Brake master cylinder help needed.......

[RichardJ]

Drum brakes have self adjusters or the owner is expected to periodically adjust manually adjusted brakes as the brake shoes wear down. Disc calipers have no adjustment for wear on the pads. The caliper pistons are fairly large and will hold a lot of fluid when the pads are worn. The large reservoir is needed so it doesn't run dry before the squealer signals the low pads.