Possible brake booster issue
 

I have '93 f350 7.3 idi
t'other day I noticed I had barely any braking power but more pedal pressure than ought to be. My abs light flashes 15 times indicating memory failure and the fuse associated keeps blowing. My brake lights are also out (bulbs are fine it's the fuse).
I still have enough breaking power to lock up the front wheels in my muddy driveway but it takes a half mile to come to a stop from 50mph.
I have put pressure in the lines then start the vehicle and the pressure does not relieve.
With vehicle off I let the open the line at the bottom as if to bleed and pushed the brake pedal and it squirted out the right amount so there are no blockages.
Am I right in thinking this a booster issue? How do I know it's not something like the vacuum pump powering the booster?

Those vacuum pumps were problematic as I recall. The better thing to do is verify sufficient vacuum with a vacuum gauge, which should read at least 19 in vac at idle. The worst part of replacing the pump is reinstalling the serp belt. If the belt diagram is still under the hood , OK, if not, make a diagram of how the belt is routed before removing it. A good time to replace the belt while it is off. I have found I usually received the correct belt when purchased from Ford. If you cannot do that you might get a couple of different sizes to make sure one will fit correctly and return the unused belts. Also , check the idler pulley for roughness or noise and it is a good time to replace that pulley with the tensioner to avoid having to do some of the work over again.