Brake Issue

[mmsidaho]

I'm looking for some advice on a brake issue I've got on a 72 C20. It's got stock power brakes with Delco Moraine parts.

With the transmission in drive, the brake pedal goes all the way to the floor and there is no braking. When I put it in neutral, I'm able to pump the brakes up to at least get the truck stopped (not normal, but at least partially working).

I pulled a wheel last night just to check what the front pad wear was like and they look almost new still, so it shouldn't be a pad issue.

I also checked the brake fluid level in the master cylinder and it was good. Does this sound like either a master cylinder or booster issue?