I just got the kit to rebuild my rear master cylinder and got everything apart, cleaned it spic and span and put it back together, the piece that looks like a large screw thing with the rubber seal on the end. when i put that piece in it fits fine until the "concave" end gets almost inside then it get very tight and almost impossible to push in, when I get it in the piece doesn't return like it should or like I think it should and cannot figure it out for the life of me.