Single cylinder/oil free 5.5hp 30gal will fill tank when started with tank empty. When tank pressure is bleed off motor attempts to start, humms and kicks breaker.

If the unloader is not working properly the motor cannot overcome the head pressure. Two things to check. Make sure tank check valve is not leaking. Easy test. With pressure in tank, unplug the compressor from outlet and remove discharge hose at tank and listen for leak or spray soapy water. If leaking replace valve or try to clean debris from valve seat. If not leaking then problem is most likely at pressure switch unloader valve. Follow 1/4 inch hose from checkvalve at tank to connection at pressure switch. When pressure switch reaches cutoff pressure the switch opens the contacts and moves small tang against valve stem to release air from discharge hose. Check for proper function and repair as needed. Good luck.