Hi Ernie, your description is correct. The check-valve is orientated so that the engine can only suck air in thru it (as you say, on the up-stroke).

Here is my thinking:

1: The manufacturers of 4cyc engines with crankcase breathers generally say you should only connect a short piece of tube (a few inches max) to the breather. I'm not sure of the reason, but had always assumed it was to do with the fact that the engine sucks then blows then sucks etc. A long piece of pipe would mean that the "blow" would not be able to clear the pipe of oil which would then get sucked back in. I could be wrong in this assumption - Hobbsy, do you know why?

2: A short piece of pipe means mess on the model. I hate mess on my model. I have a couple of engines with circulated breathers (OS and ASP) and the cleanliness is a delight. Recirculating on these Saitos is a bit of a pain due to the sensitivity of the inlet manifold to vibration (see other threads!). So really I want to connect a long pipe to the breather. not a short one.

3: My reasoning for the setup shown is this. When the engine blows, the check valve closes and the oil+air mixture has to go down the long piece of tube. When the engine sucks, the check valve opens and is the course of least resistance (shortest bit of pipe), so the engine can suck oil-free air in. Net result is the oil goes down the long tube. In practice, the "suck" phase might draw in some clean air via the check valve and some oil+air back from the long tube, but the net is the oil goes down the long tube and out to freedom.

4: Of course, the belt-and-braces technique would be to have a check valve in the long tube too, so that the engine can ONLY blow down the long tube and ONLY suck thru the short tube. But it doesn't seem necessary to do this.

It seems to work, I've now run about twelve 10oz tanks through in a mixture of ground running and flying with no problems. My long piece of tube runs through the belly pan and down the main undercarriage leg to get the exiting oil clear of the airframe; maybe I should do some ground testing with the model inverted to check it still pumps the oil out against gravity (the main undercarriage leg would be sticking upwards) (it's a pattern model so in the flying I've done its spent a fair amount of time inverted).

I suppose for completeness, I should try replacing the check-valve with a blank to see what effect running the engine with just a long piece of tube on the breather would have. Furthermore, Hobbsy could be correct that the check-valve is only helping out at low rpm.

All good fun!

HS