The leakdown tester is just two pressure gauges upstream and downstream of a restriction orifice. The leakage flow rate is proportional to the square of the pressure drop across the orifice. Of course if all you are looking for is bubbles then the flow rate is irrelevant.

You could also get an idea of the leakage flow rate by just having a quarter turn valve upstream of a single pressure gauge. Shut off the air supply with the quarter turn valve and time how long it takes the pressure to decay to a certain value.

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged