Sand and DE filters often use a Multiport valve to reverse the flow into the filter for backwashing. Multiports also offer Waste and Recirculate functions, which can be useful at times. But it also makes the multiport valve a fairly complicated piece of pool equipment.
When a pool filter is leaking, half the time it's actually the multiport valve that leaks. The usual multiport problem is leaking water, or water coming out from somewhere it shouldn't.
So then, today's post is about leaky multiport valve problems and solutions. 6 specific types of multiport valve leaks.
The first thing you need to know is that these small valve leaks aren't going to explode and drain the pool, well probably not.
Q: MULTI-PORT VALVE IS LEAKING WATER AROUND THE HANDLE
A: There is always some rain water that collects around the handle, but if water starts to ooze up and overflow onto the lid, you have a common multiport valve leak. Underneath the handle is a large Teflon plastic washer, but this is not the cause of the leak. The handle connects to the stem (with the pin), and on the stem are two small o-rings that seal up to the underside of the cover. These two o-rings, about the size of a quarter, fit onto the stem (which is connected to the rotor, aka footpad or diffuser or diverter), or they fit under the cover. Over time, they may wear out or dry out or blow out, or lose their lubrication. Cold water temperatures will make it worse, as the o-ring can shrink just slightly, or small bits of grit and debris can get wedged up around the stem o-rings, causing leaks. 
For this situation, I would first just reposition the valve. Shut off the pump and push down on the multiport valve handle fully, once or twice. Move it slightly left and right and then back to the Filter position. Use a sponge or leaf blower to remove the water sitting under the handle, and turn the pump back on and see if it still leaks. Often, just re-seating the stem o-rings will seal up a leaky mulit-port lid cover. If that didn't work, then you may have to consider a valve rebuild. The easy route is to buy the 'Key Assembly', which is everything but the valve body, or from the handle to the rotor. The much cheaper way is to remove the key assembly from the valve, and knock out the pin to remove the handle. Then you can pull off the cover, and replace the stem o-rings. Be sure that during reassembly, you orient the cover and rotor to each other in the same direction as before. Q: MULTI-PORT VALVE IS LEAKING WATER AROUND THE COVER
A: When the leak is around the outside of the top, where the lid mates with the valve body. The first thing to check is that the system is not under unusually high pressure, from a dirty filter, closed return valves or plugged wall returns. If normal, check that the cover screws are all tight.If it's still leaking, remove all cover screws and pull out the key assembly. Clean, inspect and lubricate the large cover o-ring and place back into the groove in the key assembly. Replace the key into the valve and tighten up the cover screws, then fire-up the pump to check for leaks. Valve cover o-rings don't usually go bad over time, but if it's broken, dry-rotted or damaged, you can replace just the cover o-ring.
Q: MULTI-PORT VALVE IS LEAKING WATER AROUND THE GAUGE
A: Another common leak is a small trickle of water running down below the pressure gauge on top mount sand filters. You want to make sure the gauge is firmly held in place, if the gauge were to shoot out under pressure, you could lose a lot of water before discovering the leak. One cause of this is too much Teflon tape on the pressure gauge - years of it! If you're guilty of this, peel off the previous years' layers and lay on 1-2 new layers of Teflon tape (always wrapping the threads in a clockwise direction).
Another cause of a pressure gauge leak on a multiport is that the plastic threads in the valve body are stripped. The valve threads can't be tapped or retreaded, because then the gauge threads will be too small. If you definitely determine that the threads are stripped inside of the valve, you can thread in a Heli-Coil or other 1/4 in-20 thread repair insert, which involves tapping out the hole slightly larger, and screwing in a double threaded 'helix' type of bushing, which gives you new 1/4 in threads for the gauge.
A third cause of the pressure gauge leaking problem is when the valve body is cracked just below where the gauge screws in. This is usually caused by too many layers of Teflon tape on the gauge, and over tightening or cross tightening the gauge. This splits the plastic valve body and causes a leak, but there is a simple repair. Shut off the pump and tighten the gauge more to open up the crack slightly. Clean it out with a toothbrush or small wire brush, and be sure it's dry. Squeeze Super-Glue into the crack, from top to bottom, and quickly unthread the gauge, so the crack will close up. After an hour or so, replace the pressure gauge carefully, after having removed the years of Teflon tape buildup, replacing with 1-2 layers of new Teflon tape or other thread sealant. Be careful not to over-tighten the gauge, just 3 or 4 turns, or until it feels snug.
Q: MULTI-PORT VALVE IS LEAKING WATER WHERE THE PIPES CONNECT
A: Multiport valves have 3 ports, Pump, Return and Waste, where pipes are connected, using 1.5''or 2'' threaded pipe or adapters. At the point where the Pump and Return pipes are threaded into the valve is a common spot for a leak. One cause of this is when a hot pool heater backs up hot water to the valve, which shrinks the PVC connector. In this case, a PVC replacement is in order. Another cause of leaking multiport pipes comes from sagging pipes from movement of the equipment or equipment pad, not very common but it can happen. The most common cause is over-tightening the adapter into the valve, which can cause a crack to appear later during a 'high-pressure event'. Events such as closing a return valve while the pump is running, or starting up the system with plugs in the wall returns, builds up a lot of pressure and could even blow the pipes right out of the valve.
If the valve body is cracked just below where the pipe screws into the valve, you can replace the valve body, or you can try to repair the crack with Super Glue, as described two paragraphs above. Or, replacing the entire multiport valve may be in order, if the key assembly parts are old and worn.
