One possibility is that salt crystals may be blocking the valve function or may have formed pits in the valve seals. This can occur if the valves were not completely flushed at the end of an earlier use period.

Begin by flushing the valve with warm water to loosen any crystals present. EtOH may be used for sticky solutions and dilute acetic acid may be used to loosen any mineral buildup within the valve.
 
To flush, place the valve into its open position and inject your cleaning solution from the input to the output using a 10 cc syringe. Repeat 3-4 times. Then rinse the valve using H2O. Finally, use an empty syringe to blow air through the valve to remove any residual water.
 
If the valve continues to leak after cleaning , then you will likely need to replace the valve.