I have a 64 GMC, and my brakes work fine after pumping them up too. But at stoplights, they do slowly (sometimes not so slowly) lose pressure.

It's off the painters now, but when I get it back my plan was to bleed the system thinking there was air somewhere. When you say "a bad M/C" what do you mean by bad?

There is one place that you have not looked, and it is there - only there - that you shall find the master.