Standard practice in the plumbing and pressure vessel industry is to test at 150% of working pressure. If you have a 4 psi cap, that would mean testing at 6 psi. If it doesn't leak at 6 psi, then you're good to go.


Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.