Did you just change the air dryer or the whole APU?
The APU air processing unit in old speak is the 4 way valve and controls what air pressure goes to where.
Did you fit the fat steel pipe into the hole with the large bore? once saw one where the guy had fitted it into the hole below same size thread but tiny hole and took an age to build the air up, easily done.
Did you check in the fat steel pipe for carbon build up?
If its building pressure up even in only one tank it sounds like compressor is fine, its whats telling the air where to go thats at fault.