Seacock valve for holding tank

[RMS]

I had the same thing happen to my Bosworth brand Y-valve.  Unfortunately the whole valve needed to be replaced as a result.  Rinse a lot of fresh water through the system including down the tank discharge before you dismantle the current valve.  In the future, consider using a small amount of vegetable oil on occasion to lubricate the Y-valve.  I dribbled the oil down the tank discharge.