I have the multi cooler. It originally did not have an overflow bottle so I added one. It doesn’t go up and down with expansion, only up. Learned that the hard way. You need to remove the cap and visually check every morning. The pressure cap is 50 Kpa (7.2 Psig). The cap is physically larger than any car radiator cap so finding a pressure tester is difficult.
1984 Monk 36 Hull #46
Currently in Englewood, FL