Eldorado brake issue

[cerwin]

Hi I am new to this forum, but I'm hoping someone can help me out. I have a 92 Eldorado and the brakes go to the floor. I've bled all of the lines and verified there are no leaks. The brakes will stay up when the car is off, but once I turn it on and the vacuum asist kicks in, the brake pedal goes right to the floor. I've taken apart the master cylinder and it looks fine. Before I give up and drop a hundred bucks on a new master cylinder, I was wondering if anyone else had this same problem.

Thank you in advance.

[DFBonnett]

I've never seen that problem you describe be anything but the MC. Could you get a couple of brass plugs the size of the line fittings on the MC, then disconnect the lines, install the plugs, then try the brakes? If the pedal drops then you know it is the MC. If it doesn't then you know the problem is further down the system.
FWIW
YMMV

[caddydaddy]

Also, you may want to try a junkyard master cylinder, and see if that helps. It's a lot cheaper than a new one, and if it's still in good shape, and solves the problem, then there you go!

[maxwedge]

Originally Posted by cerwin

Hi I am new to this forum, but I'm hoping someone can help me out. I have a 92 Eldorado and the brakes go to the floor. I've bled all of the lines and verified there are no leaks. The brakes will stay up when the car is off, but once I turn it on and the vacuum asist kicks in, the brake pedal goes right to the floor. I've taken apart the master cylinder and it looks fine. Before I give up and drop a hundred bucks on a new master cylinder, I was wondering if anyone else had this same problem.

Thank you in advance.

Welcome to AF, stupid question coming, is the dash brake lite on?, if you move the car does the brake seem to respond at a normal height. You can push the pedal almost to the floor if you force the pedal down, just asking, I've seen people chase this problem when there was none.

[rhandwor]

What kind of brake work were you doing. If you purchased a rotor with the wrong size bearings and installed it. This causes the pedal height to change drastically. If you don't have 4 wheel disk, rear brakes out of adjustment cause low pedal problems. Have you checked the vacuum assist does it act correctly. I have seen the front rotors put on the wrong wheel this causes the bleed screw to be on the bottom instead of the top. So you can't bleed the air out.
If you replaced the vacuum unit take the old one and lay a ruler flat on the unit and use a measure to check the distance to the push rod. Then set your new one the same exact distance they can be adjusted by turning. I have purchased some and almost always need to be adjusted. If you scrapped the old one add about 1/16 inch at a time until they are correct. Be careful as this will cause the calipers to lock if to long.
Also look for a pinhole leak in the metal lines. A weak hose to the caliper can balloon causing this type of problem

[cerwin]

I appreciate everyone's help on this issue. I've been away for a while so I haven't had time to update. The issue was a small leak in the rear left line that had eventually plugged itself with dirt. This left an inordinate amount of air in the lines. After searching I found the Eldorado has the strangest bleeding order (rear right, front left, rear left, front right.) After bleeding for a few hours. I'd pretty much given up. I slapped everything together and the brakes worked great.

Again, thanks everyone.