One issue i have is can you check the cap without having to boil water and just keep on trying it at different water temps.
There are no markings on the cap

not easily I'm afraid- it works on pressure not water temperature.

only way I can think of is comparing to new one that is supposedly working ok ... but unless you have something exotic, you might as well fit a new one.

I daresay you could come up with some contraption filled with water and apply a known pressure but that would probably be more hassle than buying a new one.

BTW - they wofk by having main plunger pushed open by water pressure and there is a central spring loaded jobbie that then opens up when the water pressure decreases to allow water from overflow tank to be sucked back in.

[Edited on 1/11/07 by 02GF74]

ta for that, think a trip to halfrauds later on maybe.
Ta