Nonsense technique that I hate with a passion - enough for me to shoot someone.
shops that pressurize a new A/C system with atmosphere air with a retired refrigerator compressor. The air contains moisture which completely destroys the dessicant in the dryer.
The correct way to use such a cheap technique is to NOT install the dryer and test the entire circuit without it, THEN install the dryer (the dryer has a maximum allowable open time of 30 seconds before the dessicant will go bad)
Have the vacuum pump already hooked up to the test ports ready to fire as soon as you tighten the flange or flare of the dryer.