It may be worthwhile checking the dryer for air leaks and checking the contents.
Undo the airbag feed pipes from the block.
Then try to raise the car, if they all blow air then the problem is further on.
If they don't, then the problem is possibly in the block.. None return valves the right way round?
Think the OP needs to go through everything again and make sure all is correct in the valve block. If the compressor is running for seven or eight minutes producing good air then stopping that air has to be going somewhere.