Hello All...Thanks for all the suggestions given to me. Really really appreciate your time...I learnt a great deal about softeners in this forum.
I tried your suggestions, but everything looked fine. So, I called softener repairman. He said that issue is with "Flow Plug (DUDC or DDUC)" in Venturi assembly (Number 37 in the above picture of valve assembly...near bottom left in the picture), and removed the Flow Plug and now the softener is back to normal.
According to him companies started putting this flow plug since last few years. After couple of years of usage, flow plug is clogging and causing water fill up in salt storage tank (unable to suck the water output during regeneration). He says it is not required at all!!! So far no issues after 10 days.
Thanks for your help here, but I think my WHES20 has a different problem. It is using too much salt and the brine tank is almost full of water before it starts regeneration. When the regeneration starts the water comes on and starts filling the brine tank - nice - except the tank is already full. The float is at the top, but the water continues to flow into the tank and out the saftey overflow taking my salt with it - not nice. I think it is the brine valve. I am going to turn the thing off and cycle it manually until I use up the existing salt in the tank and then take it apart and clean that brine valve if I can... I think it cycles almost everyday because my water is hard enough that I was borderline between the WHES20 and the WHES30.