Multiport Valves:

A Multiport Valve is a pool filter valve that has Multiple Ports. These ports or channels for water, allow the pool owner to direct the water in different directions. Filter valves need to be able to direct the water into the filter in two directions, one direction for normal filter operation and the opposite direction, which enables the pool owner to "backwash" the pool filter. The common multiport valve has six different settings. These are typically labeled as such:

FILTER: Keep it here 99%, except when backwashing, rinsing or wasting.
RINSE: Use this setting for 20 seconds after backwashing to rinse tank.
RECIRCULATE: Use this if the filter's broken; at least you're circulating.
BACKWASH: Use this setting to reverse the flow in the filter and send water   out of the waste line. Make sure valves are open or hoses rolled out.
CLOSED: Put here to close off flow from the pool, usually to work on the equipment. Do not operate pump with valve in closed position.
WASTE/DRAIN: Another filter bypass setting, but this one sends the water out of the waste pipe (hose), instead of returning it to the pool. Used to lower pool water level or to vacuum to waste

My Multiport is always leaking water out the backwash hose.!?! This will occur when the multiport valve gasket, usually called a spider gasket, has come out of it's groove, or has become nicked or worn and is allowing water to enter a port where it normally cannot. Another explanation may be that the spring that gives the footpad tension down onto the gasket has become relaxed or broken, allowing water to seep by. Another explanation is that sand or other debris has become lodged in between the footpad and the gasket, which creates a gap that allows water to flow freely into the backwash port.

How is this Fixed? First, try to reposition the footpad by pushing down on the handle of hte multiport valve to raise the footpad and possibly dislodge any debris that may be causing the leakage. Try it first with the pump on, then very gently (push down only 1/2 inch or so) try it with the filter pump on, if necessary. If this does not help, disassemble the valve by removing the screws around the lid and pulling the "Key assembly" up and out of the valve body. Inspect the gasket and the spring on the key assembly.

You may notice a twisted, warped spider gasket. If so, the most permanent repair is to remove the gasket, and replace the spider gasket. You will need to use the proper gasket for the correct multiport valve. Scrape all excess rubber from the grooves that hold the gasket. Any excess rubber left in the grooves will prevent the new gasket from seating completely into the groove. To seat the new gasket, dry out the grooves in the valve completely. Then apply 3M industrial sealant or other adhesive in dabs (not too much). Push the spider gasket firmly into the grooves of the valve body. Apply a lubricant to the top of the gasket or the bottom of the footpad, and reassemble the valve. Make sure you align the key and footpad properly to the valve body.

The Handle on my Multiport Valve is loose and I cannot turn it.!?! This will occur when the spring beneath the lid becomes corroded and broken. To replace the spring, bang out the pin which holds the handle to the vertical portion of the footpad (also called the rotor). Gently remove the lid and the spring. Replace the lid and the spring, and perhaps the o-rings and washers while you are at it. Reassemble in the exact manner that you disassembled the valve. If you don't, the ports won't line up, and dangerous pressure could result. Have a helper push down on the lid to compress the spring enough to be able to push the pin through the handle and the vertical portion of the footpad. As an alternative to this repair, many pool techs just replace the entire "key assembly" of the multiport valve. This is the handle, lid, rotor, etc., all in one.