Whipped out my master cylinder this morning to find the bore size. it definitely looks like it was leaking so need replacing anyway but unless I'm measuring the wrong bit they don;t make them that size. i measured 3/8" but they only come in 1" or 1 1/8 " *shrugs* I'll ask the helpful people at impalas.net see what they think. If anyone is interested or needs help with impala's i must say it's a really good place to go. they have helped me a lot.
Hats off to you, people of impalas.net
here's a couple pics of my master cylinder in it's sorry state and the servo