Aside from air bubbles, it's got a leak somewhere, or a wheel cylinder pin that's dislodged.
I'll bet that if you keep lookin', you'll find a leak.
Now that said... some are darned hard to find... because they might spray out into an area where it doesn't wet the frame rail, axle housing, or a wheel.
It also may be that the master cyl is leaking into the booster, and under just-some-certain conditions, mebbie the booster is pulling the fluid into the engine and burning it, it'd be hard to tell.
More than likely, you have some old seal somewhere that dried out.
Typically, that BRAKE light will have one of three things that turn it on. First is the parking brake pedal switch. Second is the proportioning valve (located down on lower of the cab, or on the chassis rail or crossmember), and the third is on the master cylinder.
The proportioning valve maintains balance between two braking circuits- typically one front, and opposite corner rear, and the other pair. the proportioning valve typically doesn't 'proportion' fluid flow or alter pressure, it simply is a shuttle piston in a cylinder, that in the event that one side experiences excess flow, the valve shuttles to 'cap off' the leaking side, and provide all flow to the remaining two corners. There's a switch contact on that shuttle that trips any time the shuttle isn't in the neutral (center) position. Once you find the problem and fix it, the shuttle must return to center before the system can be operated... typically, a light spring in the shuttle will do it automatically, but they can be gunky sticky when old.
The master cylinder switch usually trips when the piston is bottomed, as this is the indication of a serious problem.
Best way to handle it, is to get a replacement rebuilt set of calipers and wheel cylinders, front and rear flexibles, and expect to replace some hard lines as well. You'll find that the fluid in there is really cruddy nasty, so plan on running a fair amount of new fluid through 'till it's clean.
Don't short-cut it... if you have a line in there go bad in a year, you'll be tearing it all apart again, which sucks.
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