You've received some helpful advice here. If it's not fluid level - it may indeed be the brake switch causing CC to think you're applying the brakes, thus deactivating CC. You see, I just purchased a Toyo Dolphin and the cruise control is out. Sellers said it was working a couple weeks prior. When researching how to fix I found the problem may be a broken, missing or slipped out of place brake switch aka brake sensor. They're typically a plastic piece that eventually wears down or falls off, thus, the pedal drops lower and your light gets activated and . . . incidentally, CC won't work because it thinks you're depressing the brake.
Here's a link to learn more: https://psbrake.com/blog/what-is-brake-switch
