Thanks guys,

Looks like I need a nice flat piece of rubber.

I have a nice low pressure fuel gauge for my system. It actually does vacuum too, but I haven't figured out how to actually make a vacuum - maybe I'll just try and suck on it.

It maxes out at 15psi and I've never gone above about 7 so far.

Also it leaks out within a minute or so at the moment with the leaky exhaust so I'm pretty sure it will be ok.

Any other tips would be appreciated.