When the cam lobes for the cylinder you're working on are not positioned to depress either of the valves, then your cylinder should hold pressure. If the 420 top end is anything like the 560 top end, then getting air through your fuel distributer means your intake valve for that chamber is open. If the lobes are pointing away from the engine (ie not pressing down on the valves) then you should not be losing much air (if any, at least from the top end) and your guage should be pretty steady.

Good luck!