Your compression tester should have the check valve at the fitting which screws into the head and not near the gauge itself. The valve at this location allows a true reading to be taken although it will take more pulls of the rope on a small displacement motor. Try spinning the motor with a drill and see what happens.
The valve on the left is a reed style while the one on the right is a Schrader style. I prefer the reed style as little compression is needed or lost to open the valve. Length of hose shouldn’t matter with a reed style valve tester.
