I would definitely order a Toyota cap if I need a new one. I rented a pressure tester to check for leaks, and since it had a cap adapter, tested that too. My understanding is that you pump it up and it should start leaking air when it reaches its rating. Question was, what is this rating? I remembered incorrectly, mine started letting air out well above 20 psi. Then it slowly leaked down to I think around 15. I don't know if this is good or not. Thoughts?
The other reason the spec was important is that I read to test for leaks you shouldn't pressurize the system above the rated cap pressure.. which is not clear what it is. The Matrix manual doesn't even have a rating. The Vibe manual has one that's different from the cap. See below for the service manual ratings. Anyway, I pressurized the system and let it sit overnight. More on that later (this test was for internal leaks, where you take out the spark plugs and crank the engine w/out fuel injection and see if there's any liquid on some paper above the spark plug holes).
Another point of uncertainty is how much to fill up the reservoir tank (and how). The Vibe manual seems to indicate fill to the low setting when cold. The Matrix manual seems to indicate fill to the full setting. I guess one could fill to halfway when cold and be happy. I didn't observe a whole lot of expansion but will keep an eye. Vibe manual said to fill, close reservoir cap and leave radiator open, wait for engine to warm and add more fluid as needed. Well, I saw the fluid come up, rather than down, so I quickly put the pressure cap on. I think the manual meant to 'burp' the system. I didn't see a lot of expansion into the tank.
Lastly, I checked the rating in the matrix service manual. Here it is:
Standard opening pressure
74 – 103 kPa (0.75 – 1.05 kgf/cm2, 10.7 – 14.9 psi)
Minimum opening pressure
59 kPa (0.6 kgf/cm2, 8.5 psi)
15 psi is the top of the range, the 88 kPa seems to be the middle. Not surprising that the caps have some variability.. I don't know if the opening pressure goes up as they age, mine seems high.