[Lloyd Burns]

With the valves we have, sufficient pressure must be applied to the pressure port P to allow the electrically-operated pilot valve to push the main, air-operated valve hard enough to switch.

If you put cyclinder exhaust through a pressure port, with these dinky valves, the low volume-rate pressure will go out through the open port, and never build up high enough to operate the valve.

You could try to put the cylinder exhaust through one of the X (exhaust) ports on the valve via the related output port A, or B, and you would have to supply air to port P.

If you use A and Xa to switch the cylinder exhaust, then when the valve is operated, P is passed through to B, and Xb is blocked. To close the path through P to atmosphere, use a plug in B. All the while the cylinder exhaust is attached to Xa which is connected to A, and is at 0 psig = atmosphere.

But when you relax the valve, Xb, is connected to the plugged port B, so no change, and while Xa is disconnected from A, so it is plugged (not guaranteed), P is connected to A, and is passing your precious pressurized air to atmosphere. You need pressure on P to operate the valve, and you need A not blocked to open Xa to atmosphere hwen operated, so you need these connections unchanged.

There isn't any way I have thought of to accomplish blocking a cylinder with these valves.

And putting equal pressure on both sides, without blocking the air flow from one side of the piston via the valves to the other side, is almost the same as letting both sides go to 0 psig. Physics assumes that balanced forces give Fnet of zero (neglecting the difference in piston area between the sides, which doesn't help).

So go ahead and prove it wrong - stop it mid-stroke, if you can !