This shows why you need to get the macerator  - right side up-


When a macerator sticks on some hard object it will usually blow a fuse.  It is possible to back-flush the Macerator.  There is no valve in them so if you connect a water hose to the output of the Macerator and turn on the pressure, you should be able to reverse the impeller and blow the offending item back to the tank or the input pipe.  When I did it, I opened my second gate valve which goes to the original 3" drain pipe and I hope I flushed the unmentionable part into the dump and not back into the tank.... Any way it worked and saved a teardown....


  Dump Valve Repair

That Black tank valve you describe is not a Valterra. It is a Thetford and that is what Gateway is sending you. There is a kit available from Thetford but is twice the price of a Valterra valve. The kit is Thetford #03827 and includes the little brass rivets, the stailess steel side plate and seals. The Thetford and Valterra valves are not interchangable without modification. Emory Stora made the modifications and did a pictorial of his procedure. (SEE BELOW)
Denny Allen -- The Macerator Guy

Here is another way to repair/replace the OEM Dump Valve

> Presently I have the valve off the tank.  One bolt was, lower RH,  stripped and I would like some advice on how to repair these  threads in the plastic valve mounting fitting. Steve
For the bottom holes you can drill through with a 1/4" drill bit and then use some stainless steel 1/4" bolts with washers and a nut to hold the valve to the flange.  This is much more secure than the original screws.   If the top screws should be stripped you can tap the holes with a 5/16" tap and use some larger stainless steel bolts there. Emery

Steve, this is how I fixed it.  Drilled the holes all the way through and tapped them M 10. Than stainles steel bolts with a slotted small end from the inside out to clamp the valve. Works very well. Marc

Electric Holding Tank Dump Valve
by Emery Stora

Pictures are at

An electric valve is made by Phase Four Industries.  Its retail price is $99.95.  Camping World has it for that price but less 10% if you are a member of the President's Club.  The valve has the bolt circle for a Valterra valve and doesn't match the flange on the GMC holding tank.

There is not enough room for the valve between the holding tank and the cross member so I made an adapter using a Thetford female flange and a Valterra female flange with a piece of 3" ABS pipe.  It was necessary to trim 3/16" from the four standoffs on the Thetford flange.  Drill 1/4" holes through the standoffs and bolt the bottom two bolts to the holding tank flange using a Thetford gasket between the two flanges.  The top two bolts thread into the holding tank flange.  Since mine were stripped out I threaded them to 5/16" and drilled 5/16" holes in the top flange.  The picture also shows that I used a metal plate forward of the bottom of the holding tank flange to clamp the flange at that point.  The distance from flange to flange of the adapter is 6-1/2".  This brings the electric valve rearward from the crossmember.  In the event that the motor on the valve doesn't work, the valve can be opened with 1-1/2 turns of a screw slot on the rear side.

At the rear of the electric valve a 3" male Valtera flange was used. This fits into a 3" ABS tee with the side opening going to the mascerator.  The original Thetford valve was put on the back side of the tee so that the original dump pipe and 3" hose could be used in the event that the mascerator fails to operate.  It also can be used for tank cleaning and flushing before putting the GMC into storage. The rod and handle for the Thetford valve was moved to the right side frame as the Onan interferred with its use on the left side.  This required drilling a 13/32" hole in the lower face of the frame plus two screw holes for the handle clamp.

A 4" length rubber connector for soil pipe to ABS was used at the rear of the valve so that it could be connected to the outside of the ABS elbow.  This will fit the 3" pipe from the valve but will also stretch over the outside of the elbow.  This was necessary in order to have room for all the parts.  One could possibly make the adapter a little shorter and also trim back the front and rear flanges of the tee about 1/2" to shorten the assembly and then use a 3" pipe connector at the rear of the valve.

The switch for the electric valve was mounted behind the generator door below my mascerator switch.

