Been working with a friend on his 1970 W31 (manual DISC front, rear drums). He replaced every component of the brake system. All new lines, calipers, wheel cylinders, shoes, pads, distribution block and master cylinder. Bled the brakes in the proper order yesterday, and bled the master cylinder last. Wound up with a pedal that went down to the floor, but the front calipers were engaged. Got the car moving, calipers eventually loosened up. But petal still went to the floor with the brake light coming on. NO leaks anywhere. So we took off the master cylinder and put on another one that had been rebuilt (purchased at NAPA). Bench bled it, and installed it. We still have a pedal that goes to the floor, however, if you pump it 5 or 6 times it will firm up. Let it sit for 15 seconds and its back to the floor. My guess is we have 2 bad master cylinders (what are the odds), but before purchasing a third one, would welcome any opinions. Thanks!

 

Did you bench bleed the master cylinder first? I would start there, then bleed the rest of the brake system.

The fact that it firms up when you pump it sounds like air in the lines.

Edit: Sorry, missed the part about you bench bleeding the 2nd MC you got. Either way, you mentioned you bled the MC last. You need to start the bleeding process at the master cylinder, then move on to the wheels, starting with the wheel farthest from the MC.

 

There was another thread with exactly the same problem, just recently.
Turns out, the calipers were on the wrong sides - bleeders should be UP!!

 

If your drum brakes are not adjusted properly you will have a similar problem. When you bleed the brakes, have your buddy sit in the car, put enough pressure on the pedal to push it all the way to the floor when you loosen bleeder. He needs to hold it there until you retighten bleeder, and then raise the pedal. Pump up the brake again and repeat until you get a solid stream of brake fluid with no air bubbles. Recheck master cylinder for fluid level after each wheel.

 

It doesn't take a "genious", or even a genius, to diagnose this one.

So the answer is NO, you did not bleed the brakes in the proper order. Bench bleed the M/C FIRST, then the rest. Be aware that you may have pushed the differential pressure switch piston all the way to one side, so you may have THAT problem to deal with now.

 

You've gotta be really thorough with the bleeding. Starting with bench bleeding, you need to do it for a long time, making sure to go ALL the way in numerous times, shorter strokes, and different speeds. It sometimes takes me 20 minutes to get it bled properly. There's a lot of air in there that needs to be coaxed out. It gets a completely different feel when you're all done. As for the lines, I also keep going for a little bit once the bubbles stop, just to make sure. Keep the master topped off too, if it goes dry you need to take it off and bench bleed again.

 

Sounds to me like major air in the line to the rear brakes. Re-bleed the rears, run at least 1/2 Qt thru. long after bubbles stop appearing.

And, make sure the adjusters were set so that the rear shoes are just barely smaller than the drum, with the slightest drag perhaps when brakes are off. That slight drag will wear away on the first drive. Generally, adjusters way out of whack will lead to a 1 or 2 pump before full brakes condition, not 5 or 6.

 

my .03, i dont pump the pedal,i push down one pump at a time. dont want to aerate the fluid.

 

Thanks for all the feedback. We checked last night and the calipers are on the wrong sides, so we will switch those out, and bleed the brakes again.

And thanks for the spelling lesson Joe. Obviously I am no genius...

 

For future reference, the bleeder screw always needs to be at the highest point when the caliper is installed, or you'll never get the air out.

 

Win #2?
See # 3 -