Always pressure test the cap first, then the cooling system. Of course
you would need the correct tools...
If the cap is rated at 1.034 Bar (15lbs.) then I would pump it to
1.172-1.241Bar (17-18lbs.) and then watch the guage for several
minutes. If it drops then you grab a light and look for escaping
coolant as they should be easy to spot now.
But this does not always reveal more serious issues; sometimes cracked
heads and leaking headgaskets wont show unless the engine is good and
hot. Remove the cap and warm the engine fully. Ideally you should drive
it but you could loose you coolant. Using "BLOCK CHECK" tool insert
into the cap opening and begin pumping; if there are any combustion
gases present the blue fluid will turn to yellow.
If you plan to become proficient in maintaining your autos I encourage
all to invest in these tools I have mentioned. Stant makes a nice
coolant system pressure tester. There are several adapters available
to service your fleet. Bloc Check is a Mr. Gasket product, but I linked