Brake problems

I have a 94 sierra 4x4. My dash brake light is on all the time. And my brake fluid is out. So i filled it and a week later it s empty again. I cannot find a leak anywhere. All wheels and tires are completely dry. lines are dry. I had someone pump the brakes, and still cannot see any leak. What could be the problem? Could it be the master cylinder leaking inside the engine?

It has to be going somewhere. Look under master inside the engine compartment. Also look on the inside of the cab under the brake pedal. Pull the carpet back.

Also, check the rear of the master at the booster. You can loose the bolts to the master to inspect it and not break open a brake line.

Check the brake line near the rear diff as well.

//2000CAYukon

Do as 2000CAYukon suggested but also remove the wheels/rear drums and physically check the rear brake cylinders. Same for the front brake calipers.
It's not unusual for the rear brake dust and brake shoe material to soak up the brake fluid leaving the outside of the rear brakes dry.
If the one side or both sides of the rear brakes are soaked with brake fluid, you will need to replace the all of the brake shoes along with the leaking brake cylinder.
You will also need to use brake klean to clean all the brake hardware, drums (have them resurfaced after cleaning), and clean the backing plates.
After cleaning, apply a thin coat of grease on the areas where the brake shoes contact the backing plate, reassemble the rear brakes, then bleed the brake cylinder(s) you replaced.

The front calipers usually leak from the seal around the brake piston(s). Remove the offending caliper, clean area using brake klean, replace with a new caliper or calipers, replace all the front pads with new brake pads, and have the rotors resurfaced.
Bleed the caliper you replaced.

Hope this helps.

Alright, finally good weather and the snow is melting. I got another chance to look under the truck. I found some oil around the gas tank area. Yes I could barely see, and yes it was a brake line. It took me three hours to get a small piece replaced, and I bled all the line. Thank you all for your help. I couldnt find any oil before because all the snow and the salt. Thanks again.