Correct AST Pressure Cap Operation
 

Hi all,

I am trying to troubleshoot a cooling system problem with my '93 FD and I have been unable to find a good description of how exactly the AST pressure cap is supposed to behave when it is hooked up to a pressure tester (out of the system.)

When I test the filler cap, I am able to pump it up to a high pressure (> 15 psi) and it holds. From what I read, this is the correct operation of the filler cap.

When I test the pressure cap on the AST, it releases around 10 psi (Right now, I don't remember the exact number) and while it is releasing, I can't pump it back up until the pressure drops to 0. I guess I thought that the correct operation would be to just "blow off" any pressure above the pressure it is set to release at (let's say 10 psi.) But I am seeing it behave the way I described it above.

Can someone confirm what the proper behavior of the AST cap is when it is being pressure tested?

Thanks a lot,

Joe

Is this a joke. The AST presure caps are six bucks from Car Quest. Replace the thing.

No, it's not a joke. When fixing things, I prefer to understand what the problem is, rather than just blindly replace things.

I'm not sure of the reason for my loss of pressure in the cooling system, and I'd like to understand why.

No need to get snooty -- if you couldn't answer my question, you could've just moved on to the next post.