I have a 1984 f150 that I bought new. 140,000 miles. 302 engine, automatic trans. power brakes.
The brakes are very poor. they are quiet. The shoes and pads are all good. Truck was not driven for about 2 months, and now the brakes will hardly stop it. I diagnoised master cylinder, but I was wrong. I replaced the master cylinder, and bench bled it. It made no differecne. Brakes are not spongy and do not pull to one side. The are just poor. When the engine is running, the booster makes the pedal feel different, but I don't know how else to test the booster. There was some sludge in 1 reservoir of the old master cylinder, and I think maybe some particles got into the steel lines and are stopping the flow of fluid. Is this possible?
Please help.

 

Does it feel like it has low pressure when its running? mine was like that and I pulled my cylinder and cleaned it out re bled all the lines and I tightened up my rear drums and that did the trick for me but I had to replace the two hoses going to the front calipers hope this helps

 

There are 3 rubber hoses. 1 to each front caliper and 1 to the rear end.

These lines get old and spongy/soft with time. You were correct in replacing yours.