Does it have an air pump (belt driven) If not it is hydo-vac. My 71 F600 is hydrovac as well. It can be that there is a vacuum line issue, or the hydro-vac unit may have failed. Best thing first is to assure it has vacuum to it.
The booster works much the same as the pedal mounted, except it doesn't have the rod actuating it. However, it may not be a bad idea to have someone familiar with them to check it out.
Used them when the boosters went out on dads C60 dump truck... They were the only place in the country that had the diaphragms. They'd probably have any parts you would need.
That is a heavy truck! Driving up the ramps, it took enough weight off the truck that I was sliding the pick up forward until the rear tires grabbed the ramp. Pushed it forward about 5 foot!
Destroyed one load range G trailer tire on the trip, and the E rated spare next to it, as well.
The tire separated, butt is still holding air! After it happened, I had to keep going. This tire has at least 70 miles on it AFTER coming apart, and is still holding air!
The bulge in it caused crazy wear in the new spare next to it. Half of the tire is worn nearly smooth. Not side to side, but circumferentially on the half by the blown out spot on the other tire... Hard to explain with words, but I don't have a picture of the spare.