How far away should the check valve be from the engine. I suspect this could have some effect on the tank pressure. Am I right in thinking it should be as close as possible to the engine?
Also at what point do I change bearings/piston/sleeve or do I keep running the engine until there is a problem?

You didn't mention which engine, but this should have no effect on any of the 4 strokes. The heli engine check valve distance is specified in the engine instruction sheet.

Dave

Engine is 140fz. When should I replace piston/bearings etc?

If the engine still has good compression when it's hot, don't fool with the top end. If the bearings are rough feeling or noisy, it's time for a change.

Good Flying,
Dave