After the regulators are put together, the units have to be tested for any air leaks. The tow units are loaded into the fixture by pulling down the levers on the over-center clamps at the same time sealing off the air system. You turn on the air pressure switch and load up the air pressure, then turn off the air supply and look at the pressure gage. If the needle stays put, everything is correct. This is a very simple and correct way to solve an ongoing leak problem.