Recently after about an hours driving in my Sierra XR4x4 I noticed the brake warning light come on and when I tested the brakes they were very spongy. I stopped at the next garage and when I checked the brake fluid instead of being between min & max it was way up close to the filler cap. Obviously I had got a lot of air in the system somehow. I drove home very slowly with the warning light on all the time. When I got home I left it for a couple of hours and then when I checked the fluid level was back to normal and the warning light didn't come on. It's been ok ever since although I haven't been very far in it.
Just wondered what could have caused this problem and should I be concerned about it?
Nodge