The brake pedal goes all the way to the floor. I can feel the ABS working. The master cylinder is full and I can find no leaks. What can be wrong?

also can be caliper --flex brake line problem -air coming into line

if a master cylinder is bad and you are sure you have all the air out and no leaks.pedal will go to floor , running or not. if booster is bad it will cause a hard pedal. but if master cylinder is bad it will turn on a brake light on dash if it works. make sure the wheel cylinders are not leaking. I beleive that the master cylinder is bad, or you have a leak somewhere. If the master cylinder is functioning properly, the brake pedal will depress to a certain depth and no further.

I would bleed the brakes one more time to make sure, and also ensure that the brake fluid is full. But it sounds like your power booster is giving you trouble. That is the round cylinder that is attached to your master cylinder.

Does your truck have ABS? If so you may have a bad brake
pressure modulator for the abs. Its the big box by the Master that says Kelsey-Hayes on it. You can pick up the complete unit from a salvage yard at a pretty reasonable price.

I also found this on another site. I know you said that you blead the Master but check this information out. The bleeding of a new Master Cylinder is very important to get it exactly right.
http://www.ehow.com/how_4508156_bench-bleed-master-cylinder-chevy.html

well, check the brake lines for leakage after adding brake fluid to the full mark. If the brake pedal goes all the way to the floor, it's likely there is a leak somewhere after the master cylinder. If the pedal gets hard after adding fluid, and pumping it several times with the car off, then the master cylinder is still ok.

If air has gotten into the master cylinder, it may have to be bled out on a bench, or using a scanner if that's the way your vehicle is.

Bleeding the Brake System (ABS) Auto Bleed Procedure NOTE: Perform a manual bleeding procedure. If the brake pedal height and firmness results are not achieved, perform the auto bleed procedure below.
NOTE: Perform this procedure when replacing the brake pressure modulator valve or electro-hydraulic control unit.

1. Perform the manual brake bleeding procedure. Refer to Bleeding the Brake System (non-ABS).
2. Check the master cylinder fluid level often to prevent running the system dry.
3. Press the brake pedal firmly and run the Scan Tool Automated Bleed Procedure (for Gas engines) once, or the Function Test (for Diesel engines) four times. Make sure to release the brake pedal between each test.
4. Perform the manual bleed procedure again. This should remove the remaining air from the brake system.
5. Bleed the system as many times as needed to obtain the appropriate feel of the brake pedal.

There may be air in the lines. Bleed all your brakes starting at the farthest wheel from the resevoir, and finishing at the closest one. Try the pedal after its bled. If the pedal still feels soft or travels to the floor there may be a pinhole leak that may be spraying on the underbody of the car, or the seals in the master cylinder are gone filling the vacuum booster(if equipped). Air compresses more than oil so if bleeding works that may be the problem.

If the brake pedal goes to the floor with the car off, I would suspect air in the system or some other issue with the service brakes. If you feel and hear a growl in the pedal when it goes to the floor then your abs controller may be at fault. There are many possibilities. In either case if your pedal goes to the floor, then you are looking at a safety issue. Generally the service brake system (regular brake system) is the fault and not the abs. I have however seen a sticking dump valve or faulty wheel speed sensor cause an abs issue. Start with the basics first. Leaky wheel cylinders/ calipers, brake lines, or master cylinder. Verify the hydraulic integrity of the service brakes before tackling an abs issue.

brake system not purged properly, might have air inside or front brakes have leak

If no leaks.There is air in system,bleed the brakes.

Presuming you have NO leaks in the hydraulic system,it would be a bad brake master cylinder.What's happening is,the piston inside the master cylinder has rubber seals,and when the pedal goes to the floor,the master cylinder is leaking internally,past the rubber seals.If the brake booster was bad,the symptom would be an extrememly HARD,unresponsive,brake pedal,and the engine would idle terrible (vacuum leak).Good luck.