My understanding is once the hydrovac loses vacume you can use it a limited number of times before you lose most braking but when towing my truck home without the engine running I had a small amount of braking, maybe a inch of pedel, so if you have fluid in the master cylinder squirting out I would guess it's a master cylinder problem. Leaking wheel cylinders will run fluid down the wheels. I'm not sure if the hydrovac can leak fluid internally. Personally I would replace the master cylinder and all the slave cylinders if they show any sign of pitting internally. The brakes on these big trucks are marginal at best and losing your braking could be a disaster.
