Troubles starting when cold, cranks fine, found that it doesn't actually get fuel until you basically re-prime the pump. this is if its been sitting for a bit >4 hrs. This sounds plausible, could be a front seal on the IP. The strange thing is that there is a strong temperature dependence. If it stays above zero it will start almost imediately, if it dips below -10C it takes much longer, lots of cranking before it catches.
Decided to put in a priming bulb on the input line, as a temporary measure. Will try it tonight, but was wondering, what seals would leak that would cause the fuel to drain back to the tank. Is it just the main shaft seal? The fuel appears to be returning via the return line, when the engine is off.