what's going on with my brakes?

a week or so ago I noticed that the breaks on my 99 xj were getting spongy so I checked my fluid levels. The levels were okay but I noticed a tear in the rubber gasket under the cap. It gradually got worse over the week until Saturday when it got real bad. I went to two auto stores and they said the universal one wouldn't fit. The third store sold me one for Chrysler vehicles. It has a little wiggle room under the cap so I don't really trust it. Now my brakes are better but I feel like I need more stopping distance.

My questions are as follows: what caused the gasket to wear? Should I find a make specific cap and where could I find one? Would this have been better suited to post in a general auto reddit? I just figure that I have a jeep so why not post here. also who do I need to contact about getting some user flair? Pm the mods?