1999 Chevy Corvette Pressure testing coolant system

Pump it up between 15-20psi

Why would you go higher than the 15psi on the coolant cap and could going 18psi or higher possibly cause a radiator seam leakage problem?

That's my way at times depends on the rating but go ahead and pump it up to 10psi, and look around for leaks if the gauge shows any pressure loss. Common trouble spots include heads, hoses, water pump and rear heater core. If you smell something sweet when you turn the heater fans on, it may be a coolant leak at one of the heater cores, or simply the O-ring at the rear heater valve leaking. Because there is often more then one leak in the system, it is necessary to continue pressure testing until the system holds pressure.

The pressure in the cooling system builds to aprox 7 lbs at normal running temps looking around for leaks. But if the pressure builds quickly to around 15 lbs the pressure is coming from one of the cylinders through the inner head gasket. This can also show up as a coolant pressure loss. In extreme cases the cylinder can fill with coolant, and will not turn over. This is known as hydraulic lock, and if you suspect this pull the spark plugs and try to turn the engine again. Pulling the plugs from the engine allows water to escape from the spark plug holes.