I think it has more to do with general wear in the pump, rather than anything to do with the check valve. The wear leads to a lowered volume of fuel being delivered, which means that less fuel is being circulated around the injector rail and back to the tank. I think that this situation can contribute to heat buid-up in the fuel in the injection system, leading to the hot-start "vapour lock" type of problem. That's my theory anyway. I've thought about substituting a pump from a B23 engine with CIS. They work at a higher pressure so must be able to deliver a higher flow rate. Would probably just need to re-adjust the fuel pressure regulator to compensate for the higher pump pressure. Another theory I'm afraid. Really haven't a clue whether it would make any difference. The warm-weather-hot-start-stumbles drive me crazy too, so I'm willing to try anything :)
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Steve.