Update: the cap was the problem. The spec cap is apparently incorrect on
several brands -- the center vacuum vent doesn't have a spring or it has a
diaphram that doesn't correctly close here at 6000ft so there is no opportunity
to hold pressure in the first place. I put the cap from my F-150 on it and it
holds pressure, so I went and bought another one of those (13# vs 15# so I'll
lose a few deg F in the boiling point). The F-150 cap has a light spring on
the center vacuum vent which holds it closed so pressure can build on the outer
spring.

Thanks for the ideas and measurements.

Lee

 

Update: the cap was the problem. The spec cap is apparently incorrect on
several brands -- the center vacuum vent doesn't have a spring or it has a
diaphram that doesn't correctly close here at 6000ft so there is no opportunity
to hold pressure in the first place. I put the cap from my F-150 on it and it
holds pressure, so I went and bought another one of those (13# vs 15# so I'll
lose a few deg F in the boiling point). The F-150 cap has a light spring on
the center vacuum vent which holds it closed so pressure can build on the outer
spring.

Thanks for the ideas and measurements.

Lee

 

Update: the cap was the problem. The spec cap is apparently incorrect on
several brands -- the center vacuum vent doesn't have a spring or it has a
diaphram that doesn't correctly close here at 6000ft so there is no opportunity
to hold pressure in the first place. I put the cap from my F-150 on it and it
holds pressure, so I went and bought another one of those (13# vs 15# so I'll
lose a few deg F in the boiling point). The F-150 cap has a light spring on
the center vacuum vent which holds it closed so pressure can build on the outer
spring.

Thanks for the ideas and measurements.

Lee