250-500cc Rear brake reservoir Leaks

My 09 300 is great in every way with exception on one thing. I have this annoying brake reservoir leakage sometimes. Last ride of 50 miles of singe track the reservoir was almost empty. I checked at the beginning of ride and it was full. I can see seepage from around the reservoir.my friend told me his ktms has a plastic bootie when you unscrew the reservoir and mine doesn't. He said take it to auto shop for size and get a seal. Is that the issue? Am I missing a part?

It doesn't leak out every ride but when it does it's seepage "around" the reservoir but having a bitch of a time finding it. The hoses that connect to the reservoir seem fine. There's nothing obvious leaking anywhere when you apply brakes. Scratching my head....

Mine emptied in one ride but it was because it was a mudfest and I wore a set of rear pads completely out in 3 hours. The caliper piston was all the way out so the brake fluid level was at the bottom. No leaks though.

yes, mine has a rubber piece that makes the seal between the cap and reservoir. also be careful brake fluid is corrosive over time and will remove paint so if its dripping on your truck bed over time it will peel paint.

yeah theres a rubber diaphragm in there. changed fluid the other day. sounds like someone left it out last fluid change