when i test mine i only pressurize them to what the cap is rated for. the heater in my 40 is a arvin type and not original to the truck. i had the same issue as yours leaking between the case. i narrowed it down to the connections where the hoses attach. they were out of round to the point where the clamp couldnt pull the hose tight around the fittings and it would leak and runinto the truck. i got some bar stock of the same dia. and tapped it in the connections to make them round again and that did the trick. since the 40 is an atmospheric system (not pressurized) i didnt have to smoke the clamps to get them to seal once round.
