The gauges should read somewhere near line pressure. At this point the needles on the test gauges drop to zero. You cannot find any leaks in the connections of the test kit. The present USC procedure makes it virtually impossible to test DCVA's under these conditions. You need an alternate method to test double check valve assemblies.
Lets look at a few other methods to test the double check valve assembly.
Using a differential gauge you measure the pressure drop across the check valve. The needle on your gauge should indicate a reading of 1.0 psi or greater if the check valve is holding tight. Install brass fittings. Bleed air from hoses and gauge. The second disadvantage is that if the low control valve is leaking while the high control valve is open, then the needle on your gauge will fall to zero.