Unbeknown to me, my husband, bless his heart, poured some grease down the kitchen sink and clogged it--at least we think so. After I took loose the PVC pipes and saw that they were allowing water through, I knew the alleged clog was further into the pipes that I couldn't take apart. We used two plungers (double sink, no disposal), chemicals such as liquid plumber max gel, drain crystals, vinegar and baking soda, salt, boiling water, a zip it, bottle brush, and now an auger. None of those things worked, but we did notice with the auger and bottle cleaner, that when they get to a certain point in the pipes, there is an opening sound as if the drain has cleared--and at first we thought we fixed it but found out after putting the pipes back up that we didn't. We pushed against something that was springy and pushed back like a flap or something. So as I pushed the auger back and forth at this specific spot, we would hear it open and close.

Though my husband thinks that the sharp bend in the pipe is why we hear this noise, my theory is still that the flap does exist and that whatever this flap or valve is, it's completely not allowing water, let alone any chemicals, go through the drain. It's as if a cap has been put on. Today we tried sticking the auger in to open the flap and then pouring a kettle of boiling water in and some went down finally but I couldn't keep the flap open for long and the water stopped going in. What could this be? I can't find on the internet anywhere that says that PVC pipes have flaps that can close the pipe off. Nothing else in the apartment is clogged or slow draining. The kitchen is the only problem.

It's hard to get to some things since it is an apartment, but hopefully at least some knowledge will help when I call the maintenance crew--none of which are actual plumbers though they will still charge me!

I really appreciate the help!!

Though my husband thinks that the sharp bend in the pipe is why we hear this noise, my theory is still that the flap does exist and that whatever this flap or valve is,
Sorry! No flap! This is a typical kitchen sink drainage set up.(see image).
Let me give you directions on snaking a kitchen drain line.
The solution would be to remove the traps "J" bend,(see image) and send a snake up into the wall,(see image). There's a bend ablout 8 inches in but once around that you only have to put out 6 more feet of snake. Flush with a tub of hot water. Good luck and thank you for ratting my answer. Tom