have you just got a casing if so, take the lever of with tpa on it, you will see a piston take that out (it should just come out, if not undo the screw under where the hosing enters the lever and push something thin and small through) then a biggish spring. now putting it in is backwards in the new casing, spring-piston-lever with tpa in. job done then just bleed, its all fairly simple.