I use a 25 foot section of 3/4" garden hose connected to the pump which slides into a 20 foot 1-1/4" PVC pipe tied to the bottom of the GMC frame.  I use a hose cap over the threaded end of the garden hose.  To dump just pull out the garden hose, remove the cap and put it into the dump station hose (or into a nearby toilet), push the switch to open the GMC dump valve.  The valve takes 1 second to open. Pull the mascerator pump switch.  It will dump the tank in about 3 minutes.  Then turn off the pump switch, push the valve switch to close the valve, put the cap on the end of the hose and slide it into the storage pipe.  About a 5 minute job now and no sewage drips, no rubber gloves and no hose to rinse out.  I bought a 50 foot length of 3/4" hose from Home Depot and cut it in two, connecting the cut end of the section with the male connector to the pump.  I store the other half with the female end in my propane compartment.  If I am more than about 20 feet from a dump point I can connect the other half of the hose and reach farther.  I have only used the second half of the hose once in the last year but it allowed me to dump where I could not have with a 20' hose.

Emery Stora

Kitchen drain into holding tank Repair

I had posted about a week ago that my kitchen sink line going into the holding tank had broken loose and that I was getting a large amount of sewage fumes into  the interior of the GMC on my last trip.

I decided to drop the holding tank and have now put it back together. Jim Melberg was of great help giving me the size of the bathroom sink pipe "O" ring for the slip joint.  He said a 1-142 which is 3/32" x 2 3/8" x 2 9/16".  The seal company didn't know what the leading 1 was but they had a 142 that was the right size.  My "O" ring was missing and might have never been put in.

If I had known just how the kitchen pipe attachment was fastened in I would not have dropped the tank.  The repair could have been made from the top.

In my 1977 GMC the pipe is behind a plastic cover in the air compressor cabinet just across from the side bath.  After removing the cover, the easiest way to gain access is to take out the compressor and to remove the floor of the closet.  Then, the easiest way to dissemble the pipes is to cut the pipe just below the top elbow.  The fitting to the tank is a sink drain trap type collar. You might need a basin wrench to take it off; however, in my case, the rubber seal had deteriorated and the pipe was pulled out.  I suspect in most cases it can just be pulled up and out.

Mine had a steel collar screwed to the floor.  Apparently once there was some sort of trim or flashing under this to close off the hole in the floor but it had deteriorated and disappeared.  I pried off the collar and discarded it.

I used a 1-1/2" pipe thread to ABS pipe fitting for the tank, a 3-1/4" length of 1-1/2" pipe, a 1-1/2" rubber coupler, a 1-1/2" street elbow, a 4" piece of 1-1/2" pipe and then a 1-1/2" to 2" rubber reducing coupler.  The 2" end of the coupler fits fine over the outside of the top elbow.

It was not necessary to have a cleanout tee since the rubber couplings can easily be disconnected if necessary to clean out the pipe.

Pictures of this assembly
Look under Kitchen Sink/Holding Tank

If anyone is getting a lot of sewage fumes into the interior of the GMC this connection might be the thing to check out.Emery Stora

Ardemco, supplies Tanks
1685 Babcock, Costa Mesa, Ca 92627
(714)631-1814 or 800-253-0115

Web Site
advertises in the GMCMM on a regular basis.

Click for Detail

Replacing the Gate Valve by Gil

Black Tank Repair
Last trip out I discovered a small crack in the holding tank. Removal of the
holding tank was a lot easier than I expected, thanks to Scott Nehoda and
allowed me to fix a leak between the sink and the holding tank by way of To fix the crack in
the tank a friend loaned me his "MINI-WELD" model 3 airless plastic welder.
I called the company (1-800-633-3047). The model 3 is no longer available
and has been replaced with a model 5 at a price of $199.95. They directed me
to the web site at . When I first opened the
box I couldn't see $200 in there but after the repairs were completed she
was worth every penny. 15 minutes prep time and 5 minutes welding time. Took
longer to read the directions than to do the work. The web site has a lot of
info. Like how to identify what type of plastic you are dealing with and
like that. Hope this helps you guys because it sure did me a lot of


We dropped our tank ourselves, in the driveway. First we jacked up the air
system as high as it would go. The way we dropped the tank was to get a
piece of plywood that fit between the brackets that hold the tank up there,
and held this up to the tank with a floor jack. We then loosened the holding
brackets and lowered the tank with the jack. Install was just the reverse.

