Its a process of inspection and elimination, if you employ a garage it depends on how long it takes them to locate the leak (labour charges). If you're lucky you hear it hissing at the leak (with the engine running) Start by checking the the main vacuum feed to the servo - this is a rigid plastic pipe going to the servo (big black circular thing located in the engine bay, directly behind the brake pedal area. i.e. about 18" under the brake fluid reservoir. This pipe is known to split. The other item to check is the servo unit itself, and leaking servos are quite common. With the engine running stick your head in the drivers footwell and see if you hear hissing from the area above the brake pedal (IIRC there is a small rubber flap/valve there that leaks) OK all - sorry for long delay in updating on this ....first we were on holiday ...then mechanic...you know how it goes ! Anyway, I knew it wasn't a problem caused by new brake pads being fitted as the problem had already shpwn itself prior to these being done....and anyway I know my mechanic's no monkey! After lots of discussion with Ford Technical and others he knew who dealt with Ford Diesels it was narrowed down to TWO - either a specific pipe that is direct link to servo (think thats what you talk about above) OR the brake servo pump itself. Mechanic had been warned that both were a bugger to get to and work on and if he went for the "cheap" option (pipe around