Pump under sink in a wet bar allows water to pass through a first check valve to an upright vertical drain pipe, which has a second check valve. The second check valve closes. However, the first check valve apparently is remaining open, because when the pump is off the water between the two check valves flows up through the sink drain back into the sink. This check valve has a screw type plug on top which is difficult to get to since it's in a very tight cabinet, but could be accessed in an emergency. Any suggestions?

It sounds like the flapper in the check valve is stuck from residue buildup. Opening it up and cleaning it all is required, replacement would probably be more of a headache but might be necessary if the flap itself is damaged.

quote; However, the first check valve apparently is remaining open, because when the pump is off the water between the two check valves flows up through the sink drain back into the sink.

This is a bit confusing, because if the sink drain is between the two check valves, what keeps the water from flowing into the sink when the pump runs?

Chances are both checks are bad atm, but when the pump cycles, it forces the first one closed with the surge of water alone. That would be my assumption anyhow. Regardless, the checks still need cleaned or replaced.

Sorry for the confusion, hj.

The first check valve is downstream of the sink drain, in a HORIZONTAL drain pipe.

The second valve is still further downstream, UP into a VERTICAL drain pipe.

I believe the backflow into the sink is water which is trapped BELOW the second valve, far enough up into the vertical drain pipe, to create the backflow through the open first valve when the pump is off.

Checked drains shouldn't backflow anyhow since air can't move through the pipes. Maybe I am picturing it wrong, but you should need both checks allowing water past them to have this effect.

How about a photo, because it still does not make sense how the sink could drain if it is going into a pipe full of water.

Before I resort to a photograph (not sure how helpful that would be anyway) let me simply state the obvious.

Water from the sink needs to be elevated about 8 feet from the basement to the common household drain system.

The first check valve (which is the problem valve), is located just beyond the pump outlet in a horizontal drain, which then connects to a vertical drain pipe (about 8 inches away) to facilitate lifting the water to the main drain.

The second check valve is in the vertical drain, about 6 inches above the point where the two drain pipes are joined.

When the sink is draining and the pump is operating, BOTH VALVES ARE IN THE OPEN POSITION.

When the pump stops, THE SECOND VALVE (in the vertical drain) CLOSES. Since this valve is at a higher elevation in the system than the first valve, water is trapped BENEATH the second valve.

This trapped water then flows back toward the sink, THROUGH THE FIRST VALVE, WHICH IS STUCK IN THE "OPEN" POSITION.

If it was so obvious, it would be fixed already.

quoe; This trapped water then flows back toward the sink, THROUGH THE FIRST VALVE, WHICH IS STUCK IN THE "OPEN" POSITION.

That would be logical, if it were not for the fact that IF the "second valve" is really closing, and depending on other factors, the water would normally NOT drain back because of the suction on the second valve. In other words, water cannot drain from a pipe, UNLESS, air can get in and displace the water that is draining, which probably means BOTH check valves are leaking. From your description, I assume the water from the sink drains directly into the pump. If so the only check valve needed is the first one and it appears to need replacement.



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If the sink drains into a pump basin, how big is the basin, that the returning water fills the basin and backs up into the sink?

and why wouldn't the returning water cycle the pump again.

From the description, I assume this is an "inline pump" that is turned on when the sink is draining. Therefore it does not have an automatic switch or a sump basin.