I know master cylinders can fail in two ways, by leaking and by the brake fluid by passing the piston seals. My question is how ever is there a way for the master cylinder to fail resulting in having to press the brake pedal down further to make a vehicle stop but it isnt leaking nor is the fluid by passing the piston seal?



I ask cause on my 78, I used the mityvac I got to flush the old brake fluid out and I had the rear passenger drum brake was apparently not working it was blocked up. I put vacuum on it but nothing came out, I pressed hard on the brake pedal and felt a snap in the pedal and then the fluid started coming out. After I did this my brakes felt alittle worse than they were before. Before the ceramic pads stopped the car decently but I thought I had air in the lines since it required more brake pedal effort compared to the semi metalic pads I had. So I bled the brakes after flushing the system and I am not getting anymore air out of the system but the brakes require me to press them down 2" before the pedal gets firm and then I have to press down more than that to mak the car to stop. I even have to almost stomp on the brake pedal in my opinion to get the car to come to a complete stop. The car doesnt creep once its stopped and the brakes are not required to be pressed harder.

I just dont know if its the cermaic pads require more brake pedal effort, or if its the master cylinder failed in a way I never heard of before. All I know is since I flushed the brake fluid out and put fresh in the brakes seems like they only work if I hit the pedal harder than I am acustom to. I also started thinking maybe that one blocked up wheel cylinder resulted in firmer pedals and a 78 Mercury has this soft pedal. But I just feel the braking isnt right for some reason.

If anyone can help thanks in advance.

 

Around that time frame your vehicle should have a proportioning valve with a pressure differential valve in the center. The purpose of the differential spool valve is to shut off either the front or rear circuits if the brake pressure is not equal on both circuits due to a system failure and prevent a long pedal travel. While normally self-centering, with age they get stuck to one side often while bleeding.

I have a feeling that is what has been happening to you. After the new pads were installed it appears to me the valve shuttled and shut off the rear brakes. This would cause less then ideal performance (no rear brakes) and this is what caused you not to be able to draw fluid from the rear wheel cylinders.

When you pressed on the brake when bleeding you got the valve to shuttle to the opposite direction. Sometimes tapping on the valve will get it to center, or with older vehicles sometimes you have to bleed a little from the opposite axle to get it to center. If you search this site you will find some answers about using a little clip to hold the spool valve in the center position when doing brake bleeding.

That's not to say you aren't correct that a frozen wheel cylinder will give you a higher pedal. It will due to the lower brake fluid volume without the wheel cylinder pistons moving out. Often consumer comments from that is the pedal is high but the vehicle does not stop as well.

 

There is a Propornating valve and when I got the car due to a leak that the car was being drove with, the valve was locked to the rear. I tried everything to get it unstuck but I fixed the leak and bled the rear brakes since I rebuilt the driverside rear wheel cylinder. I didnt touch the passenger side rear. The driverside rear with vacuum sucked just fine, given the brake fluid was black as oil, but when I got to the passengerside rear wheel I got nothing. I even removed the bleeder valve thinking it was stopped up. It was not stopped up so the blockage killing that right rear brake cylinder was between the hardline T on the rear end and the wheel cylinder itself.

I have yet to check the propornating valve to see if it recentered or not, but I do know yesterday I rebled the front brakes got a little more air out of the system and when I drove the car today after my orignal post the brake system is a little more decent than it was. Car actually stops with less pedal effort on my part. I am how ever thinking about checking the prop valve by pulling the brake warning light switch out and see if the rod centered itself or not. If not I might try to find a replacement and replace the whole unit with new and see if that doesnt work. The only other thing I can think of to get the best out of these brakes would be to replace the hard lines and replace the wheel cylinders in the back. The calipers up front are new along with the rotors and pads.