Grey Tank repair success thanks to Plasti-Mend

[RRman]

Over a year ago we once again overfilled the grey water tank (water wouldn't drain out of the shower)at a without full hookups campsite and water ran out of the belly of the RV. We never travel with the tanks even partially full. Since then as long as we dumped before full or left the valve open, no more leaks until recently when it started leaking even with the drain valve open.
Took down the belly covering and found a 1" crack at the outlet of the tank connection to the pipe fitting. Ordered Plasti-Mend Black for ABS tanks (available from Amazon or directly)and applied per instructions. Its time consuming to apply 4 coats. Passed the full tank leak test, but then I discovered a 1/4" crack in the tank "rib" farther down that bottom edge from the outlet, almost at the corner.
Used more Plasti-mend and the tank passed the second leak test.

Very pleased with Plasti-Mend. But why can't tanks be out of modern materials that could presumably reliably handle the weight of mild over-filling their designed capacity? They fail so often!

[Ozz]

What happens is the drain pipe it rigid, the tank fills then drops a little with the weight, and the drain pipe acts like a lever, straining the tank spud, this cracks the tank.
The cure is to install two flex couplings in the drain line, flexing it when the tank raises and droops a little.
Ozz

[Art-n-Marge]

Ozz is right on. Sometimes you have to fix more than the obvious. You fixed the cracks but may have not fixed what causes the cracks. You should probably install the flex couplings to prevent any more cracking and/or shore up the braces/brackets to prevent any sagging from being filled up.

I've read that some of this bracing seems subpar for the weight of the tanks it's got to support. I've never understood why this is allowed but RV manufacturing standards don't seem to be the best.

[DQDick]

Once again it's do it as cheaply as possible. Putting the couplings in would probably cost $4.00 but when you take that times 15 rigs a day and and 257 days a year you get $15,420. If it cost more than $4 then the price goes up. So the leave it for us to do.

[RRman]

As far as I could see the only support for the tank was the flanges (?) built in on the top of the tank as it rested on some steel "floor" supports . Assuming the fittings are glued together and so close I wonder how hard it would be to retrofit flexible fittings?
I considered trying to support the bottom of the tank, but there was no readily apparent structure in the belly to easily connect to.
Is there a link to an example of retrofitting flexible fittings?

The second leak at the bottom corner opposite from the drain fittings in the molded "rib" presumably would not be affected by a flexible fitting, but reflects the low strength of the ABS?

[Ozz]

Chas, here is my repair, my drain lines may be different than yours, but I have two pictures that show the 2" line, just put the couplings there one close to the 90 that comes through the black cover, and one downstream about 2 or 3'.
I didn't have the couplings in place when I took the pictures.
Is this where yours cracked?

https://picasaweb.google.com/Jimsue13/20120113HoldingTankRepairs?authuser=0&feat=directl ink

[RRman]

Ozz, yes, it was on the rounded outlet part of the tank on the bottom. My repair turned out like yours. We'll see how long it lasts as I've buttoned it back up for now. I would think the Plasti-mend and mesh would provide enough strength for the failed areas to last at least a while...
We'll see.

[Ozz]

Chas,
Best thing to do until you get the couplings installed is to leave the valve open if you can, while in a RV park, but mine cracked again before I got the couplings on while I was in the learning curve. If it cracked once.. it will again.
Ozz