Pressure too

I have the same problem as Brian Mullins, but just recently two things have been changed. 1. The pipes have been replaced with copper one throughout and 2. I had to replace the shutoff value as the previous one had perished. Not being the most handy of handymen, I went to a plumbing supply shop and bought a new value to replace the old screw in and diaphram one. They gave me only a screw in replacement and told me that that was all I required and to throw the diaphram away. Well since then the pressure has decreased dramatically. Everything I have tried (busting arm muscles in the kitchen to increase pressure in the pipe did only one thing – give me an arm workout!). I think more water is going out than is coming in and the pressure is dropping that way. Any advice would be most appreciated.