LUVTruck.com

I stuff a block of wood or something under the pedal when bleeding the brakes. You generally don't want the pedal to bottom out during bleeding or you can create probs with the master cylinder. The piston and cups have a range they tend to stay in along the cylinder. If you let the pedal bottom out on the floor during bleeding then you're pushing the piston out of that normal range and potentially into buildup or gunk on the inside of the cylinder. Since I started doing this, I have not had a bad master cylinder. Two of the three LUV master cyls I've replaced went within a month or so of bleeding the brakes to the floorboard.

I've heard of people ruining their master cylinder during brake bleeding. When people bleed the brakes they usually crack the bleeder open untill the pedal hits the floor. Under normal operation the brake pedal never goes near that point and rust can form inside the bore of the master cylinder. When the pedal is pressed to the floor, the rubber o-rings go further into the bore to the rusty area putting microscopic cuts in them, thus causing fluid to seep past them. I don't know if this is your problem, but thought it might be something to consider.