I noted that my check valve leaks. When you disconnect the hose going to the cylinder it will drip a few times each hour. After a few weeks it adds up. The spring and bearing are all fine and it's clean in there. I got a generic check valve - actually two different kinds. I connected them to a container of oil and tested it with gravity feed pressure for a few days and no leaks. I added the extra check valve to the oil hose so that system now has the original check valve in the pump as well as the extra check valve on the feeding tube from the pump to the cylinder. It still leaks. It leaks less - maybe about 10 or so drips a day but after sitting for a few weeks, it smokes pretty heave for the first few minutes. It just seems that the check valve should be more reliable than that. I can only blow through it in one direction. Maybe the oil can just slowly leak through it but it seems like they should be more effective than that.

Yamaha makes a check valve that some have used.  Check with DEET.

Could fit a small inline fuel tap from oil tank to pump.
But knowing my luck i'd forget to turn it on in the morning.
Or adapt a vacuum operated one like fuel taps on various models. You're right though, the original shouldn't leak & neither should an accessory one.

You might consider stretching the spring slightly to improve the design flaw. How about another 1/8”

It also might not be the check valve at all? 

Why would generic one not leak at all under gravity test and then leak on bike?

Maybe oil tank vent is plugged, pressure builds up, and forces oil past valve(s)?

(I've seen a lot of plugged and pinched tank vents.)

Maybe oil is heavier than it used to be in 1970 hence stronger spring required. Na not that.
Wonder if you stop the pump with the plasic wheel so the pump plate is 'down', right down with cable wheel pulled around so no oil can enter the shaft to pump up the line. Na not that either.
Sorry, just having a stupid day.

I have heard of members using a clothes pin clamp to reduce the chances of going into the cylinder or carburetor. But as others have mentioned in the past a shut off valve is not recommended.

Good point #5 & we must've been typing at same time.
But another point, would a blocked vent cause a vacuum & stop flow.
Or is the 'flow so slow' that air bubbles could travel backwards through the system to let the oil flow fast enough from tank to pump.
Hell i dunno. I'm outa here for the day.--could re-seat the ball.
Deet is probably watching & gonna sort us all out shortly.

I recently got a DT400 and it has an off/on valve in the oil line as apparently it would leak out the oil into the sump overnight.
I am wondering it its possible to - re-seat the ball bearing valve in the pump but i havent been down that path yet.

Having said all that, aquarium suppliers have some good little check valves but i am not sure of their compatability with oil instead of water.