Six months ago i had almost the same problem (water pump leaks): i have tested the caps (2, new)

Boths opens at the correct pressure of 1,4 bar (indicated on the cap).

For testing the cap with few tools (!):

With a cold engine disconnect the small tube from the radiator at the small input reservoir tank.

Connect one pump for tires with pressure gauge (such stanley) at the reservoir tank small input

Tight the small tube from radiator

Start pumping air in to the tank until the pressure gauge does not increase more.

The reading is the opening pressure of the cap.

Listening to the breathing tube of the reservoir cap, you feel the air out.