Identify piece please

[Rophle]

Can someone tell me what this is under the bathroom sink and why it's leaking when I try to clean out the tank?

[woodwalker]

It is a back flow valves so you don't get sewer water back into the water system. Undo both sides remove and add Teflon tape to the threads and rretighten. Hopefully that works

[Rophle]

Thanks for identifying it for me. It's leaking out of the top of it, though, not the connections. What would cause it to do that?

[2slick]

Those things are prone to fail. I actually replaced mine with a Watts brass unit that can be rebuilt, and added an inline check valve. It is a vacuum breaker/checkvalve. Basically, it is there so when you flush your tank, and shut off the water source, it does not let black water siphon back through the flush line. I bought mine off Amazon, and they have them basically just like yours.

[harley]

Also, too much water pressure will make it leak. try the teflon tape trick and use less pressure.see how that works.

[Rophle]

All of that makes sense.
I probably had the pressure too high to begin with, but after it started leaking and I was checking to see where it was coming from I had the pressure low and it still leaked water.
I'm assuming the high pressure to begin with damaged the unit, and then even with low pressure the valve failed. Does this seem plausible?