i've seen this happen 3 times in the entire time I've done my own work and working at the dealer. There is no detailed description, but i know if there is no master cylinder pushrod clearance which should be 0.0mm if i remember correctly (booster pushrod is either too close or touching the master cylinder pushrod) that is when i found the 3 prop valves with their rubber caps off and leaking fluid. Without a description, i would presume that the rubber is there to let you know that if it ever came off there is a lack of clearance between the 2.