the two vales look the same but there must be a difrence otherwise i know high pressure flow is from a pump and low is gravity fed so whey they sold seperatly as high or low when they identical .only difrance when fitting to cold storage tank in loft is use a washer each side[not provided] of tank then use plastic nuts [provided] after fitting a new float valve with arm it still keeps trickling out causing it to overflow exits over my nieghbours and my front doors so i tried a high pressure valve and now going to try a low pressur valve but they look identical

mr c o connor, November 2010

The only difference is the plastic seat inside the ball valve (the hole bore going through it are either larger or smaller) As you point out, use the high pressure for mains water and low pressure for gravity. I think what your are neglecting to do is to adjust the ball valve (arm) for the water level to finish below the overflow outlet. If it is brass then physically bend it down with your knee half an inch or whatever it needs. If it is plastic then use the adjustment screw near the body of the valve.
Good Luck.

Pete the Plumber., December 2010