I decided to replace the rear calipers on my 79 300td, as the pistons were siezed. Got a good looking pair at the junkyard out of a 79 450 SEL for $18. Installed them today, put in fresh pads, began to bleed the system, but couldn't get any pressure to build up. The pedal would begin to feel "firm" at first, but then it would lose pressure/push to the floor. I checked for leaks around the calipers but found none. Bled the other side just for kicks, didn't help. There seems to be a lot of air in the lines.

Did I somehow kill my master cylinder, or a gasket within it, with all that pumping? It's full of fluid.
I'm at a loss.

thanks in advance for your input.

 

I did pump it that many times. Looking into replacing the master...I guess.

 

If the master worked before, there's nothing wrong with it now. Either try to get a head start on the issue with a power bleeder or just keep plugging away at it. You may very likely have to bleed all four brakes just to get all the air out. Make sure you're doing it correctly: pump several times, hold the pedal down, open the bleeder valve, close the bleeder valve, release the pedal and repeat, occasionally checking the fluid level.

 

I don't think air could get into the front...the front and back are on different lines. The more I pump, the less pressure I get. I've bled many a brake line...I just don't know what else it could be, other than the master?

 

If you got that much air into the system then it most likely traveled to the master and was pumped out into the fronts as well.

 

Bled the front yesterday, then the rear again= nothing doing. I'm not getting any pressure -no matter how many times I pump, I can push the pedal to the floor. I'm at a loss, any suggestions?

 

Yup, if it doesn't work after all that = master cylinder.

 

just replace the master cyl, it's a cheap easy and quick fix! good luck

 

Funny your M/C died just when you fixed the rear caliper.