By loose, do you mean like in the pic above, where the cable is just dangling? If so, that can't happen by itself if the bike is just sitting there, not being used, someone would have to be doing that. Animals can't generally turn those things, at least not without leaving marks on them, and I can't imagine any of the ones I've had or dealt with trying (even ferrets). That would leave people, so if there is someone other than you with access to the bike, I'd question them about it (or put a camera up somewhere that would record motion and catch whoever it is).cwah said:For the front brake, I spend my time adjusting the caliper but it's loose the next day. I don't know how to fix that.
If you just mean loose as in "it doesn't brake as hard after it's been used for a ride", that could just be abnormal pad wear. Or if you are braking hard from a high speed (15-20MPH or more), many bicycle brakes are not designed to stop all that much weight from that high a speed, and wear could be very high on the pads, causing them to be farther away from the disc rotor surface, and causing you to have to squeeze a lot harder on the levers than before, to get the same braking. But that won't be a sudden thing, it'll be gradual throughout the ride, until at the end it could take more than enough extra squeezing to notice.
Another possibility is cable "stretch", where the cable actually has a snapped wire or two (or more), and each time you pull, the cable stretches enough on the remaining wires to require a lot more pulling force to transfer the same amount of length change/force to the caliper. Sometimes the stretching is permanent, leaving the cable looser and looser each time you brake, and sometimes the cable is elastic and goes back, so it doesn't feel loose when you're not braking and are checking things.
If the caliper itself actually feels loose on the attachment lugs on the fork, you might have a problem with the threads in the lug or the bolts holding it on, if they have to be retightened a lot. Locktite will fix that if it's not a thread problem, but if the holes are deformed for any reason (side impact on the caliper itself, for instance) they may never hold the bolt in right.