Brake problem

This morning, I got the bike out to ride to work and I noticed the brake lever travel was abnormally long.

The travel in the lever has been getting more and more, and I've been dialing the adjustment slowly down to "1", the lowest setting to reduce the travel. This morning it was almost touching the grip, not good.

The reservoir was full of transparent fluid, nothing obvious (leaks). I removed the cap to find the membrane or rubber boot creased.

It's a cylindrical shaped boot that (I'm guessing) moves up and down as well as seals the reservoir. It the inner portion that moves, was creased as it might look if you pushed it down on the edge versus the center.

I popped it back up, put the cap on and the brake lever travel was better but still not right.

I changed the fluid over 2 years ago and just started having an issue within the last 6 mos or so. Why, all of the sudden, would it do this? It was working fine.