Bad master cylinder?

Ok, I posted this over on JU but I never really got any constructive responses, so here goes:

Ok, heres what I got... 98 XJ, 102k, with the following just replaced: rear brake shoes, spring kit, self-adjusters, and wheel cylinders. I bled the entire system as well.

My problem before was that the back wheels wouldn't even lock up on packed snow/ice. It was solved with the new wheel cylinders, shoes, etc...

My problem now is this: If I nail the pedal at 40 - 45, the pedal goes about half way down and all 4 wheels lock up, as they should.(I don't keep them locked of course) Now, if (at the same speed) I hit the pedal firmly but not enough to lock anything up, the Jeep does a nice nose dive and comes to a quick halt. But, as I'm stopping this way, if I keep pushing harder on the pedal, it'll go d@mn near to the floor with no increased stopping power going to the brakes.

Now this problem was most likely there before, I just didn't notice it. I just came across it while testing the brakes after the rebuild.

Thanks...

-Chad

Classic textbook master cylinder failure. The cups on the master cylinder are designed so that the harder and faster you push, the more they expand, so often it will give you a firm pedal when you hit it hard, but when you push more gently, the cups will leak fluid and the pedal will sink. One of these days you may hit the pedal and the initial leakage will be enough that pressure doesn't build up and force the cups outward. The pedal will drop to the floor and...well, if you're a quick thinker you probably won't die, but you need a new master cylinder now.

Ugh.... As I expected. Anyone else ever experience the same thing or something similar?

Did you use the pedal to bleed the system ? If so did it get pushed to the floor during the process ? Most likely you damaged the seals on the MC by pushing them past the crud that builds up there.

Nope, gravity bled the whole system.

I had basically the same thing happen when I "acquired" my 90 model. changed out the front pads, the rear wheel cylinders and shoes. If I nailed the pedel, it would stop but if I applied lighter pressure, the pedel would sink to the floor.

Changed out the brake master cylinder and it is mucho better now. Make sure you follow the instructions and bench bleed the master before installation. The installation itself is not that hard, but it can get messy and then you have to bleed all four wheels again.......so get plenty of brake fluid.

Hope that helps!