This is probably more of a general plumbing issue but, I have a very slow sink draining. The clog is not in the trap, but somewhere between the trap and the grey water holding tank-closer to the tank. I have used a snake, plunger, and various solutions but still can't manage to clear the pipe. The shower drains fine and the holding tank is not clogged.
I'm perplexed!
Thanks.

Do you only have one waste tank? Is the sink draining into the same tank as the show(er)? Does it drain fine for the first 20 seconds? Does the snake make it to the tanks?

Have you actually hit a blockage with the snake? If not, you might have a clogged roof vent. If air can not escape, as fluid or waste is going into the tank, it will act like a blocked drain and drain out slowly.

I've seen some cases where on a longer drain run the vent is "Tee'd" into the drainage line.

Remove the vent caps off of the roof and use the snake in there. The vent could have gotten blocked when the tank was full of waste.

Keep in mind, because of age and UV degradation, the vent caps might break when you are removing them.

Which sink? Is the rig level? If Kitchen sink and not level, you may be asking water to run uphill.

The lines from the kitchen and bathroom sinks join together and then have a short flat run to the tank in my bus. I have had them plug up about once a year or so. (we fulltime so they get a lot of use.) I have used Liquid Plumber, Draino, etc. and 2 different snakes with no luck. It turns out it is never the traps. Just a foot or so before the pipe enters the tank is has a flex pipe used to make a slight bend to solve a clearance issue. It also has a slight dip in the line right there. These combine so that soap residue builds up over a period of time and forms a waxy substance that blocks the line. I have to take the pipes apart there and clean them out. I just did this again last week. This time after cleaning i decided to experiment and sprayed the insides with silicon to see if it slows or stops the buildup. And no,....the way it is set up i can not change the plumbing at this time. If/when i ever need to replace the tank then i can do something a little different that may cure that problem.

quote:

Just a foot or so before the pipe enters the tank is has a flex pipe used to make a slight bend to solve a clearance issue.

Flex pipes are nasty. The ridges will catch everything. Perhaps there is room to put a secondary catch trap in line to settle the solids and waxy stuff prior to hitting the flex pipe. These used to be used a lot in galvanized sewer pipes for the same reason I presume.

Nope, no room to put in another trap anywhere. If i ever have to replace the tank i can do it a little differently and do away with the need for the flex pipe.

You can also use a rigid plastic pipe. With a heat gun or good hair dryer insert the pipe at the shortest point of connection between the curve. Then heat the pipe and form it or bend to mate to the other pipe. Allow to cool, remove and then use appropriate cement to complete. The pipe can be bent and squashed to get around any conflict points.

Shadow man,

Have you tried pouring lots of boiling water down the sink?

You can always put some baking soda followed by vinegar down your drain. Works sometimes instead of Drano. Good entertainment anyway.

Many thanks for the suggestions everyone.
There are actually two tanks: a grey water and blackwater. The shower and sink drain into the grey and the bathroom sink and the toilet drain into the black. Bill, I did try to snake out the roof vent but there was no obstruction that I could tell. The snake is about 15 feet and it went down all the way.
Noble97monarch the coach is level thanks.

Shadow Man when you took your pipes part, how did you get to them?
Did you drop the grey water tank to access them?
Perhaps if I were to drop the grey water and run the snake in from that end?
Sounds like this flex pipe could be the issue.
I have run the snake from the vent under the sink and the snake seems to go all the way and stop against a pipe or something firm.
I'll also try the baking soda, vinegar, and more hot water than I have used.

Jack, i have an SOB, a MCI bus. I have only one tank for black/gray and i can access my pipes easily because they are in one of the bays. Sorry but i know this is of no help to you. Hot water doesn't do anything to this no matter how much you use.....by the time i know i have a problem it is too late, the pipe is plugged and will not drain.

While I agree with Shadow man that hot water won't help with a kitchen sink that is totally clogged, boiling water will help with a kitchen sink clog that is only partially blocked and has some flow. This is because the clog is made up of fats that have congealed as they cooled on the way to the tank.

If you poured a teakettle worth of "boiling " water down your sink occasionally, you may stave off another clog. This solution would not work with a bathroom sink clog as they are almost always due to hair, which won't dissolve in hot water.

20 years of owning apartment houses, I've seen and fixed every kind of clog you can imagine!

It has been a long time, but I used Pure Power black tank enzyme to clear a clogged toilet chute on a trailer that did not have a straight-down chute. It took a day or two.

Thanks again for getting back Shadow man, Nobel97, and Bill H. I will try the hot water, as well as some of the other remedies here before I drop the grey water tank.

quote:

Originally posted by brandman:
I will try the hot water, as well as some of the other remedies here before I drop the grey water tank.

Just in case, get some PB Blaster and shoot the mounting hardware. Before you start the tank drop, be sure the hardware is free to turn so you know what you are getting in to. I squirt my grey, black and gas tank mount hardware once or twice a year, just in case.