but it still was bypassing fluid. I dont drive the car much now (engine trouble) but before I start Im going to replace the MC. Edit: New ones available from parts america/advance auto.
Every few years my late father would pull the master cylinder out and rebuild it or clean it weather it needed it or not One thing he would always notice that fine dust would get into the reservoir and would turn into a slick dark gray paste and the cups would ride over it and cause pedal fade
Great info guys, thanks. I'll probably just buy a new one, ask my buddy down the road to help me bleed the system. Now if I could just get a day off... Larry Long Live the A-body!!
