Brakes to the floor with a NEW master! How?

[1965 220se]

I have a '65 220Se 2-dr coupe.

PROBLEM: The brake pedal went to the floor. So, I replaced the old ATE with a new ATE master. Bled this thing 3 times, no air is currently in the system. The pedal still goes to the floor.

Sometimes I can get the pedal to fee like it's got some pressure, but as soon as the car is started, the pedal goes to the floor.

BRAKE BOOSTER: The vacuum booster diaphragm sounds like it could have a small leak in it. But here's the thing...even if the booster does have a leak, shouldn't the master's hydraulic action take over and apply the brakes like a manual car would?

DIAGNOSIS: With the wheels off I can see the calipers gripping and the rear shoes moving outward, but the pedal still goes to the floor with the wheels on.

QUESTION: Even if the booster isn't operative, shouldn't the pure mechanics/hydraulics of the system at least stop the car like a manual car?

Help!!!

[william rogers]

Did you bench bleed the master cylinder before instaling it?
William Rogers........

[1965 220se]

No, I didn't know I had to...heard of doing this but no one (until now) could tell me how and I thought the air would come out during the normal bleeding process at each brake...silly me.

Can you elaborate on how to bench bleed the master? I'll be happy to go through this process as I'm stumped as to why I can't get any pressure out of this thing with a new master!

I'd also appreciate anyone's tips on bleeding MBs. I've owned 21 cars and never had a problem with bleeding. But, I've never installed a new master so I'm still learning after all of these years/cars.

Thanks!

[BobK]

To bench bleed, you need to mount the master cylinder solidly on the bench. Some times you can just clamp it in a vice. Be sure it is level or you will likely never get all the air out. Next, you must connect short lines to the output ports on the cylinder. Some cylinders will come with some plastic fittings and tubing. If not, you can make some bleeding lines by buying short lengths of brake tubing with the proper fitting already on it. If both ports use the same size fitting, just buy a 24 inch length with the proper fitting at both ends. Cut the tube in two equal lengths and bend it so that the tubes will loop back into the top of the master cylinder reservoir. You fill the reservoir with fluid so the ends of the tubes are submerged. Then simply pump the master cylinder until no bubbles come out of the tubes. Keep adding fluid as needed to keep the ends of the tubes covered. What this does is remove all the air trapped inside the cylinder. You may be able to leave the bleeding tubes installed until the cylinder is mounted and you are ready to connect the lines. After lines are connected, bleed the entire system to get out any air the entered when the cylinder was removed. Start with the right rear wheel and work your way forward. Amazing how well air can hide in a brake system.

[william rogers]

BobK ,that's written better than any manual or instructions that sometimes come with a new cylinder..........
William Rogers.......

[gidpor]

Again, this info was very informative for future(not so distant) use - but brings up a question.
From the above instructions it sounds like you need to bench bleed the master cylinder if ever it is removed from the car.
(IE to be able to reach the bloody sway bar bushings on a 126). If the same unit is reinstalled, does one still need to do that, Or are we talking about bench bleeding a new part only ?

[BobK]

To be safe, if you pull it out, rebleed it. Nothing worse than a too low pedal at the wrong time. It does not take much air in a system to be felt. Just the nature of master cylinders that they are a pain. The valving of the pistons and cups makes it hard to get all the air out. It seems they are never level when mounted in a car and there is always some high spot where an air bubble will hide. I get nervous that air will sneak in while I connect the brake lines. That's why I like the idea of leaving the bleeding line connected until the last minute. I also made up my own power brake bleeder to be able to bleed brakes by myself. I tend to run a lot of fluid when am flushing a system and bleeding the air. A five dollar quart of DOT 4 fluid sure beats a $150 caliper. Even that's no guarantee that you won't get some corrosion down the line.

[1965 220se]

Thanks to all! I agree, I've never read anything better written or more informative prior to these writings.

It's sad and funny at the same time that such a small amount of air has caused me all this time and frustration. Hard to believe some air in an unbled master really allow the pedal to function so poorly that the pedal will really go all the way to the floor!

I'll take your advice and let you know how it goes. Thanks so much again to all of you. I'm off to bleed the master.

Thanks, Philip