I think I have yet another bad master cylinder...
Last summer my brakes acted 'funny'... sometimes they worked, sometimes tey didn't. So I replaced the MC and everything worked well.
Then about a week ago I snapped a caliper brake line. The brakes ran dry, and although I didn't think so at the time, I may have pushed the pedal to the floor many times. I ended up replacing both rear calipers, and with the help of the forum, bled the brakes.
But my brakes aren't 'right'. It seems that sometimes my brakes work better than others. The first pump is always the best, and sometimes they fade. This isn't as bad, but is similar to the problems of last summer.
My computer is doing something strange so forgive me if this comes out a jumbled mess...
Back to the brakes. I kept my old MC, thinking it might be original. I am glad I did! I took it apart and plan to rebuild it. It was full of gritty black 'peanut butter'! The onlt thing I can find wrong with it is the seal closest to the front of the car is a little thin on one half. Maybe half a milimeter. Is this enough to cause failure? Could the gritty sludge be the culprit? Should I hone the MC before rebuilding it?
Thanks! :)
Jim

 

Jim...Any sludge is bad sludge plus any wear or deflection in the seals is call for at least a full kit in your MC. A good flush of the entire brake system is also called for by the sound of it.

 

Did you bleed the master after you ran it dry? You should. That alone should help you out right there. But you may have to bleed the whole system all over again.

If you are going to flush it all out. Use a lot of denatured alcohole. Drain the whole system of all the old fluid, fill the master with the alohole and open up the bleeders and pump it though. Keep refilling the master with the alcohole until it comes out almost as clear as it goes in. Let the car sit for a day or two to allow the remaining alcohole that may be in the lines and metering blocks to evaporate. I did that to mine and switched over to the dot5 fluid while I was at it. Never a problem since.
Just my .02, good luck
JoeB

 

Welcome to the wonderful world of master cylinders. As you noted, the sludge in there is probably the culprit. Should you hone it? Only if the bore of the MC has pitting or any sort of scratches. If the bore is in good shape you can just put a rebuild kit in there. My experience with rebuilding MC's is only a history of three: a non-power MC out of my 66, two power MC's out of my 77 Buick and my 72 Corvette. All three had black primordial goo in the bore of the MC and I was surprised that the brakes worked at all. In the case of the Buick and the Corvette, I would coast up to a stop, hit the brakes and then panic when they went to the floor. :eek: Pumping the brakes rapidly helped. So I nurse the cars home and the brakes worked perfectly. :confused: Boy am I confused. I put in a $20 rebuild kit and everything works fine. My daughter has the Buick at college and two years later the brakes are still perfect. One year on the Corvette MC rebuild and the brakes are perfect.

I would rebleed the brakes a couple of times since it sounds like you have air in the lines. Have you ever replaced the rubber lines on your Corvette. Now you know why the mechanics tell you to bleed your brake system every other year. My mother-in-law passed away a year ago and left us a 22 year old Mercedes diesel. Just to be sure I sucked out the old brake fluid and bled the brakes. OMG, the black crud that came out of there couldn't have been clean brake fluid at one time. :eek:

Gary

 

I think you (might) change your mind on the "rebuild" when you price a rebuild "kit" versus a rebuilt master (i have just been down that road). I would like to know why a "kit" is $35 but a rebuilt MC is $25 ??? I guess it`s the same reason a "parts store" gets $40 for caliper seals and a sleeved/rebuilt caliper can be had for as little as $59. ...redvetracr

PS: I know caliper seals are $8 per caliper at VB, BUT it frosts me when i "need" them and have to wait for the UPS truck seeing how the parts stores are 10 minutes away!

 

You should hone the bores if there is any wear at all. If you dont hone them and they have a wear ridge, you will damage the seals on the pistons when you bleed the brakes. (when you bleed, the pedal goes to the floor. This pushes the piston into a virgin area of the cylinder).
Any time the pedal drops to the floor it will cause damage to the master cylinder unless it's a new or rebuilt unit.

 

I am getting readu to start on my brake system after I finish the suspension and IO am going to go with a new\rebuilt one from VB. Is there any reason why you want to rebuild what you have instead of just replacing it? I looked at rebuilding mine and for the price of a replacement it just is not worth it. Just my 2 cents. Good luck :cheers:

 

Here's the way how I understand this can happen. When your pedal went to the floor, the piston in the MC travelled farther than it normally does. This area of the piston normally accumulates dirt and when the piston travels this far, this dirty part of the piston can make it past the seal and into the fluid reservoir.

When you bleed your brakes, you should place a 2 x 4 behind the pedal to keep the pedal from traveling all the way to the floor.

 

Thank you everyone!
I wanted to rebuild the original cause I thought it would be cheaper, and more fun. My last MC cost about $100, and the rebuild kits were about $30. So I called the parts store, and they couldn't get me a kit, but a MC costs $21. I am afraid what a $21 MC might be, but it is worth a shot. I'll find out tomorrow!
Now I am thinking I just have really bad luck. And really bad brakes. And what is that black stuff anyway?
Thanks again!
Jim