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.