Weird cooling system happening

[Bradjon]

I am getting ready to g to bill be to install a new water pump and injectors in my90 . Water pump seal is starting to leak just a little .about 12 oz a day . When I drive the car it doesn’t leak in fact doesn’t start leaking till about 1 hr after I shut it off . There stranger thing is she has sat for 3 days and went to fill turned the radiator cap and it went poof like the system is still pressurized? Huh I don’t understand had to add 24 oz of water but the system is still under pressure. Have taken the car on a 80 mile drive as kinda a test to see if would be ok to make the 460 mile trip and she ran between 87 /89 celcius and seem ok to go any idea why it still holds pressure after 3 days ? Help