sounds like your compressor has had some abuse.
normaly when they are correctly piped up, if the NRV is knackerd the unloader in the pressure switch will dump the tank when the motor cuts out.
and usally hissing / blowing oil from the dipstick means the rings are gone, but if had no unloader or no return valve fitted air is going to get past the valves and piston, probaly at enough of a rate to make it hiss
normaly when they are correctly piped up, if the NRV is knackerd the unloader in the pressure switch will dump the tank when the motor cuts out.
and usally hissing / blowing oil from the dipstick means the rings are gone, but if had no unloader or no return valve fitted air is going to get past the valves and piston, probaly at enough of a rate to make it hiss