Holding Tank Repairs

It is a sinking and sickening feeling when you look under your RV and see a holding tank leaking.  A fresh water tank leak is annoying.  A gray water tank leak is unpleasant.  A black water tank leak is disgusting, unsanitary, unhealthy and illegal.  Depending on the location and extent of the damage, the tank may be repaired or may have to be replaced.  Your local RV service center is likely to recommend replacement regardless of extent or location of the damage.   That is because they make more money on installing a new tank and it is less likely that the leak will come back.  Also, black water tanks can be very difficult to repair.  If you have properly maintained your tanks, gray water tanks should be fairly clean and not TOO unpleasant to work on.   Black water tanks, especially those that may be at least partially full or poorly maintained, could be VERY unpleasant to work on. In any case, you will usually need to completely drain the affected tank and let it dry before you can begin repairs.   Some repair kits claim you don't have the drain the tank, but for your own comfort and reasonable sanitation, draining and flushing the tanks before you crawl around working on them is a very good idea.   Be sure to inspect the leaks and mark the damage before you drain the tanks. You will need to know where to make the repairs after they've stopped leaking.  You also need to know what caused the damage so you can mitigate future problems.  Impact damage is usually pretty obvious and mostly requires operator care to avoid a re-occurrence.  Freeze damage or stress cracking can be more difficult to diagnose but should still be considered and appropriate remedies applied to avoid additional failures.  I had a gray water tank that developed a couple of pin-hole leaks without any evidence of impact damage.   I have to assume they are the result of stress cracking or deterioration of the ABS plastic the tanks are made of.  The unit is more than 25 years old and I suspect the tanks have become brittle.   Extended storage of the unit with the tanks empty may have allowed oxidation of the plastic from inside the tanks (the outside is coated with 2" of foam insulation, which could have masked some impact damage).  Tweaking of the coach body can loosen connections and even crack the tanks.

Replace or repair?  This decision must be based on the specific circumstances for each situation. Replacing tanks on many older units is not very feasible since matching tanks may be difficult or impossible to find.  You might be able to adapt a standard tank if you can find one about the right size and with the inlets and outlets in about the right place.  Sometimes you can buy a "blank" tank and cut the inlet and outlet openings where you need them.  Be sure you can get proper fittings to seal the new openings.  You will need fittings for input, output, and vent openings.   Pipes are usually attached to custom-cut openings using rubber fittings. You may be able to glue flanges to ABS tanks. In general, repairing will be quicker and less expensive than replacing holding tanks but the repairs may not be as permanent as a replacement.   If the damage is visible and easily accessible and not too extensive, repair may be a viable option.  Extensive damage, such as ripping a big Titanic-style gash in holding tank or tearing it completely out of the vehicle or dump-valve damage that shatters where it attaches to the tank would be good indicators that the tank should be replaced.   Small holes, stress cracks, and leaking fittings can usually be repaired if they are accessible.   The leak is going to drip from low spots, but the actual hole may be some distance away.  Carefully inspect the flow and trace it back to its origin.  Block it with a tool, rag, or your (gloved) hand to determine if it is leaking where it drips or if the leak is actually higher up. Its a very good idea to wear rubber or latex gloves when working on holding tanks, especially the black water tank.

Replacing a holding tank can be a complicated, time-consuming and expensive process.  Your rig may be out of service for several days or even weeks while the work is done and required replacement parts are obtained.  If you want to try doing it yourself, inspect the existing installation carefully to determine if you do in fact have the tools and experience to tackle the job.   Tanks that are located in enclosed compartments or protected by underbelly plates will be a lot more work than exposed tanks.  If you have ANY concerns about being able to handle the job, take it to a qualified RV tech.  In the long run you'll save money and frustration.   If you decide to make the repairs yourself, try to determine the cause of the failure before you remove the old tank.  Damage caused by freezing or by road debris or some kind of impact should be pretty obvious and you need only avoid the circumstances which caused the problem to prevent further damage.  Stress cracking is another common problem and may require some creative re-engineering during the new installation.  Waste is largely composed of water, which is heavy: 8 pounds/gallon.   The contents of a 60 gallon holding tank is nearly 500 pounds!  Add in the summer heat from the pavement and the vibration and bouncing when traveling and you have a LOT of stress impacting the plastic tank.   Eventually it may weaken and crack. If your tank has failed due to stress cracking you will want to explore alternatives for reinforcing the support system to relieve the stress to prevent future failures.   I know of an RV tech who put a couple of extra 2x4 supports under his holding tank after it failed.  If you have to replace a holding tank, seek a standard sized tank that matches your original as closely as possible. Pay attention to the location of openings and mounting requirements as well as tank size.   I once saw a holding tank fall completely out of a travel trailer.   It was originally only supported on both ends by the lips on the lower edge of the frame, which worke great for an empty tank.  Road heat and the weight of nearly full tanks caused the tanks to sag until they slipped out of their tracks.  Some creative cross-bracing (why didn't the manufacturer do that in the first place?) solved this problem so the new tank wouldn't suffer the same fate.

