Dump valves are pretty reliable unless they freeze or get damaged by mishandling or impact with something. However, they will sometimes need repair or replacement in normal use due to ordinary wear and tear on the seals or getting debris trapped as they are closed. You can also break the internal slide or bend the shaft if you don't pull and push the valve straight in and out. If you discover sewage accumulated inside the dump cap between dumps, you probably have one or more leaking valves, assuming you closed them correctly and completely the last time you used them. The color and smell of the accumulated fluid will tell you which valve is leaking. Foul smelling stuff that is blue, green, or brown is coming from the black water tank. Some holding tank chemicals might even turn it orange! Grayish, soapy water that is nearly odorless is from the gray water tank. The black water valve will be large, 3" valve. The gray water valve may also be a 3" valve but on some rigs it is a smaller, 1 1/2" valve. Having a clear plastic dump cap instead of a black one will let you see if there is anything behind the cap before you open it allowing you to avoid a nasty surprise. Always make sure both valves are fully pushed in before deciding you have a problem with the seals. Sometimes a bit of debris may get caught in the valve when it closes causes it to leak until it is opened and flushed again. You may be able to clear debris from seals using a brush or carefully cleaning the groove with a bent wire. A persistent leak is a strong indicator that the seals need to be replaced Having a clear plastic dump cap will let you see if there is leakage before you open the camp and get doused with nasty stuff. Having a dump cap with a hose fitting allows you to slowly and easily drain the accumulated sewage safely into the dump hose or a container before you open the cap and get a big uncontrolled "whoosh" of nasty stuff all over you and the ground. The space between the valves and the cap can usually hold a quart or two of nasty leakage. Dump your tanks before attempting to work on the valves. After you get home from dumping and parked where you're going to be working on them, put a container under the valves and remove the outer cap and open the valves and leave them open for a while to let everything drain out and stop dripping. This will help avoid getting sewage up your sleeve or dripping in your face while working on the valves.
Sometimes you can clean debris from the seals by carefully scraping the groove with a bent piece of wire. Be gentle and careful if you try this. You only want to remove the debris and must avoid damaging the soft rubber seal. Often even soft debris that has been stuck for some time may have already damaged the seals, forcing you to replace them to correct the problem.
To avoid problems with your dump valves, always pull and push the handles straight. Any angle on the handle could bend the shaft, damage the seals, or crack the slide. Once any of these things has happened you will have to replace the valve. Fortunately they aren't very expensive (around $20 each at even higher priced RV parts stores and even less at discount outlets) and they are usually pretty easy to change.
Always wear protective rubber, nitrile, or vinyl gloves when working with sewer hoses and dump valves to avoid exposure to chemicals and nasty waste products. Then thoroughly wash your hands after you have removed and discarded the gloves. Coveralls are a good idea too, and be sure to wash and disinfect your work clothing when you're done, especially if there was any spillage.
Sometimes the problem is due to worn seals, which can be replaced without replacing the entire valve. However the effort is pretty much the same whether you're replacing just the seals or the entire valve. Seal kits will be a little less expensive than complete valves but since the valves are fairly inexpensive, I prefer to replace them rather than just change the seals to avoid any extra labor if just changing the seals doesn't solve the problem. Some valves can only use their own branded seals and if you get the wrong ones, they will leak. That is one reason I prefer to replace the whole valve so I don't have to worry about matching old seals. You'll need to dump and flush the holding tanks before beginning any repair. The valves are blade valves that are fastened between flanges on either side -- one on the outlet from the tank and one on the pipe that leads to where you attach the dump hose. They are secured by 4 bolts -- one in each corner of the square part of the flange on the valve. Remove the 4 bolts, then carefully pull out the valve. Remove the old seals and clean the flanges. Install the new seals on the flanges. Make sure to put the large end of the seal over the lip on the flange. Then very carefully slip the valve (new or old) into place, taking care not to dislodge or distort the seals. This can be tricky. Replace and tighten all 4 bolts and you should be good to go. New valves should come with new bolts, another benefit of replacing the valve and not just the seals. Always hold the nut and tighten or loosen the bolt head because the nut is knurled to prevent it from slipping. Turning the nut will grind the surface of the valve. Tighten the bolts until the heads begin to bite into the plastic flange. Once the bolts are tightened, close the valve to make sure it operates smoothly. If there is any resistance or it won't close completely the seals have probably slipped and you'll need to take it out and reinstall them properly. At least partially fill the tank with clean water to test the installation. Sometimes (often) the old bolts will be so badly rusted you can't unscrew them to disassemble the valve. If there is room you may be able to cut the bolts using a hacksaw or a die grinder with a metal cutoff blade. Since you will be replacing the old valve you can cut right through the valve itself. Cut the the center of the bolt through the middle of the flange of the valve itself and be careful not to damage the flanges on either side of the old valve. The flange on the valve itself should provide enough buffer to prevent you from damaging the flanges on the tank and pipe. If you damage those other flanges you'll have a lot more to repair!
