Cat, use a little bar keepers friend on the valve seat / faces and burnish the valve seats in... they leak from new, or at least mine did, just from rough machined surfaces... Blacken the seat and valve faces with a sharpie pen and then burnish them with the bkf... compression loss past the piston on a single ringed piston is common when the engine is turned over slowly... due to variations in ring tension and fit, you'll get some leakage... with a wet cylinder, and rpm the dynamic compression is what counts.

Next time you take the engine down blacken the cylinder walls and piston ring faces, then work the pistons in the cylinder... they're not perfectly mated, and take quite a number of run hours to get those rings to seat... those cylinder sleeves in the boxer are hardened, and being a single ring take a long while to bed in.