Water – Insinkerator Instant Hot SSRV-1 Spitting Air — Need Ball Valve

My Insinkerator Instant Hot undersink water dispenser is about 8 years old and it has begun spitting air when running water. It is not a steam/overheating problem, because it only spits air when running water out of it, and it still spits air if the heat is turned off and it is running cold water. Thanks to another post on this site, I dis-assembled the unit and narrowed my problem to one thing: a bad one-way ball valve. The ball valve is at the bottom of the plastic water return/expansion tank at the top of the unit. It appears that the ball valve seats in a black rubber seat, which has disintegrated with age. Now the ball valve no longer seats, causing the unit to suck in air when it is running and spit the air out the faucet. Does anyone know where I can get a new expansion tank? My unit is Insinkerator Instant Hot Water Dispenser Model SSRV-1.

Here is the trouble-shooting explanation of how this unit works, and why I need the part: The tank is not pressurized with water, except when you press the dispenser. The water pressure goes straight to the dispenser, and when you press the handle it runs down into the tank, coming in at the green-circled pipe. The water eventually fills up the tank and pushes hot water out the middle tube up to the hot water faucet. When water rushes into the tank at pressure, it runs past a venturi (green circle) which creates a vacuum and sucks water in from the grey tubing that connects to the bottom of the plastic overflow/expansion tank. So long as there is water in the expansion tank, it is being sucked into the metal tank together with the cold water. But when there is no longer any water in the tank, a little floating ball drops to the bottom of the plastic tank, and stops air from flowing into the grey tube. So, the ball valve acts like a one-way valve when there is air in the line, but is stays open when there is water. That is because, if there is air line the line, it will be sucked in at the venturi (green circle), creating bubbles in the top of your tank, which will spit out when you are dispensing hot water.

You can confirm that this is your problem, by disconnecting the clear tubing from the red-cicled inlet pipe, and hold your hand over the inlet while someone else runs the dispenser. If the spitting stops, you know that air coming in through this plastic tank is the source of your problem.

So, while I am waiting for this part, can I just crimp off the clear tubing that is coming into the unit attached at the red-circled inlet? Based on other posts about this unit, I think that two things may happen if I do this: 1) I may get a momentary slug of cold water when I first press the dispenser, because this is the tube that water runs down after the dispenser is shut off; 2) If the unit boils a bit at the very end of a heating cycle, or expands the water, it will force hot water out the dispenser rather than running into this overflow tank.

My next question is, if this overflow tank is not offered anywhere for sale, can I buy a one-way air valve that is open in water? In other words, a ball valve that floats, like the one that is sealed inside this tank? So I can install it inline with the bit of grey tube that is showing? Any other ideas? Thanks.