CCM,
That is an interesting point. I can see the point you are making about the bleed by problems.
I wish I was sure of the difference between cracking pressure and the ratio of pressure needed to operate lock valve pilot spools...I think these are different issues, but that is only an assumption on my part at this point in time.
I know on one particual prince valve the RD1400 has two check valves inside, and a pilot spool that opens the check valve on the side which finds oil flowing in the direction against the check valve. This valve has a pilot ration of 4 to 1. Which I assume means that it takes about 500 psi to operate the lock valve in a 2,000 psi system.
A check valve may be a ball held against a seat by a spring. The spring pressure would have to be overcome before the ball could be pushed away from the seat, and fluid would flow. To me the pressure required to lift the ball off the seat would be the cracking pressure.
But since the lock valve has two check valves hooked up in opposition, cracking pressure [if I am defining it right] is not an issue, because the same pressure that would cause one check valve to crack, would push the other check valve's ball tighter against the valve seat.
Now the important point is that because of the blead through you mentioned, if there is no check valve, it is likely the cylinder will be moving all the time, even when the control valve is centered, if the cylinder feels any force on it.
Now is this true for an open center valve, like are common on tractors?
Anyway, I guess what I am trying to say is that I wish someone would define cracking pressure and pilot ratio in terms we could all understand... /forums/images/graemlins/tongue.gif