So on my 2000 3.8 5spd, I replaced the timing cover gasket, water pump gasket, EGR gasket, and valve cover gasket, the valve cover gaskets, and the main seal was bad and leaking so I changed it too. I flushed the coolant system, while running the air full blast on heat, and even filled it up beforehand with the vent open until the bubbles stopped coming out, closed the vent and then went and started it with the cap off, and revved it to around 2500 and topped it off. Now when I get it warmed up it starts to leak at the top of the cap so I got a new pressure relief one and it’s doing the same. Should I have closed the cap before starting and not let any air get sucked in as I was starting it or what? Sounds like air in the system to me, but who knows. It’s not hot, I’ve checked the oil for water and there is none. I even took an oil sample to verify, and there was no coolant or water. So what’s the deal? The backstory on this car is, I bought it from a mechanic friend of mine that gave it to his daughter who drove it for a while, he’s a well known and great mechanic, but anyways the clutch and quadrant went out. He stopped working on transmissions before this, and her being her went and bought a new Stang. So this one sat for 3-4 years. When I got it I put a new billet quadrant in it, new cable, clutch, fuel pump, and filter. I then got tired of the oil leak on the driveway so I fixed that problem. Bought a new thermostat and gasket today(can’t get it on, been raining all day and it’s 43 degrees outside), so my question is what am I doing wrong here? Was the steps I took wrong on the flush, and fill to get all the air out and it’s causing my radiator to become pressurized? I noticed when it’s running it won’t go over into the overflow tank which is strange, but when I shut it off that loss of pressure sends it to the tank. Seems kind of backwards to me, but this is the first Ford I’ve ever owned so who knows. What’s y’all guess? What should I do or check next?