A while ago I had to apply incredible pressure to by brake pedal and ever since it has been very soft and will travel to the floor. The pedal does not sink at a stop and I am not losing brake fluid. Pumping the pedal will not increase the pressure. What is the weakness of a master cylinder and could this actually be my brake booster?

 

Same problem here! I'm curious to know.

 

i have the same problem and was told that the booster was leaking vacuum and thats why the pedal hits the floor. mine does it randomly though.

 

well to me it sounds like not a slave cylinder problem...it's the brake booster, its what the guy said above me said. it's a vacuum leak, replace your brake booster and you will have your full potential braking ability back...have a nice day!

 

From reading in another forums, some guys had the same problem and it was the MC. I don't want to replace the brake booster and then find out it was the MC.

Any clues on how to check the functionality of the MC?

 

Anyone know if brake boosters are rebuildable? I know there are gaskets in there somewhere.

 

first replace the plastic valve and vacume line, see if that fixes the problem. If it doesn't pull the master cylinder out and bench test it, IE drain it completely fill it up and prime it. Step one, remove master. 2, drain. 3, re-fill. 4, secure the master in a vice or clamp. 5, take the end of a hammer and depress the little metal cylinder on the back of the master. 6, repeat 5 untill no bubbles appear and you have less than 1/8 inches of depression. If it bottoms out and never gets hard to do this then you have a to replace the master ($50-60). If it works then you have a booster problem and replace that.

 

Did you change your brake pads recently, or mess with the bleeder valve on any of the calipers? If so, did you bleed the system of air bubbles?

If you are positive that your not loosing any fluid, the problem is most likely an internal Master Cylinder leak, or the booster.

When the car is at idle and the brake pedal is pressed several times, does the idle fluctuate as if you have a vacuum leak? If so, check the booster first.

 

Does it actually get to the floor on those occasions or is the pedal just real spongy and the more you push the closer it gets to the floor?

If it's the first thing than the booster sounds like the best guess to me, if it's the latter, than doing a full brake flush might help. Whether it's air in the system or that the fluid is just too old and full of water, etc. like mine was.

 

Thanks, all this info is helpful. Like I said originally, this happened immediately after I depressed my pedal as hard as I could. When I did this, I pushed the pedal as hard as I could and at this time my brakes were fine and the pedal did not go to the floor, but after a few seconds of this pressure something gave and my pedal went to the floor and ever since it has been spongy and will travel to the floor if I push hard enough. My brakes cant even lock up anymore so my abs wont kick in when it needs to. This sort of makes me think its my master cylinder now, because if my brakes aren't locking up this probably means there isn't enough pressure in my lines. Ill try the bench test and go from there. Thanks.