Click for DetailAs I recall, we had to loosen the plumbing fittings for the kitchen, which
in our 75 palm beach, was in the shallow little cupboard directly across
from the toilet. It was located under a wooden cover that is in at a slant.

The GMC Maintenance manual gives instructions to disconect the nut with a basin wrench.  That fitting is just below the white collar used to seal off the hole.

The fitting is exactly like the fitting used on a sink drain or sink trap.  It was a metal collar which threads onto the plastic fitting that is in the top of the tank.  Since my pipe was cracked I didn't have to loosen it but on my coach there is a plastic cover in the bottom cabinet, left side.  Looking down into this there is a white plastic collar that seals off the area.  This is held down with a few sheet metal screws around the perimeter.  It look like the easiest thing for me to do the get things back together is to take out the small floor in that cabinet to give me more room.

I also am going to eliminate that sink drain type fitting and use one of the rubber fitting with the stainless steel clamps.  That should make it a lot easier to get back together.  I'll probably cut back some of the existing elbow and clean out fittings and use either a rubber elbow or a couple of straight rubber connections with a plastic elbow.Emery Stora

The other drain was under the toilet, which we also removed for an overhaul.

Mine was attached with steel screws. (rusted totally)  I used a carbide tip
in my die grinder and ground the heads off.  By doing this I could lift the
ring enough to unscrew it off the threaded holding tank 3" nipple.  I have a
feeling I was concerned with the heat caused so I think I used a wet cloth to
cool things down as I went on grinding.  I would suggest using ss screws for
the replacement.    After grinding and ring removal there was enough stud
screw left that I used vise grips to remove those.   Scott

The correct "O" ring is Precission Associates number
1-142 which is 3/32" x 2 3/8" x 2 9/16".  Now, where
one can find this hummer is another question.  I used
up all my "sample" options with PAI two years ago and
their minimum order is way beyond my means.Jim

It really wasn't too bad a job, and based on time that it took the pros, we
did a pretty good job. We "welded" a couple of cracks that had been caused
by the mounting brackets, which are too narrow, and therefore apply pressure
to too small an area. When reinstalling, I made these brackets much wider.

I've seen posts that indicate it is better to replace than repair, however
both my water tank and holding tank have been repaired, and have both held
fine. I'm sure the plastic is getting old and brittle, but for now, all is

By the way, we also moved our holding tank discharge to the left side. Too
many dump stations were hard or impossible to reach with the rear discharge,
and many campsites had hookups I couldn't get too, meaning I had too pull
the coach up a few feet to dump. I guess I could have carried a longer hose,
but I found that to be a storage problem.

So there you have the "poop" on how I did it. Bottom line is that  it isn't
too bad, as long as you have a helper, more for the bulk than the weight.
Perhaps I have taken the wrong path, but it has worked for me. Good luck.

Tony Badalamenti


I found that my holding tank would leak when partly full or when I was parked with the rear
end low as on a driveway.

After fooling with it for a year I decided to have the tank replaced.  When it was removed it became obvious what was wrong.  The tank had a crack on the rear passenger side. The crack was about 6" long and was located on the top edge of the tank near one of the plumbing fittings.  The crack extended down the back side of the tank about 2". It also extended along the top of the tank right into a reinforcing rib.

So when the tank was partially full or when the coach was on a slant, some of the contents would leak out.  Also stuff would leak out into the reinforcing rib on the top.  That stuff on top would accumulate in the rib and after a time would get ripe and smell up the inside of the coach.  I fought the smell
problem for a year

I suppose the crack could have been repaired.  A new tank to replace the 24 year old tank sounded much better to me, however.

The tank really just drops down.  The pipes from above are just inserted in rubber sleeves.  The major problem with installing a new tank is locating and cutting out the new holes for the plumbing.
I guess not all coaches have the plumbing installed in the same locations.  The new tank uses RTV rather then the stock rubber connectors to seal the plumbing.

After watching the professionals install my new tank, it is really fairly simple if you have the right tools and have the coach up in the air where you can get at the tank.  I'm still glad though that I had it done rather then do it myself.  I guess I feel I have better things to do with my time then mess with a old and smelly holding tank.Richard