What could be wrong with the air conditioner? on 1997 Chevrolet G Series Van (G30)

We purchased our van used and the front and rear air conditioner always blew out hot air. My dad put in a whole bunch of R134 refrigerant and it kept sucking it down. It got barely even cool and then went back to hot air again. When the air conditioner is turned on I can here the compressor on. What could be wrong with it and what would be the most expensive fix?

You need to have a AC diagnoses from a shop as you probably have a leak in the system and to find a shop near you here
I swear my husband had a diagnostic done where they put the dye in the lines and the mechanic couldn't find a leak. Is it possible he missed something? The air conditioner blows hot air on warm summer days and barely cool to warm air on cooler nights.
Sometimes finding these leaks can be tough, Bring it back to the shop that repaired this