I have no idea what to do. I have a front load wash machine. I am using about 9 feet of 2 inch flexible drain tube (like a vacuum hose), which runs behind the wall on a slight decline to the sink drain.

The water DOES leave the tube completely, but sometimes the water backs up at the junction of the drain tube from the wash machine to the flexible drain tube going to the sink drain.

I am stumped, I cleaned the pipes (snake etc) an there has never been a washer attached, so lint isn't a problem.

The flexible drain tube is at about 3 feet 5 inches high and the wash machine is 3 feet tall.

The sink drain is about 2 feet from the ground, so the drain tube is declining about a foot through the 9 feet of drain tube.

Any advice! I wa sthinking of trying a check valve, and using a different type of drain pipe maybe?

Hey Johnny, I can't find much fault with the way your drainage's set up.
Most washers have liquid left in the vertical section of the hose between the hook and the washers drain connection. This liquid drains back into the washers sump when the washer pump shuts off. As a rule this is not noticeable But if you wish to reduce the amount flowing back into the washer and since you have a front loader why not cut the hose back to 1 inch over the top of the machine and reduce the amount of liquid left in the hose when the pump shuts off. You will still get some back flow but not as much. Good luck, Tom