The exploded view of the clutch linkage in GM's literature shows a gasket between the clutch master cylinder and the firewall (pn 15592279). This gasket is no longer available from GM. My local parts stores can't get it either. Has anyone else had this problem? Also, I have heard that I should not use a replacement clutch master cylinder (Raybestos, etc). Can anyone tell me why? Is is really worth twice the price to get one from GM?