Matt,

I think your system is operating normally except for a leak from the low side to the high side. What I think is happening is that over time, when the fluid expands due to hot ambient temps, the low side pressure builds and starts to leak into the high side. If the spool valve is in correctly, which I think yours is, the high side is basically a dead end -- no return leak path to the tank. I have pressure gauges on both the high and low circuits. I have noticed that when I open the dump valve with the gear down and pressurized, the pressure goes up on the high side.

I would look for a slow leak from the low side to the high side. You might start by replacing the dump valve, especially if you used the valve that came with the kit. (I installed a better Swagelok valve.) You could also have a very slow leak in one of the cylinders that only leaks one way. I had a main cylinder that leaked from the low to the high side when I powered up the master before a flight, but it did not leak in flight with the gear up. The pump would blip every couple of minutes until I raised and lowered the gear once. After that, it would hold pressure the rest of the day.  Leaks do not always leak both ways.

One other thing you should check is the down-circuit pressure relief valve setting. The pressure relief valves are adjustable. When I fist set up by pump, I set both pressure relief valves. It's a lot of trouble because you have to remove the tank to change the settings, and then you have to run the pump to check the new setting  (by raising the pressure switch setting and noting when the pressure relief valve opens).

Mike Reinath
LNC2 360 at 650 hrs TT
San Jose, CA (RHV)