If the drip is very slow or oozing, you can safely ignore it for awhile. If it begins to saturate the ground under the equipment you may want to take action. You can replace the PVC fittings and some pipe, which you can find at any home or hardware store, along with PVC primer and glue. On the new fitting, smear RTV silicone sealer on the threads and then wrap it 3x with Teflon tape. Hand tighten, then turn with large pliers for another full revolution, being careful not to over tighten. Reconnect the pipes with a coupling or union.
Q: MULTI-PORT VALVE IS LEAKING WATER OUT OF THE WASTE LINE
A: Water is supposed to stop coming out of the waste line after you stop backwashing the filter, but eventually you will notice that water is leaking out the backwash line all the time, not good. First thing I would do is to shut the pump off and push down on the handle fully, turn the valve slightly left and right, and then back to Filter position. Sometimes small bits of debris get stuck between the rotor and spider gasket, and this flushes out the valve. Turn on the pump again to check if that helped. If it still leaks after repositioning the rotor onto the spider gasket, shut off the pump again and remove all of the cover screws, and pull out the key assembly.
Inspect the spider gasket (wagon wheel shape) for wear or damage (note that some Pentair valves have the spider gasket on the bottom of the rotor). If you find that the spider gasket looks good but is just loose, break out the Super Glue or silicone, and use dabs to glue any loose rubber down into the (dry) grooves of the valve body. If the spider gasket looks too far gone, badly twisted and torn, you should replace it. Be sure to use the correct gasket (or use a Valve Go-Kit) for your valve, as they are all different sizes. Scrape out the old gasket with a sharp, small flathead screwdriver, removing all tiny bits of rubber, and glue the new gasket in place (flat side down), using dabs of Super Glue or silicone in the (dry) grooves. Tighten the key assembly in place over the new gasket (no lubrication needed), and you're done replacing a spider gasket.
Another cause of a backwash line leaking is that the spring inside of the key assembly has failed. When this happens, your valve handle is floppy and there's no tension when you push down. Chemical corrosion can break down the stainless steel spring over time. Remove the key assembly and look under the lid at the spring to check for rust that has broken the spring's coil. To repair, you can just buy the new key assembly, already assembled, or you can break down the valve and replace the spring. To do this remove the key assembly from the valve, and knock out the pin to remove the handle. Then you can pull off the cover, replace the spring, and while you're in there, replace the stem o-rings too. Be sure that during reassembly, you orient the cover and rotor in the same direction as before.
One more thing - please don't repair a leaky backwash line by gluing a valve on the backwash line! I've done myself before, in a pinch - but it should be avoided; these problems above can cause water to bypass the filter, resulting in poor filtration, and - one time in the future someone is going to forget to open the backwash line before backwashing, which can cause even bigger problems.
Q: MULTI-PORT VALVE IS LEAKING WATER WHERE IT CONNECTS TO THE FILTER TANK
A: Slow seeping weeper running down the filter tank? Top mounted multi ports attach differently than side mount multiport valves, so let's take them each separately.
Top mounted valves used on sand filters, attach to a flange that is molded onto the top of the filter tank. First determine if the leak is coming from between the tank and the flange, or the valve and the flange. If the flange is leaking where it connects to the tank, I'm afraid I don't have much help for you. But if the leak is between the valve and the flange, the first thing to check is the tightness of the bolts or the clamp band (turn the pump off first). Repositioning the clamp band and re tightening can help in some cases. Unlike my many cautions above to not over tighten PVC fittings into the multiport valve body, when tightening clamp bands, there is no such thing as too tight. Tighten down clamp bands until they squeak and creak, often times this will fix a leak between the filter flange and filter valve.
If you can't get the leak to stop, you can pull off the valve to inspect the flanges and flange o-ring. To do this you may need to cut the pipes with a hacksaw, and glue couplings to reconnect the pipes afterwards. If you have unions, or install unions instead of couplings, you can quickly disconnect the Pump and Return pipes (for valve repairs, sand changes, or other repairs). Replace the flange o-ring if it looks dry rotted or damaged. Use lots of pool lube on the oring and flanges to improve the seal and tighten it all up again. If it still leaks, or weeps, check to see if return side pressure can be reduced, by changing filter sand, and/or reducing resistance after the filter with less equipment, straighter runs or larger diameter pipe.
Side mounted multi-ports, used on sand and DE pool filters, connect to the filter via two (2) Bulkhead Unions, and two sweep arms. DE multiports and Sand multiports are not interchangeable, because they operate in reverse. For sand filters, water enters on the top bulkhead, but for DE filters, water enters the tank through the bottom bulkhead.
For leaks on side mount multiport valves, determine if the leak is between the tank and the bulkhead or the bulkhead and the filter. To disconnect a side mount valve from the filter, open the air bleeder and drain most of the water from the filter tank, then loosen the bulkhead union nuts. Pull back on the valve and/or push on the tank to pull them apart, and be prepared to support the valve after pulling it off (cinder block or bucket), to avoid a lot of weight on connected pipes. Inspect and lubricate the o-rings on the valve sweep elbows. Filter tank Bulkheads can be tightened up by turning the large nut inside of the tank, using very large pliers or a strap wrench. Bulkheads also have gaskets and spacers on both sides of the tank wall, that may need service or replacement at some point - which may be after you've tightened the bulkhead, but it still continues to ooze water. Bulkhead parts are considered Filter parts, not Valve parts. 
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