Carrier pressure relief cap.
 

Gents, not seen this little item talked about much on the forum, I have made up a test rig to check the lifting pressure, which should be around 5 p.s.i.g. (When at ambient temperature), What I have found is to get these caps working correctly is you need to change the rubber sealing seat, I have found fitting an 'O' ring in its place works. According to the British workshop manual to adjust the pressure requires you to unsolder the internal screw and wind clockwise to decrease, anti clockwise to increase the lifting pressure. You also need to check the brass bellows is not holed. Another issue is getting the correct size gaskets for these, the hardest one to source/make is the one around the bellows.

Hope this is of help to someone, can bring the test rig to war and peace if anyone wants to set there's up.

Checking the vaccum settings is project for later.

Kevin.