This new
MotorCraft RS62 cap cost less delivered to my house from eBay than a Stant at Wal-Mart or a parts store, so I got 2. The old ones were holding 0psi because their rubber seals for the radiator neck were hardened. Even after replacing one with a silicone seal that I made, it still only held 11psi.
For more info,
read this article that fully explains the cooling system.
