After suggestions on a little hydraulic conundrum. I was looking for a solution to the slowly dropping blade on my 70CI. The real problem is leakage through the spool but, given the age, fixing this was not going to be easy. My solution was to fit a pilot operated check valve to the return from the bottom of the rams. The idea being that it allows free passage when the blade is being lifted but blocks return flow to hold the blade up. When hydraulic down pressure is applied, this activates the pilot and allows the return flow. Theory is good but in practice, the down pressure activates the pilot, this allows the return flow (as per plan so far!) but as soon as return flow is allowed, this takes the pressure off the pilot (whoops, didn't think of that) so the check valve shuts, this causes pressure on the pilot..... Result; very jerky action when lowering. The basic problem is that the blade doesn't actually require pressure to lower, the weight does that, so the only time there's pressure on the pilot is when the check valve is closed. When it opens; no pressure! I'm sure there must be a simple answer to this but I haven't figured it yet. Best I've come up with is another valve to the top of the rams that needs pressure to overcome it so the pilot line sees a constant back pressure. More cost, more complications. Must be a smarter way!
You need a counterbalance valve, in place of the lock valve you installed. They're designed to work as a braking valve with overrunning loads.
Or just overhaul the spool and not try to reinvent the wheel
Thanks for that. I've researched a bit further and that's exactly what I needed. Should have asked the question earlier as I hadn't heard of a counter balance valve but knew about pilot check valves. Fortunately, I have been able to solve the problem this afternoon by adding a check valve, with a needle valve bypass, to the upper side of the rams. This doesn't appear to have slowed the rams down but the slight restriction offered has given sufficient back pressure for the pilot to control smoothly. It was a cheap fix.
Using external components to hold load is not really reinventing the wheel. Overhauling the spool would be ideal but when they get to forty plus years old, some leakage is almost inevitable and the only real cure for worn metal is replacement. Given it's an assembly with the tilt and the ripper, this would be a very expensive solution to a fairly minor problem. The spools are working fine otherwise.