I tried to get one of these from Volvo and was told they are not available, only the green 150 KPA.

My aftermarket 75 KPA apparently built up enough pressure to create swelling of the metal tank. I replaced the radiator (it had well over 300,000 mi so it was not worth fixing)

I noticed the upper radiator hose was "rock hard". I tested the valving in the black cap just making sure they were both free. A 1,000 mile trip last weekend indicated no problems. Did the hose squeeze test! Thought I had cured the sticking valving?

This week I again noticed the hose was "rock hard" again so I switched back to a 150 KPA cap. The hose yielded more readily seems to me?

Anyway the valving is different between the black and green. It would be great if I could determine just how much pressure is in the system but I'm thinking it's well above 11 psi! Possibly over 22 psi (150 KPA)

If you're using the aftermarket black cap do a squeeze test on the upper hose, I believe it should yield with very little finger pressure.

When I get some spare time I will rig a gauge to the system and report my findings.

Be aware that a 75 KPA cap might not be giving you the correct pressures.