i have a 3" wide wet patch that runs down my servo and bulkhead will this be the master cylinder and how do you test it to see if it is leaking also when i topped up the reservoir the cap had metalic grey looking brake fluid on it what will be causing that and finaly as my servo is very rusty what ather models will a servo be interchangable on all help appreciated as i have spent all day changing brake pipes and cables and am sick of king brakes