You said the metal piece on the exp. tank that the cap attaches to is loose. If it's still loose, you're system will lose pressure. Been there. Not sure how the cap resolved the loose neck on the exp. tank?
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.