So a quick update. Tried the leak test again this time with the exhaust gaskets in place. Lo and behold, now the cylinders now will hold pressure.

This is where the real mystery begins though. Left cylinder holds pressure just fine. 5 psi for 10+ minutes and there was not any pressure drop at all.

Right cylinder is a different story. After about a minute it will drop from 5 psi down to around 2 psi. Clearly leaking. The weird thing is that with the right cylinder pressurized, I can see tiny air bubbles escaping from the apex of the left reed valve. If i give the left reed valve a little push, it gives way with a 'pop' and all the air rushes out. So air is obviously getting across from right to left cylinder.

Can the centre seal leak just one way?

Also, i blocked the carb manifold on the left cylinder assuming that I could eventually get enough air to leak across and equalize left and right. This was in the hope of eliminating any other leaks. I could never get the pressure to equalize I'm assuming that there is more than one leak on the right side.

I hope someone is able follow my thought process here. Would love to hear some advice from anyone with some experience on this topic.

Cheers