I have a 20 year-old GI unit that worked well until last month. During the regen cycle, when it gets to the brine draw step there is no vacuum on the brine draw tube. In fact, it actually leaks a little water out of the tube when I disconnect it from the float valve in the salt tank. However, if I move the bypass valve into Bypass position and then slowly return it to Service position, the vacuum is established and it draws fine. I have disassembled the brine valve and cleaned all the nozzles, jets, and screen (several times!). What gives?