one way valves

[mpk490p]

i have a showerforce 1500xt with a cold water tank, the tank keeps overflowing, i have checked the inlet and float and they are fine so i fitted a one way valve on the cold feed, it seemed fine but then started to overflow again recently, again checked everything and its defenatly coming across from the hot side (i have a hot water tank that sits higher then the cold one) what are the chances the valve has just failed? it is the shower? should i fit another valve?
any help apricated