No reason to panic yet it could still be just a bad cap, the cap vents at 16 psi.
You say the system held pressure, ok to what pressure did you test to? Did you test with a test pump?
Normal operateing pressure is from 12 to 15 psi with 15 psi crowding the limit.
You can test system pressure with the same hand pump, leave it hooked up but do not pump,run the engine and read the pressure it should not exceed 15 psi. A rapid rise in pressure to above 15 indicates you are getting combustion gas in the coolant useually do to blown egr cooler or head gaskets. Further testing required.