Folks,

I thought I would take some time here to outline my recent experience with the parker oildyne pump so that it would save some of you the same time and expense i have had to deal with recently.  My pump had been sitting in the aircraft for a number of years through the early construction phase without exercise.  Once I began swinging the gear and flying I experienced various issues:

1. Gear would not retract - despite operation of the dump valve 
2. Gear would not lower unless dump valve was actuated
3. Pump would screech and make various noises
4. Pump would turn on and off frequently during flight

After experiencing all of these issues I decided to make it my mission to learn as much about the system and eliminate all of the failure modes.   Lessons learned:

1. The pump has many applications in the marine industry for which there is some excellent documentation
2. The two way dump valve supplied with the kit is poor quality and cannot be trusted (can leak internally and externally)
3. It is a very good idea to install pressure gauges in both up and down hydraulic lines
4. The original pump is slow to raise and lower the gear
5. Nobody knows more about the equipment than Parker/Oildyne
6. Heat will cause pressure increases in the lines to the point that the pressure switches will drop out
7. Seals inside the pump will eventually leak through wear and tear especially after sitting for a long period
8. There are numerous failue modes to the system, the goal is to eliminate as many of them as possible

Following 2 instances of not being able to raise my gear in flight I had had enough.  After studying all of the related posts on this site I then elected to call Oildyne.  The people there were kind enough to share with me their engineering drawings and their recommendation of the correct configuration for the pump internals for the LNC2 application.  Contrary to some of the posts to LML the preferred spool type is a symmetrical one that is centered using two additional springs.  The attached drawings provide a good understanding of how the spool, check valves and relief valves work.  Armed with the data  I set about modifying my hydraulic system as follows:

1. Removed the original pump adapter from the pump motor - Parker p/n781021
2. Removed the relief valves from the pump adapter and installed on a new adapter - Parker p/n 781047
3. Replaced the gear pump module for the high output upgrade - Parker p/n 634218 
4. Replaced the dump valve with a high quality swagelok 3 way valve that dumps to the tank B-42XHF2=BKB
5. Replaced one of the pressure switches for another (was replaced under warranty - had a 500psi hysteresis)
6. Installed a small pressure gauge for diagnostic use
7. Installed a new fluid reservoir. New design does not have the mounting hole in the bottom

A 781021 can be converted to a 781047 by performing the following steps

1. Remove the asymmetric spool in the pump adapter and replace it with the symmetric one along with the two centering springs
2. Remove center relief valve as shown in drawing - it is unnecessary

Net results:

1. All gear legs move together and at about the same rate
2. No screeching - sounds smooth with positive cut off - I believe the center relief valve to be responsible for the occasional screeching
3. Emergency dump valve dumps pressure on both sides of the pump to zero with return to the tank
4. No internal leaks -  Hence no cycling of the pump in flight
5. 100% reliability on gear operations

All mods to pump done for $250

Notes:

To inspect the check valve poppet as a possible leak down cause, remove the hex plug, on whichever side of the power unit is having the problem. Inside there is the spring and the poppet.  Check the rubber face seal of the poppet for any chips which might be embedded. On a rare occasion the face seal itself, which has a 'stepped' design, may delaminate, which can cause a leak.  When finished doing all this, retorque the hex plug to 450-600 in-lbs.