Hey guys. I have an issue with my brakes on my 1979 G20. When I apply the brakes at a stop the pedal loses pressure and pushes to the floor, then the van creeps forward a bit.

I checked the and both reservoirs are full and I replaced the mater cylinder gasket this morning but the problem still remains.

I've read that signs of a brake booster failure are the inability to press the brake pedal, therefore I assume that I may have a faulty master cylinder. Anyone care to chime in? Thanks.

 

Even if the vacuum booster bleeds out, you should still have some mechanical pressure on the brake.....unless that is problem is combined with air in the system.
To test the vacuum booster I think you pump the pedal a few times until it becomes hard then start the van and the pedal should go to the floor.