Disc brakes defectuous .... repeated

Hello,
I never post anything on this forum, being rather connected CAP. But I have a basic mountain bike that I like a lot.
My problem is this: regularly, my rear brake does not brake anymore. In fact it loses in effectiveness gradually, until the big fright signaling that it is totally inoperative. And it happens on two outings pretty much. Buddies thought of an air bubble, but I brought it in review, it starts again.
The type of store explained to me that a rubber could be eaten by the brake fluid and that I had to activate the brakes even when I do not tighten up ..... that is to say that I handle them every time I pass my bike, even if I do not ride on it .... but I find this strange explanation: first because the front brakes do not have this problem, while I only use it when the rear brakes are in carafe (so much less), then because I've never heard of something like this, finally because it has restored it to me there is a year and it starts again this year ... weird all the same.

Does someone have an idea? I like my bike and I miss it.
thank you very much