A compression tester is common;

most shadetree mechanics will have one.

But a leakdown tester is usually only seen

in professional shop environments.

A professional leakdown tester

applies 150 psi of air pressure

(150 psi is a high to middling

compression reading, by the way)

to a cylinder at top dead center.

The leakdown tester has a gauge

which measures the percentage

of pressure loss

through poorly seated valves

or past worn-out rings and cylinders.

Using a twelve-volt tire pump

as an air source

is a poor substitute for shop air,

but this relatively low-pressure test

will still reveal badly seated valves

and can be a useful rule of thumb.