Some small leaks might be temporarily repaired using a wet patch roofing tar. This is not a suitable permanent repair. The underlying cause must be diagnosed and repaired, but if there is a small drip around the junction of the valve body and the flanges it mounts to, sealing it with tar might let you finish a trip and then make appropriate permanent repairs when you get home. Using wet patch sealant avoids having to wait until the tanks are drained and dried. Wet patch roofing tar is intended to be used in rainy conditions and may not be resistant to the chemicals and other contaminants in sewage. While it may stick to wet surfaces, it may not stick to greasy, soapy surfaces or those contaminated with human waste and holding tank chemicals. Clean the surface as well as you can before attempting to apply wet patch. I like Henry's wet patch cement, available in 10 oz tubes to fit a caulking gun at most home centers.
Maintaining dump valves mostly consists of keeping them lubricated so they operate smoothly without any tearing or excessive wear. Lubricate the shaft of the valve with a silicone spray. DO NOT use WD40 as it will dissolve the grease that helps seal around the shaft and will make the valve harder to open and close. The plastic "paddle" that actually opens and closes usually doesn't require any direct lubrication but some holding tank treatments contain valve lubricants or you can buy special valve lubricant to put into your holding tanks. Valve lubricant is usually dumped down the toilet when the tank is empty so it goes directly to the valve. You will have to put lubricant down a sink or shower drain to lubricate gray water valves. Choose the drain closest to the gray water tank and put it in when the tank is empty. You may want to open and close the valves a time or two to get the lubricant into the seals before adding waste to the tanks.
Dump valves may have metal or plastic handles. These handles sometimes get bent or broken, especially the plastic ones. If the valve is otherwise in good shape, the handles are easily replaceable. Open the valve, then grip the shaft with cloth protected pliers to avoid damaging the shaft, while twisting the handle to remove it. Then screw on the new handle and tighten it and close the valve.
Some dump valves are located away from the outside edge of the RV and are operated via extension cables. If you have valves that are difficult to reach you may be able to replace them with cable operated valves for added convenience. When replacing existing valves with cable operated valves, use new Bladex/Valterra valves. They are specially designed to operate easily with cables. Follow the installation instructions carefully to ensure proper operation. Some ultra-luxury units even have electrically operated dump valves. To me that is overkill and unless you have physical problems that prevent you bending over to reach the dump valves or to pull the handles, I don't think it is worth the expense and it introduces extra electro-mechanical parts that can be additional points of failure. If you have cable or electrically operated valves, make sure to push the handles all the way in when you are done flushing your tanks. For some reason it seems easier to forget to close them than it is to close the directly operated valves and the results can be very nasty the next time you take the cap off the dump port to connect your sewer hose! It may be harder to tell if cable operated valves are fully closed because there is some flexibility in the cable, altering the "feel" you get when closing valves with direct handles. Always close all valves with a firm, smooth, quick motion to ensure the valve is fully closed. If you encounter resistance there may be debris interfering with the operation. Open the valve and inspect the seals and remove any debris before trying again.