Repairing a holding tank can save you both time and money.  Tanks may be made of various types of plastic.  Most RV holding tanks are made of Acrylonitrile Butadiene Styrene or ABS plastic. This is the black plastic that plumbing pipe and fittings are made of.   Translucent white tanks are Low Density Polyethylene or LDPE.  This is the milky white plastic that most fresh water tanks are made of as well as the milky gray plastic that some holding tanks are made of.  Be sure you get the right solvents and repair materials for the type of plastic your tank is made of.  ABS reacts to solvents and can often be chemically "welded" to make permanent repairs.  LDPE tanks are injection molded or thermal welded and are impervious to almost all solvents.   Minor damage to an ABS tanks can often be repaired using ordinary ABS cement (available at any hardware store or home center). Plasti-Mend Black Repair Kits can be used to repair more serious damage to ABS plastic tanks.   The kit consists of cleaner, PM-Black solvent/repair compound, and fiberglass mesh patches.  Follow the instructions in the kit carefully.  Mostly it is common sense.  Wear an appropriate respirator to prevent respiratory damage from inhaling toxic fumes and rubber gloves to protect your hands.  Clean the area to repaired carefully.   Let the repair cure according to manufacturers instructions before using the tank.  There are also kits made to use on LDPE tanks, but since LDPE is resistant to most solvents and adhesives, making permanent repairs can be difficult.  ABS tanks can often be repaired and back in service in a few hours.  If you can't get the Plasti-Mend Black Repair Kit, you might be able to fix small failures with ordinary ABS plastic cement.   I've also heard of creating your own repair compound by mixing ABS shavings with acetone until it forms a paste.  You can create ABS shavings by scraping or drilling common black sewer pipe, which is readily available at any home center.   You should drill a small hole at the end of any long cracks before repairing them to avoid them continuing to open up.  For large holes (bigger than 3-4 inches), a rigid patch made of ABS is recommended.  For smaller holes and along cracks, use fiberglass mesh to reinforce the repair and hold the repair resin in place until it cures.   By the way, Plasti-Mend Black repairs are reported to be stronger than the original ABS tanks.   Some repair kits claim to work on all types of plastic holding tanks and can be used without emptying the tank.  Both are obviously nice features. Personally, I would drain the tank anyway, especially the black water tank.  I don't want raw sewage dripping on me while I'm making repairs!   Even gray water can be unpleasant and annoying. Sometimes you can repair small areas of black ABS tanks using only ABS cement.   Thoroughly clean and dry the area around the leak and apply a generous coat of ABS cement over the crack or small hole -- until it is completely covered and let it soak into and fill the crack.   Let it dry per manufacturer's instructions, usually about 15-20 minutes and apply a second coat.   For added confidence, wait 15-20 minutes and apply a third coat.  The ABS cement reacts with the plastic to chemically "weld" the damaged parts back together.  Obviously this won't work with big holes. Small holes (up to 3-4" can often be repaired using fiberglass mesh.  For holes larger than 3-4" you'll need some ABS plastic patches to glue over the hole.   You might be able to cut suitable pieces from ordinary black sewer pipe. If you need a flat piece, try heating the piece you cut from the pipe with a heat gun and putting a weight on it to flatten it out.

Be prepared to make multiple repairs.   The random nature of many types of damage to holding tanks may make it difficult to identify all the problems at one time.  You may have to patch up what you can find and then continue to inspect, evaluate, and repair other leaks as you find them.   It is not uncommon for a novice to have to repair the same area more than once.  Failure to completely cover all the damage or to sufficiently seal the edges of the patch will require secondary repairs.  After your repairs have cured, put enough clean water in the holding tank to inundate the originally damaged area to confirm your repairs are sound.  Buy a kit that is designed for more than one repair or purchase a larger container than you think you need so you have enough for redoing any spots that still leak.

