Master Cylinder
 

I'm on the road so I won't be able to respond to this for probably a long time, but I was wondering what could effectively ruin a brake master cylinder aside from high miles. I recently changed my pads and didn't realized not to take off the brake lines and I'm pretty sure that at one point there was a large amount of air in the system. I bled it completely and the pedal was still sinking to the floor so I was convinced I was doing it wrong so I took it to a local shop to have them do the exact same thing and bleed the entire system but it is still soft.

Their expert opinion is that I should replace the master cylinder but at their price I thought it would be better to check the forum. I can say with a pretty high degree of certainty that I thought that it was pretty thuroughly bled twice and when the lines were draining that it was dry at one point. Anything that anyone can tell me would be greatly appreciated.

On a side note, I won't be able reach a computer for a while so if you come along and read this and the answers that were posted seem like it could use a little more attention, a free bump would be appreciated.

Did the master cylinder ever run dry of fluid when you were bleeding? If so you may need to bench bleed the master cylinder. You must be absolutely certain the master has no air in it before you can bleed the rest of the system.

I'm short on time right now to explain but do a google search and you should find some info...

I once heard someone say that bleeding the brakes can lead to master cylinder failure. Obviously, that is no reason not to bleed your brakes, but it seems plausible that it might kill the master cylinder under certain circumstances. The explanation was that you push the pedal in much further during bleeding than you do in normal use. If there is some corrosion or dirt or something inside the master cylinder, it can take out the piston seal(s) when you push the pedal to the floor. I think I experienced this with my daily driver. I changed the brake fluid and bled the brakes, and afterward the master cylinder went bad. I replaced it with a reman from the local auto parts store and the pedal was firm again after that. You may have experienced the same thing with your car.

-Max

sure it's not the brake booster? how does the feel compare to when the car is on/off?

The booster has nothing to do with the pedal sinking to the floor.

Sometimes the master break cylinder can bleed internally and there's nothing you can do but replace it :(

I have the problem of my break pedal sinkign to the floor as well. It was this way when I bought it from the previous owner.

I put in hawk HP+ pads, bled the system, and still the pedal goes to the floor with very little resistance.

You probably have already done this but whatever.
Go around and check to make sure none of your calipers are leaking. Mine was and the brakes went right to the floor. I ended up rebuilding them all and now all is good.

it the pedal slowly sinks to the floor and there are no signs of leaks, and you are confident that all air has been removed from the system. then it could possibly be the cups, inside the master cylinder. if the cups go bad, the brake fluid will slowly pass by them causing the pedal to slowly sink to the floor. you could contact the dealer and see if you can buy new cups and pistons. i would say replace all the cups, the primary and secondary pistons and any springs inside the cylinder. i would try to do that then buy a whole new master cylinder, cause the cylinder itself is most likely good, just the internal parts are whats causing the problems