Pressure in cooling system after sitting

[Monsterbucks]

Urgent help needed. I have a 2000 Mazda B3000 3.0. Recently noticed I am loosing coolant. When I went to put coolant in system was holding pressure after engine cooled down even days later. What can be causing excess pressure in the system? Replaced head gaskets 3 yrs ago with new radiator a month old. Possible some water in oil not 100% sure

 

[Uncle Gump]

Define holding pressure... because if it's leaking coolant... the cooling system really can't hold pressure.

 

[RonD]

Welcome to TRS

Rad cap has two valves

The larger one with the big spring that lets coolant flow OUT to the overflow tank
Then the smaller valve in the center of that cap that allows coolant to be sucked back IN to radiator from overflow tank


As coolant heats up it expands in volume, that's what causes the pressure in the cooling system.
When pressure in rad gets to 14psi(rad cap rating) the larger valve opens and allows coolant and pressure to flow OUT.

As coolant cools back down is shrinks in volume so any coolant that flowed out needs to be sucked back IN

Check overflow tank to make sure its not clogged up at the bottom, coolant can flow in but not back out

Test or replace rad cap