Hi Brian,
Based on your description, I believe you have two problems.
First, the gear is held in the up position by fluid pressure acting on pistons in each landing gear actuator. When you gear sags down, that means there is a leak somewhere in one of these cylinders. There could also be a leak past one of the valves in the powerpack (the gear up check valve) but I’d start looking at one of the cylinder actuators.
These can be tested for leakage by putting your airplane on jacks, then plugging the fluid gear up and gear down hoses at one of the actuators. Then put the gear up. If the other two gear legs stay up, then the leak is past the o ring seals in the actuating cylinder you’ve removed from the system. If the gear still leaks down after you’ve removed one cylinder from the system, re-connect that cylinder and do the “block off” procedure on the next cylinder. Repeat until you’ve located the leaking cylinder.
You may find that more than one cylinder is leaking.
These are relatively easy to rebuild; it might be worth your time to rebuild all three to get them all back to square one condition. The instructions are in Chapter 6 of the service manual..
If isolating the gears to see if the actuating cylinders are the problem; and are going through the isolation procedure with all three cylinders and the gear still falls out of the wells, the problem is the gear up check valve in the power pack.
The other problem I see is a malfunctioning pressure switch. This is a simple on-off switch activated to off at a pressure of 1400 psi; and on at a pressure of 1100 psi. The part number of this switch is 587 847. List price for new is over $2100. I’ve located used serviceable one at a couple of salvage yards for around $500. Reputable salvage yards such as Preferred AirParts and Wentworth Aircraft, to name advertisers in our magazines offer a short period where the part can be returned for a refund.
If you mechanic is handy, he will be able to test this switch using a nitrogen bottle to supply the pressure and an ohm meter to determine when the switch is opening and closing.
Let me know what you find.
Best,
Steve
