weak brakes on 72 chevy truck

[Omarsmgff]

Hello all! My neighbor has a 72 chevy truck with disc/drum brakes, he wanted me to replace his master cylinder due to the fact that his rear reservoir fluid level kept going down. I checked calipers/wheel cylinders for leaks all was ok, lines are relatively new, no leak was found. I took out his old master cylinder and did find the rear seal was bad and there was rust between the m/c and booster. I got the correct rebuilt m/c for disc/drum and i bench bled it until no more bubbles came out. So i put the new m/c on and bled the brakes until only brake fluid came out of the fittings (with the truck off). I also bled the proportioning valve that is right next to the m/c. I found no leaks. The pads and shoes have plenty of life left and I also adjusted the drums.
Here's the problem, the brakes are slow to stop the truck. If I put the truck into gear I have to really press the pedal so the truck wont move. The pedal is firm but it takes and unususal amount of force to stop. The brake light does not come on, so i would assume the proportioning valve is not tripped. I did a search on here to figure it out and still have not found the cause. I did try gravity bleeding since i read about it here but it made no difference.
The booster worked fine prior to the work being performed and it was left in place. There is plenty of vacuum from the carb. I read on here that there is a valve on the proportioning valve that needs a special tool to operate it in order to properly bleed the brakes but have no idea if that is what i need.
Im stuck!!!!! please help me!!!!!
thanks, omar