GMC Repair: 90 gmc heater, air cond, truck watch

Question
I have a 90 gmc two wheel drive 1500 truck.it has a/c also.the air cond. does not seem to work.the heater is only slightly warm but when i rev the engine up,the heater becomes much warmer.the a/c is fully charged.any idea why i cannot get more heat?does the heater work on vacuum?all of the heater controlls seem to switch correctly floor to defog etc.could the heater problem be an air lock in the cooling system?

Answer
hello reg
when the truck is cold remove the rad. cap start the truck watch for movent in the rad. if none then change the water pump bad . and also check the rad to see if hot botton and top could be pluged . even bad heater core let me know what you find pete