[ttyR2]

Most compressors have a check valve mounted in the tank. Once the tank is up to pressure (whatever that is for your specific compressor), the motor shuts off. The compressor itself still has residual pressure in its discharge line, but doesn't get any pressure back from the tank. The unloader valve is designed to release that pressure so the next time the compressor starts, it effectively has zero load on it for a revolution or two, enough for the motor to come up to speed. If the unloader is stuck and doesn't work, the compressor will try to start under load and unless you have a really good motor and plenty of power, the motor won't be able to get up to full speed, drawing too much current, and tripping the breaker as you have been experiencing. Usually you can trace the unloader valve setup by looking for a smaller tube than the one that exits the compressor and goes to the tank.