John, I just finished the rebuild of the master brake cylinders on my twinco. The total cost of the 5 "o" rings, each cylinder, was $2.50 from Spruce. The most difficult part of the process is removal of the top clevis bolt to remove the cylinder. The replacement of the "o" rings is straight forward, but care should be made when replacing the gland nut back on the piston rod as there is an inner "o" ring that can be crimped or cut on installation.
Good luck
Jim Lynch
13586