Starting a thetford 42070 water valve replacement might seem like a total nightmare if you've never messed with RV plumbing before, but it's actually a pretty straightforward DIY job that can save you a ton of money at the repair shop. If you've noticed a slow leak behind your toilet or, worse, a puddle forming on the bathroom floor every time you flush, the water valve is almost certainly the culprit. It's a common issue, especially after a long winter or if the lines weren't perfectly winterized, but honestly, it's just one of those things that wears out over time.
Why These Valves Tend to Give Up
Before we get our hands dirty, let's talk about why you're even doing this. The Thetford 42070 valve is a plastic component that lives a hard life. It's responsible for holding back the pressurized water in your RV's plumbing system until you step on that pedal. Most of the time, these valves fail because of freezing. If even a tiny bit of water stays inside the valve during a freeze, the plastic expands and cracks. You might not even see the crack until you de-winterize in the spring and suddenly realize your "home on wheels" has a leak.
Other times, it's just mineral buildup. Hard water can be brutal on these small plastic parts, causing the internal seals to get brittle or stuck. Whatever the reason, once it starts leaking, there's no "patching" it. You just have to swap it out for a fresh one.
Getting Your Tools Together
You don't need a massive rolling toolbox for this. In fact, you probably have everything you need in a kitchen drawer or a small bag under the seat. Here's the short list of what you should grab before you start:
- The new Thetford 42070 water valve kit (obviously).
- A flathead and a Phillips head screwdriver.
- A socket wrench (usually a 1/2 inch, but check your mounting bolts).
- A bucket or a few old towels. Trust me, there will be water.
- Maybe some gloves, because, well, it's a toilet.
It's also a good idea to have a new closet flange seal (the big foam or rubber ring at the base) on hand. Since you're likely going to have to pull the toilet off its mount anyway, you might as well replace that seal while you're in there to prevent future floor leaks.
Step 1: Shut Off the Water and Drain the Lines
The biggest mistake people make is forgetting that the RV water system is pressurized. If you start unscrewing things while the pump is on or the city water is connected, you're going to get a face full of water.
Turn off your water pump and disconnect the city water hose. Then, go to the toilet and hold the flush pedal down until the water stops flowing. This relieves the pressure in the lines. Even after doing this, there will still be some "glug" left in the pipes, so throw a towel down behind the toilet to catch the drips.
Step 2: Removing the Toilet
Now, some people try to do a thetford 42070 water valve replacement while the toilet is still bolted to the floor. Unless you are a yoga master with tiny hands, I don't recommend this. It's way easier to just take the whole thing out.
Most Thetford Aqua-Magic V toilets (which use this valve) are held down by two bolts at the base. Pop the plastic covers off, unscrew the nuts, and gently lift the toilet up. Before you walk away with it, you'll need to disconnect the water supply line from the back of the valve. Usually, this is just a hand-tightened nut, but you might need pliers if it's been on there for years. Once it's free, take the toilet out to a picnic table or just a clear spot on the floor where you have room to move.
Step 3: Out with the Old Valve
Flip the toilet over or lay it on its side so you can see the back. You'll see the water valve assembly right where the pedal mechanism connects. It's usually held in place by a couple of screws.
Take your screwdriver and remove those mounting screws. You'll also notice a small plastic "arm" or link that connects the flush pedal to the valve. Be careful with this part—you don't want to snap the plastic. Gently wiggle the old valve out of its housing. It might resist a little if the rubber seals have gotten stuck, but it should pop out with a bit of persuasion.
Step 4: Installing the New 42070 Valve
This is the part where you want to pay attention. Take your new valve and make sure the seals are seated correctly. A little bit of silicone grease (often included in the kit) on the O-rings can help everything slide into place and create a better seal.
Line up the valve with the flush pedal mechanism. There's a specific way the "drive arm" of the valve fits into the pedal. If you don't get this right, the toilet won't flush, or the water won't turn on when you step on the pedal. Once it's lined up, press the valve firmly into the body of the toilet and put those screws back in. Don't go crazy tightening them—it's just plastic, and you don't want to crack your brand-new part.
Step 5: Putting the Toilet Back
Before you set the toilet back down, take a look at the floor flange. If the old seal looks flattened or gross, scrape it off and put the new one down.
Lower the toilet onto the bolts, making sure the supply line is reachable. Reconnect that water line to the back of the new valve before you bolt the toilet down tight—it's much easier to get a good grip on it while the toilet can still wiggle a bit. Tighten the floor nuts evenly. You want it snug so it doesn't rock, but again, don't overdo it or you'll crack the porcelain or plastic base.
Step 6: The Moment of Truth
Now for the nerve-wracking part. Turn your water pump back on or hook up the city water. Don't flush yet! Just let the system pressurize and look behind the toilet with a flashlight. Is it dry? Great.
Now, give it a flush. The valve should open smoothly, water should swirl around the bowl, and most importantly, it should stop completely when you let go of the pedal. Check for any tiny drips around the connections. If everything looks bone dry, you've successfully completed your thetford 42070 water valve replacement.
A Few Pro Tips for the Road
If you're still having issues after the replacement, like the bowl not holding water, that's actually a different part (the blade seal), though it's often confused with a valve issue. However, if the water just keeps running into the bowl, you might need to adjust the way the valve is seated or check if the pedal is sticking.
Also, a little maintenance goes a long way. If you're storing the RV for the winter, make sure you blow out the lines with air or run plenty of RV antifreeze through the toilet. The 42070 valve has a tiny internal cavity that loves to hold onto water, and that's why they break so often.
Honestly, once you've done this once, you'll realize it's a 20-minute job the next time around. It's just one of those "RV owner rites of passage." You saved yourself a couple hundred bucks in labor costs, and you don't have to worry about a soggy floor anymore. Grab a drink, sit in your lawn chair, and enjoy the fact that your plumbing is actually working the way it's supposed to. Wrapping things up, it's a simple fix that keeps your adventures going without the unwanted indoor pool.