The slide-type selector valve is probably the most durable and trouble-free valve currently in use. Some manufacturers refer to this type valve as a piston or spool type. Figure 8-10 shows a cutaway view of a typical four-port slide-type selector valve. The main parts of the valve consist of a body, sleeve, slide, detent springs, and the necessary packings and gaskets. The valve body is cast aluminum alloy. It has four fluid portspressure, return, and two cylinder ports. A large bore has been drilled lengthwise through the body, and all four fluid ports connect into this main bore at intervals along its length. There is also a drilled passageway in the body that runs alongside the main bore. This passageway is used to connect one of the cylinder ports to the return port. A hollow steel sleeve (3) tits into the main bore of the body. Around the outside diameter of the sleeve are six O-ring gaskets. As the sleeve is inserted into the main bore, these O-rings form a seal between the sleeve and the body. This creates five chambers around the sleeve, and each chamber is formed by two of the O-ring gaskets. Each one of these chambers is lined up with one of the fluid ports in the body. The drilled passageway in the body accounts for the fifth chamber, which results in having the two outboard chambers connected to the return port. The sleeve has a pattern of holes drilled through it to allow fluid to flow from one port to another. A series of holes are drilled into the hollow center of the sleeve between each O-ring gasket.

A steel slide (5) or spool is machined so the largest diameter portions have a close tolerance fit in the sleeve. Typically, the slide has three raised, machined portions known as land areas. These areas usually have several grooves machined into them around the circumference, breaking each area into several lands. The lands (and grooves), in concert with the close machined tolerances, provide for easy, smooth operation, long service, and no leakage. 

One end of the slide is connected to the control handle in the cockpit through mechanical linkage. When the control handle is moved, it will then position the slide within the sleeve. The slide lands then line up different combinations of fluid ports, thereby directing a flow of fluid through the valve. On the end of the slide, next to the eye, are three grooves called "detents." These detents are used to leek the slide in the exact position needed to properly direct the fluid flow.

The detent spring (6) is a clothespin-type spring, secured to the end of the body by a spring retaining bolt (7). The two legs of the spring extend down through slots in the sleeve and fit into the detents. The slide is gripped between the two legs of the spring. To move the slide, enough force must be applied to spread the two spring legs and allow them to snap back into the next detent, which is another position. 

Because of the very close fit between the slide and sleeve, the most common cause of failure or malfunction is the presence of dirt or foreign matter. Foreign matter could result in binding of the slide, scratching the machined surface, and damage to O-rings. Originally, these valves were provided with protective boots on both ends of the slide to prevent dirt or corrosion from getting on the exposed machined surface, where it would be carried into the valve when the slide was moved. These protective boots usually are missing on valves currently issued, leaving the machined surface exposed. As a preventive measure, in place of the boots, a light film of hydraulic fluid should be applied to the exposed areas of the slide. Primarily, this oil film is to prevent corrosion, but it helps to prevent any entry of foreign matter into the valve. Proper linkage adjustment is necessary because linkage that is too long or too short will prevent the detent spring from locking the slide in the correct position.

If it becomes necessary to test this valve under pressure to determine the cause of malfunction, it is important to first check the MIM for the particular installation. A slight amount of internal leakage is permitted in the working positions, and this should not be mistaken for faulty operation.