Even with the check valve in the end of the hose, the accuracy on small singles can be significantly affected. 

The hose volume still makes a difference and a standard schrader valve will definitely reduce the pressure the gauge sees from the very short duration low volume "pulse" of pressure from a small displacement piston.  (Getting help to push the bike fast in first gear to spin the engine faster than the kick starter can overcome some of this)

And yes, the best use for the compression gauge is "comparison" on the same engine.  

So that gauge pictured has 2 Schrader valves?
One inside base of gauge and 1 on side for relief?

Something does not add up.

No. As I wrote, it has a Shrader valve in the gauge base BUT the fitting in the spark plug hole is open.

My foot on the Kickstart tells me exactly how good the compression is. But I have a genuine yamaha compression gauge I'll post a pic of if I can--my google photo free thing is full. Might start using Imgur.

So toss it. Why waste time shortening hoses and mickey mouse crap.

That Shrader valve must have a very light spring on it and must be flush to combustion chamber.

c

I found a solution for a cheap compression tester that gives me assurance that my bike is at least is 120 PSI, so why toss it? I don't test compression every day, only have one bike, and it gives me a baseline, so in the future I'll know if it drops. I like turning lemons into lemonade.