I think a got bad brake Master

I'm happy you found that, but it doesn't explain the repeat failures. The wrong diameter master cylinder, calipers, or wheel cylinders will cause some symptoms like the brake pedal traveling too far, easy rear-wheel lock-up, or the rear brakes not doing their share of the stopping.

The smaller reservoir is for drum brakes, (rear) because very little fluid is needed during the life of the shoes. As front disc brake pads wear, the pistons move out of the calipers to self-adjust. Brake fluid fills in behind the pistons so the level goes down in the reservoir. Professionals never top off the brake fluid during other routine services like oil changes because when new pads are installed, the pistons have to be pushed back into the calipers and doing that will push the brake fluid back up into the reservoir. That fluid needs a place to go.

When you work with any part that touches brake fluid, be absolutely certain you don't get a hint of petroleum product in the brake fluid. Doing that may result in more repair expense than what the truck is worth. That includes engine oil, transmission fluid, power steering fluid, and axle grease. Most of us even wash our hands so we don't get fingerprint grease in there.