losing brakes

Hello

I'm having a wierd issue with the brakes on my 78 PW. when I start moving in reverse the brake pressure goes away and the pedal goes to the floor. I would think that this would be a master cylinder issue, but I have put a new master, brake booster, right caliper along with new rotors and pads. I can't seem to find any leaks at all. Do you think that the new master cylinder is bad? It was doing this before I replaced all the brake parts, along with a sometimes hard pedal and sometimes loss of pressure. That's why I replaced all these parts in the first place.

The vehicle is a 78 PW W200, 440 auto.

Any input would be great