Toilet keeps filling?

I believe that my toilet has an Armitage shanks float valve system. About a month ago the toilet developed a problem whereupon after flushing the toilet would keep filling. I changed the washer for an Armitage shanks float valve diaphragm washer as this seemed to be the closer in similarity to our existing washer although the center was red on the new one where as the plastic center was white on our old one. The flush seemed to work okay for 1 to 2 weeks but it started over filling again. I bought another washer (armitage) again worked for 1 week. Today toilet is over filling again valve won't shut off. Anybody got any Ideas, I really do not wan't to change the washer every 1 to 2 weeks. Any help would be greatly appreciated