Last night our old Camry leaked out part of it's coolant after the radiator developed a crack in the top plastic housing (at the seam where the cooling fan bolt attaches). Looking it over today I rented a coolant pressure tester/pump to pressurize the system to find that leak. I was surprised when I also checked the radiator cap and found that the cap was not releasing pressure as I pumped it up to 30psi. As you know, normally the cap will release the pressure at it's rating - in this case 13psi. In the past all of my radiator caps would go bad because they would not hold pressure so this is the first time I've seen a cap that would not release the pressure. This cap was purchased new at the local Toyota dealer about 3 years ago and has less than ~15,000 miles on it ! Was this over-pressure the probably cause of the crack in the radiator plastic ? I'm still puzzled that a radiator cap can fail like this to allow so much pressure to build - - am I overlooking something here or is just replacing this cap with a new one the next thing to do (after repairing or replacing the radiator) ?