brakes on 1994 f-250 ford pickup

If you can pump the pedal rapidly a few times and hold it and it stays high, that is typical of rear shoes out of adjustment. Sometimes the star wheel adjuster gets rusted tight and it won't adjust. Also look for either of the parking brake cables rusted and partially applied. That will also keep the adjusters from working. If one should release later it will let those shoes retract and suddenly you will have a lot of clearance between the drum and shoes.

caradiodoc

how many inches of vacuum should the pump produce ?

If you're referring to an engine-driven vacuum pump, those are only used on some diesel engines. 20" hg is more than enough to run a vacuum booster. If there wasn't enough vacuum, the pedal would push very hard and it would not feel mushy or go to the floor.

Internal leakage in the master cylinder can cause a mushy pedal too, but you usually won't be able to pump it up. A clue is that the pedal might hold while you hold steady pressure on it, but it will suddenly start to fall away when you ease up a bit and vary the amount of pressure.

When the low pedal is related to rear shoe adjustment, you'll get a nice hard pedal after a few pumps and it will stay up as long as you hold it, then after releasing it and immediately taking another stroke, it will sink too low again. It takes some time for the rear shoes to retract. When you pump the pedal you're taking a new bite of fluid and pressing the pedal before the shoes have fully retracted. Eventually you have enough fluid in the system to push the shoes out far enough to contact the drums. That's when the pedal feels firm and high. When you release the pedal for more than a few seconds, the shoes have time to fully retract. With the next pedal stroke the shoes have to again move so far that the pedal goes down real far.