Brakes build pressure, then loose it

I just replaced the master cylinder on my 53 3A, and am having a problem with brake pressure. I have to pump it about 3 times before i get a firm pedal, and once it let off the brakes, i have to pump it all over again. If i build up pressure by pumping, and hold the pedal down, it does not fall, which would indicate that i do not have a leak. This would make me believe that there is air in the system somewhere, however i have literally ran at least 2 quarts of brake fluid through all the lines and am no longer getting any air out. Do you think that i just need to bench bleed the MC, which is something that i did not do? Or is the MC that i bought defective? Thanks!

 

Try bench bleeding the master cyl. Then bleed your brakes again. Hope that helps.

 

Whats a good way to check the residual valve?

 

I had the same problem, bled a pile of fluid thru it and while I did get some bubbles, it didn't really improve.
Put new 11" on the front, bled, same symptoms.
Re-adjusted the rear 9"ers last week, BAM! Firm pedal, no pumping, no loss of pedal. My guess is that the rear shoes had too far to travel on the initial push on the brake pedal, so you'd have to pump or build pressure to make the shoes come into full contact on the rear drums. If you still have 9' brakes all around, adjust them up good and snug. Visualize that single m/cyl plunger is pushing fluid to all 4 corners. It can only move a given amount of fluid on each stroke. HTH.

 

If you're all adjusted up drum-wise , check the pedal pushrod and see if you have adjustment room there.

 

Thanks for the tips guys! I have 11" on the front and the original 9" on the rear. Ill adjust them tonight and see if that helps.

 

The brakes just needed to be adjusted. Im suprised how close the pads need to be in order to make for a firm pedal!

 

Yup, you got it. As the stock single m/c only pushes a given amount of fluid, the larger volume of the wheel cyls on the 11" brakes (as opposed the the stock 9"ers the m/cyl was originally used for) makes it mandatory to have the brakes adjusted up tight. That's my theory anyways. Glad you got it sorted out!

 

Im oh so excited that my brakes finally work! Thanks for the help! Time to do some wheelin!!