I have several vac. pumps , but the one I like the best is an old "Crosley" [ remember them?] refrigerator compressor.
I have a plastic tube on the discharge [high] side that I put in a clear glass of oil.
When the bubbles stop for 5 min. , I have a good vac.
[29"]
When I have the time and want to be through, I pull the vac, cap the system, and recheck the next morning.
No leaks should hold the vac.
Any system that does not pull down in 1/2 hour has a leak
or a bad vac. pump. The pumps capacity is easily checked
by capping the suction hose end and pulling a vac againt the gage.