General repair guidelines.   Before attempting any kind of repairs, examine the damage carefully and try to determine and mitigate the cause.  Make sure it is the tank leaking and not a joint or a nearby water line.  Leaking joints can often be tightened or damaged ABS plumbing can be sealed with ABS cement or Plasi-Mend Black.  If the tank itself is leaking, carefully note or mark the location and extent of any tank damage.   Drain the tank and let it dry thoroughly before beginning any repairs.  This may take several days before it stops dripping.  Place a bucket or other container under the dump valves and leave them open (after dumping the tanks) so residual fluids can drain out and the tanks can dry more quickly.   Wet surfaces from leaking contents will nullify the affects of most solvents or adhesives you may try to apply.   If the tank cracked due to stress, install additional supports to relieve the stress before making repairs.  Some RV holding tanks are not well supported during their original installation.  I've seen tanks that deformed enough from reflected heat from summer roads to sag enough to drop out of their supports entirely!   Once you have identified the leak, clean the area thoroughly for at least an inch or two around the leak.  Make sure you have removed dirt, grime, grease, insulation, and paint all the way down to the fundamental plastic of the tank.  You may need to use a Scotchbrite pad, sandpaper, or steel wool to get down to clean plastic.   If you use an electric drill or grinder to prepare the surface take care not to go too deep.  Do not attempt to apply chemical solvents to LDPE tanks. There are some repair kits that claim they will "stick" to any tank, but LDPE is not reactive to most solvents and most adhesives will function only as a temporary repair at best.  You might be able to make repairs to LDPE tanks using plastic welding techniques, if you, or your RV tech has the right equipment and training.   I've seen some temporary repairs attempted using silicone or even latex caulk.  That might slow a leak enough to get you home from an outing, but repair will not permanently bond to LDPE and the vibration and pressure will soon displace the sealant and you're back where you started -- or worse!  Now you have to clean off the caulk before you can attempt permanent repairs. Chemical ABS repairs can be permanent, often becoming stronger than the original tank wall.  Read and follow the repair kit manufacturer's instructions very carefully.  Failure to do so will, at best, result in a failed repair and could cause further damage to the tank or create a health hazard for you. Let the repair "cure" for at least the minimum time proscribed in the instructions before using the tank again.  Once you have repaired the plastic, be sure to replace any insulation you may have removed to make repairs.   Spray foam "gap filler" is an easy way to replace foam insulation you have removed and can often be used to fill gaps between the holding tank and nearby structural members to minimize vibration and provide some protection against extreme temperatures.  If your original holding tank was uninsulated, you might want to spray the whole thing with foam.  Not only does it provide a thermal barrier, it can absorb the impact of small debris kicked up by your tires that might crack or penetrate the unprotected plastic tank.

Temporary repairs of some small leaks might be accomplished using wet patch roofing cement.   Wet patch formulas are designed to stick even when the surface is wet so they can be used without draining and drying the tanks.  However, such repairs are not suitable as a permanent solution.  Vibration, expansion and contraction from heating and cooling, and pressure build up are likely to cause the patch to eventually fail.  Contaminates in sewage may also affect adhesion.  Wet patch formulas are designed to work on roofs even in the rain, but are not intended to deal with soap, human waste, and holding tank chemicals so they may not stick to leaking plumbing as well as they do to roofs.

Leaking dump valves can usually be repaired or replaced.  It is not a very expensive nor terribly difficult task, usually well within the skills of a good do-it-yourself mechanic.  Each valve is held in place by only 4 bolts.  However, they often become rusted or may be in a location that is difficult to get tools on so the task may be more daunting than expected.  I've seen situations where the old bolts had to be cut off to remove the leaking valves.  Since the valves were going to be be replaced anyway, an angle grinder with a metal cutting blade was used to cut through the center of the bolts in the middle of the valve itself, taking care not to damage the flanges on either side.  It is possible to replace leaking seals without buying new valves if the valves themselves are still in tact.  The process for replacing seals or entire valves is essentially the same except that you put the old valves back in place with the new seals.  New valves come with new bolts and nuts.  When installing the bolts and nuts, hold the nut with a wrench while turning the bolt to tighten.  The back side of the nut is fluted to grip the plastic as it is pulled by the bolt as it is tightened.  Turning the nut as it gets tight will scrape away at the surface of the flanges so it doesn't grip as well and may vibrate loose.  If the flanges are damaged they should be replaced.  That is usually fairly easy where the flange attaches to the tank with a large hose clamp, but some flanges are an integral part of the tank or the dump fittings and, in this case, it may require replacing the entire fitting or modifying the tank since the flanges cannot be removed.

No more leaks!