Had to replace my thermostat not too long ago ,and until I properly bled the system my pressure bottle cap would pop off also (brand new bottle/cap). I went with a cap that is meant for a Volvo something or other (sorry can't remember right now)as others on here have done. Also I jacked up the rear of my truck as high as I could ,and took out the temp sensor at back of cyl.head(4.0 liter) then refilled Through pressure bottle opening slowly until coolant only came out at the temp. sensor hole. Do this when engine is cool. Lowered truck ,and filled pressure bottle to half ,put cap back on-good to go ever since.