On my old ASI ast, I had to turn to close, push down real hard on the cap, and turn again to seal it. Is this the same on the Petit ast? I can't seem to get it to seal and coolant leaks out the top nipple when the car is running. Tried searching but got a bunch of errors.

 

Te square looking caps w/o the red pressure release like the pettit ones need to push down and twist twice to completely lock up.
If I remember correctly the pettit cap works the same.

 

the one i got from them has the red release lever, same as my old ASI cap, just 13 lb instead of 16 lb. I thought it was the construction of the AST neck that determines if you push it down twice or not?

 

Yes, if you look at the underside lip of the AST itself, you can see how far the cap needs to be turned to lock. It has to completely be turned until it hits the higher part of the lip. Sorry, dont know how to explain it better than that.

 

Because your AST is already on the car, try angling a mirror to see how the underside of the lip locks and seals. Play with it a while. When you get it right you'll know it.