On air compressors, you "usually" find a check valve between the tank and compressor that will not allow air to back flow. There is an unloader in the air line from the compressor to the tank that is designed to purge pressure from the line so the compressor can start without a load.
Problem is the unloader in some of these small compressors does not do a very good job - or the check valve leaks.
If you have a copper, aluminum or steel tube that runs from the compressor to the tank you may be able to install an in-line bleeder valve or "T" with a manual open-close valve so you can open it to relieve pressure, start the compressor, and then close it to refill the tank. This is one way to overcome high head pressure that will not allow the electric motor to start without bleeding all of the stored pressure from your tank(s).
