jsimpson

I replaced the master cylinder, brake calipers, and replaced the rubber brake lines with stainless braided lines; booster is good. I bled, bled, and rebled, but I still have only space for a big toe under the brake pedal depressed. This is sitting in the driveway after bleeding, NOT after driving, so there has been no chance for the rotors to pump air through my calipers. I don't know what else to try. :confused:

Larry B.

Will the pedal "pump" up? If it will not and the pedal is firm but just low, then you can adjust the rod that pushes the master cylinder. Maybe the new mastr cylinder is a little different than the original. :jester

jsimpson

The pedal won't pump up, and the master cylinder does NOT have an adjustable actuating rod, nor can I find one. The original master cylinder had the same problem; that's why I replaced it. I'll probably spend another day (and another quart of fluid) bleeding again tommorrow.

1980 razz

it took us afew times bleeding before we got it up where i wanted it,but it has stayed high and the breaks are great now..dont get frustrated i bet theres just some damm air still stuck in there..lol

Chuck Gongloff

Some master cylinders, usually those used with power brakes, have separate bleeders for each "chamber". Did you bleed those before you started with the system? Also.....no offense intended....many don't know or realize that there are 2 bleeders on each rear caliper :) I've always had good luck "gravity bleeding" everything, then going back and pressure bleeding to finish it off. Hope this helps. Chuck

Tom454

Rebuilt (or even new) master cylinders for older cars will sometimes sit on the shelf for a long time before they are sold. Although a rust prohibitor is used, they still will rust as they sit on the shelf. This includes the internal bore. I have seen this happen many, many times. I went through 3 master cylinders for a 76 Trans Am before I got one that worked. When you install a master cylinder, you are supposed to "bench bleed " it before you install it. Although (in my experience) it does not matter where you bleed it (on or off the car) it is important that you do bleed the master cylinder properly... and this cannot be done by "gravity". The cylinder must be repeatedly "pumped" to get the fluid into the voids between the multiple pistons inside the cylinder. The pistons (all machined into a shaft(s) ) also have one way "flapper" valves and passages through the pistons which must be free of air. So if you do not properly bleed the master cylinder, you will get a weak pedal. But... it will usually pump up. Since yours does not even pump up, I would suspect a bad master cylinder.


Some do have bleeders on them... but these are few, and most of these have been replaced by non-bleeder units. The master cylinder off my own 70 has bleeders... I am sending it to get it sleeved while I drive around on a substitute. Without bleeders, you can still bleed a master cylinder on the car. One method involves buying or making two inverted flare tubes (installed where the lines connect) to recirculate fluid as you pump the pedal. The other is to open a caliper bleeder to one chamber at a time and pump about a pint of brake fluid through each chamber (if you have multiple chambers, my 66 is a single chamber). Keep the reservoir full at all times, continuously topping it off. This will work fine. If the cylinder will still not hold pressure after this, then it is probably defective. The rust acts as sandpaper and grinds up the sealing boots between chambers, on the inside of the cylinder.

I have also found rebuilt master cylinders that have been honed past the working limits of the standard internal cup size (I've even done this myself just for grins). Even though the parts were new, and had no rust, the cylinder failed under pressure.

In rare cases, I have found it necessary to bleed a master cylinder and then use inverted flare plugs to seal off the ports for testing. This eliminates the entire rest of the brake system as a fault, and lets you test only the master cylinder. I will be doing this exact test next Sunday on an 80 that I have been working on. The brakes are doing exactly what yours are, even after a thorough & proper bleeding, and with a rebuilt (off the shelf) master cylinder.