Front brake problem - advice needed...

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Another problem with the old Kawasaki GT550.
The front brake lever has got a bit floppy. This reduces the efficiency of the brake but it is also causing the brake switch to be open (on) as the lever isn't returning to close the switch.

Now I suspect this is a common problem. The piston doesn't seem to be returning.

So I'm thinking about getting a kit to refurb the master cylinder (e.g. this kit from wemoto - clicky link). I'm hoping this will keep the lever sufficiently pushed out (alternative is a complete reservoir from a breaker - however I'm thinking this may suffer from same problem)

This will need sorting before the next MOT so what I'm looking for is some advice and instructions on refurbishing / rebuilding the master cylinder reservoir pistons using the kit. I've never had the need to do this task before so any help appreciated. It's the sort of thing I don't mind taking my time over but really don't want to cock-up (both from financial and safety point of view).

Thanks in anticipation for any help.
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