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kfen: As long as your 325 uses an unloader on the cylinder head to bleed air off from the HP cylinder at shutdown, the exhaust valve will serve the function of the check valve by holding air pressure back from entering the head. Not my favorite system to troubleshoot, since the tank must be blown down before disconnecting the exhaust tubing, but it works ok, and saves a few bucks on a check valve. Any high pressure air in the LP cylinder and interstage tube will travel through the compressor pump and bleed off at the HP unloader tower port on the HP intake valve, unloading the compressor. JL, I am not sure why your compressor is not unloading. If you have pressure at the tower fitting, it should activate the plunger through the diaphragm and puck in the tower. Check to see if the fitting is plugged with hard water deposits. Make sure there are no rips in the diaphragm (I think you did already but they can be tricky to find sometimes). Make sure the unloader port on the tower is not plugged. It is a 1/8" NPT port on the circular "stem" part of the unloader tower. This may have a 1/4" copper line (which could also be plugged) leading from it to a fitting (which could also be plugged!!) just above the intake filter where the HP air is vented into the head after unloading. This is a fairly quiet setup and may be hard to hear. You can remove this line completely and vent the air to atmosphere, where it can be readily heard. Remember, the hydraulic valve that loads and unloads the head must get its signal from the tank, not above the check valve. It is also possible that the hydraulic unloader valve is partially plugged and is simply not passing enough air. You can pull the air lines off it, and the compression fittings, and remove a small screen inside it. This may be plugged with rust or scale, preventing adequate airflow from reaching the unloaders.
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