1987 Ford Bronco Brake pedal goes to floor after bleeding.

Brakes problem
1987 Ford Bronco V8 Four Wheel Drive Automatic unsure miles

As noted I have an 87 Ford Bronco. After sitting for at least a year I noticed the master cylinder was empty although the system had no leaks. A couple years before that I replaced a rear wheel cylinder, a caliper, pads, shoes, front brake hose, and it hasn't been driven much since that work was done except for ocassionaly starting, running it for a while, or moving it around in the yard. (Because of gas prices)! Therefore the parts should not been worn out. Anyway, last year I drove it a bit because of bad snow. Thats when I noticed the master cylinder empty, or almost empty. I refilled the system and bled the whole system. The pedal would come up but after driving for a few minutes it would go to the floor. I assumed a new master cylinder was needed. I brought one, bench bled it, installed it, and bled the whole system again. I still have the same problem. During bleeding the pedal does come up, the fluid flows freely, and I belive the air has been removed from the system as I no longer see air bubbles during bleeding. What could be the problem? I must note that the bleeding instructions that came with the master cylinder was slightly different than the HAYNES manual for my truck. The instructions in the box said you should plug the two line ports with the plastic fittings during bleeding and don't remove them until the Master Cylinder is installed, and remove one at a time to install the lines. The HAYNES manual says you should cover the ports with your fingertips and while the plunger is being pushed in you should allow some fluid to excape the ports. As you release the plunger your fingertips should close the ports to prevent air from going back into the master cylinder. (The master cylinder has the removeable plastic resivoir, with the twist on cap. It also has a valve between one of the lines going into the master cylinder, and the master cylinder itself. It also has a plug on the side for a low fluid warning etc. Which was never plugged up). I bled it according to the instructions that came with the new master cylinder. I have given some thought to the brake booster but I'm unsure if that could be the problem. I belive the adjustment on the booster is correct. However when I push the brake pedal I hear a decent amount of air inside the passenger compartment, from under the dash, at the brake pedal. The thing that confuses me is I have heard that sound for years even when the brakes had no problems. The truck origionally had a 302 in it but currently has a fuel injected 351 which I belive to be a early or mid 90's engine. It was in the truck before I brought it from a dealer in 2000, and usally when I order parts I get 92, or 93 which always has been the right ones. My brain is racked on this one! Help.