Help!! 1962 Cadillac brakes acting up.

[domer]

 I had the booster and master cylinder rebuilt on my 62 Caddy. I reinstalled the complete unit this afternoon.  I used the one-man brake bleeding device but my brakes are still acting up.  With the car off, I seem to have correct resistance on the brake pedal while depressing.  As soon as I start the car the pedal drops to the floor and I have zero resistance whatsoever on the brake pedal.  When I shut the car off, I pump the pedal a couple of times the brake pedal resistance begins to build up again.  I do need to bleed the brakes some more or is the brake unit still defective.  The mechanic did mention to that unit "should be ok" but that he had a really hard time coming up with the parts and that he had never seen a 62 caddy with dual chambers on the master cylinder.
Thanks
Dominique

[Coupe]

Somebody correct me if I am wrong here I am going from memory, 1962 was the first year for dual chamber master cylinders in Cadillacs. My '62 4 window has the a dual chamber master and booster by Bendix. Cadillac used either Bendix or Moraine systems in '62 so it is possible your mechanic did not use the correct parts for the system in your car. Did you rebuild the wheel cylinders also? If not are you sure they are not the leaking? Are you using DOT 3 or 5 brake fluid? DOT 5 (silicone) can take a lot of effort to get rid all the air bubbles.
If the wheel cylinders are good I'd try bleeding the system again with help at each corner. Start with the right rear then left rear, right front and finally left front. If that doesn't solve the problem I would take the master apart and make sure there is not any pitting in the bore. If there is pitting it will have to be sleeved.

A note of caution, for you or your mechanic. Be sure you have your shop manual handy to be sure everything goes back together correctly. When I bought my '62 the brakes had less than 5000 miles on brand new shoes and drums and they were shot because the self adjusters had not been reinstalled correctly.

[35-709]

From the description there is still air in the brake system somewhere.  Did you bench bleed the master cylinder before installation on the car?  It will take much longer to get the air out of the system if you just put the MC on and started bleeding everything at the same time.  The MC should be bench bled first!  I have always had much better success using 2 people to bleed the brakes rather than using a "one man".

[domer]

 I did some more bleeding on the brakes with the help of a buddy.  I raised the back end of the car on jack stands and started the bleeding process from the rear.  The brakes work as designed now.  Thanks again for the help!!!
Dominique