Greolt was the first, or one of the first, to describe converting a compressor to a vacuum pump.
We have an old two cylinder compressor that had been made redundant so when a need came to make a vacuum hold down for machining some 1/4" polycarbonate I coverted it to run as a pump.
Thre quarter copper now connects the inlets of the cylinders to the port at the top of the tank; the original intake filters are installed on the outlet ports as mufflers, which works very well. The vacuum outlet is on the side with a 3/4" ball valve and a length of swimming pool suction hose.
The vacuum platen is just fabricated out of 1/2" aluminum plate and bolts directly to the machine table. The height is needed because this is a Haas Super MiniMill with the raised Z axis so normal length tools will not reach closer than about 3 inches above the machine table. The aluminum platen is covered with a sacrificial polycarbonate with O-rings seals. At the front is a domestic water filter housing which acts as a primary trap for coolant that gets suckes into the system.
With the main tank ball valve closed the pump can pull 26 inches of negative pressure; with a part in place leakage means it stabilizes at about 22 inches.
The tank is 60 gallons and as is visible in the picture the compressor was rated at 12.5cfm. It can pull the tank down to 20 inches negative in about 45 seconds and it takes almost 2 minutes to get down to 25-26 inches which is more or less consistent with the rating. It had been used as a compressor for over two years. Initial purchase price was almost $800 but that had been effectively amortized over its use as a compressor so the vacumm pump only cost a few hours of work.