Sounds like the system lost pressure. Most likely it is the pressure cap but you should pressure test the system to see if it will hold it.
Just don't presume it is only the cap.
Stant makes a little kit for around $120 that will pressure test the cap and the system too.
It's worth the investment. Works on all cars.
A lot of times if it isn't the cap, it is the flange on the tank the cap seals too.
You can't usually tell on the cap by just looking at it. You have to pressure test it.
They can be bad brand new out of the box.
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