That's a real problem because it sounds like it's either the valve, a blockage in the line or it could be that the vent devise under the sink is stuck closed and needs to be opened up. The vent valve is a plastic cannister that's on the top of a pipe under the sink area
Dick, Joyce, Diego, Picatso and Gustav
2017 3720 RL, MorRyde pin box
2017 3500 Dodge Maxi DRW 4X4, Pullrite Hitch, IS, Disk Brakes, 3rd AC, Winegard Traveler, Bathroom door mod, Dometic 320, couch for desk swap, etc.