Thanks ever so much for your help guys, I now have a beautifully proportional braking system. The O rings were sticking then jumping out, I think the units have been idle for 2 to 4 years since manufacture and there was a tiny thin layer of rubber sticking on the side of the unit which was causing them to stick. They now move out and in freely. Also, using a pressure gauge to see what is really happening when setting up the radio to move the valve made that part so easy.

cheers

Harry