For your coolant system to work properly, it must hold pressure. A new radiator cap will guarantee this pressure is not only held, but is held to the correct level. The replacement, locking and OEM cap have a 13 psi (.9 BAR) pressure rating.
Toyota Part number: 16401-16020
A performance radiator caps (TRD & Mishimoto) raises the pressure of the cooling system to 1.3 BAR, effectively raising the boiling point of the water in the system. This is good protection for high performance engines being driven hard, requiring safer cooling.