The following schematic and plumbing diagram describe how to connect the Thomas pump to do double duty as a compressor and as a vacuum booster pump. Air can flow freely through the open input valve into the pump. The vacuum is held in the vacuum tank by the check valve. The pump cannot draw air through the closed input valve so is pulls it through the check valve from the vacuum tank.
Air leaving the pump can flow freely through the output valve and is exhausted through the muffler.
The compressed air is held in the air tank by the check valve. Compressor mode - The pressure switch closes activating the pump relay. This starts the pump and opens the input valve. The output valve remains closed. The changeover relay activates the pump relay which activates the pump and now the output valve is opened and the input valve remains closed.