This is from the Piper maintenance manual for the Arrow.
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DESCRIPTION. Hydraulic fluid is supplied to the landing gear actuating cylinders by an electrically powered reversible pump located aft of the baggage compartment at right side station 156.00. The fluid reservoir is an integral part of the pump. The pump is controlled by a selector handle located on the instrument panel to the left of the control quadrant. As the handle is selected to either the up or down position, the pump directs fluid through a single line to a manifold and from that manifold to each individual actuating cylinder. As fluid pressure increases at one side of a cylinder piston, fluid at the opposite side is directed back through another manifold to the pump reservoir. The two manifolds and their connecting lines serve either as pressure or return passages depending on the rotation of the pump to retract or extend the gear.
On the manifold through which pressured fluid passes during gear retraction, there is located a pressure switch which opens the electrical circuit to the pump solenoid when the gear fully retracts and pressure in the system increases to 1800 plus or minus 100 psi. The switch will continue to hold the circuit open until pressure in the system drops to 1500 plus or minus 100 psi, which at this point the pump will again operate to build up the pressure as long as the gear selector handle is in the up position. The down position of the gear handle has no effect on the pressure switch.
The hydraulic pump is a gear type unit driven by a 14 volt reversible motor. To prevent excessive
pressure on the hydraulic system due to fluid expansion, there is a thermal relief valve incorporated in the pump that will open at 3000 plus 300 or minus 200 psi, thus allowing fluid to flow to the pump reservoir. Other valves in the pump system channel fluid to the proper outlets during gear retraction or extension. In the base of the pump is a shuttle valve that allows fluid displaced by the cylinder piston rods to return to the reservoir without back pressure. This shuttle valve has a delivery pressure of 400 to 800 psi during the extension cycle. Also in the system is a bypass or free-fall valve that allows the gear to drop should a malfunction in the pump circuit occur. To prevent the gear from extending too fast, there is a special restrictor fitting on the
side of the valve.
The emergency gear lever, used for emergency extension of the gear, manually releases hydraulic
pressure to permit the gear to ?free-fall? with spring assistance on the nose gear. The lever must be held in the downward position for emergency extension. For a description of the landing gear and electrical switches, refer to Section VII, Landing Gear and Brake System.
Hydraulic
High Pressure 2400 ± 200 psi
Low Pressure 600 ± 200 psi
Flow Rate @ 1000 psi 60cu. in. per min.
High Pressure Control 2400 ± 200 psi
Thermal Relief 3000 + 300, - 200 psi
Hydraulic Fluid MIL-H-5606
Pressure Switch
Open (OFF) Pressure 1800 ± 100 psi
Close (ON) Pressure 300 ± psi below opening pressure
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It looks like 400-800 psi is used for extension, but as others have said, there is no mechanism to increase this as it leaks down. It also looks like there is a .020" bleed hole that connects the low pressure 'down' side of the system to the hydraulic reservoir. If this is so, it would quickly return the pressure in the low pressure side of the system to zero after the gear is down. This is not a problem as once the gear is extended, the 'J' hooks and springs will keep the gear in the down and locked positon without the aid of hydraulic pressure.