Why do my brakes pump up after bleeding and still go to the floor when i start the truck?

it seems like the master cylinder is not pumping any fluid. Here are some possible reasons why this could be happening:

Air in the brake lines: If there is air in the brake lines, it can prevent the master cylinder from pumping fluid. You may need to bleed the brake lines to remove the air. Here is a video that demonstrates how to bleed brakes on a Chevy Suburban.

Master cylinder failure: If the master cylinder is faulty, it may not be able to pump fluid. In this case, you may need to replace the master cylinder. Here is a video that shows how to replace the master cylinder on a Chevy Suburban.

Brake booster failure: If the brake booster is faulty, it can cause the master cylinder to not pump fluid. In this case, you may need to replace the brake booster. Here are brake booster tests: How to Test a Brake Booster: 4 Simple Diagnostic Tests (wikihow.com)

If it goes to the floor and no brake warning lites or anything comes up on the dash, check the brake booster--engine vacuum and vacuum to the booster. Just my opinion. The master cylinder usually bolts to the booster.

Are you loosing brake fluid after you tighten that bleeder screw back? Your brake booster helps aid in the vacuum of the car - if you have an internal issue with that brake booster - you should check any vacuum lines coming off that booster. If you can answer with your year, make and model I can get you a more in depth answer - there are other things that can be causing your issues as well - from the master cylinder to other things. Good Luck. There's also a way to test that brake booster - do you have a pressure/vacuum gauge?

Michael:

You must start bleeding the brakes at the wheel farthest from the master cylinder (usually the right rear), then the next farthest from the master cylinder, then the next, then the closest. If your master cylinder is at the left front of the car, start with the right rear, then the left rear, then the right front, then the left front. If you don't bleed the brakes in the correct order, you are just shifting the air in the lines from one line to another. Make sure that you close the bleeder before letting the brake pedal up, and the engine should not be running when you bleed the brakes... Make sure that the emergency brake is off. Make sure that the master cylinder does not run out of brake fluid at any time that you are bleeding the brakes.

Booster is okay, you have to bench bleed the MC, first. So, since it is installed, bleed it with a kit from the parts store, little plastic hoses screw in to the brake hose fittings, and put other end under fluid in reservoir. pump slowly, especially wait at end of release, for fluid to refill piston chamber....pump until no bubbles...

FROM THIS POINT you must not allow fluid in reservoir to go low, or start all over again....

Now bleed the brakes and you will eventually get pressure.

Are you losing any Brake Fluid? Is it visible anywhere at the four wheels? Is it visible anywhere else?
The Master Cylinder may be defective. When you bled the Brakes did you maintain proper Brake Fluid Level in the Master Cylinder? If you didn't: and air entered into the Master Cylinder there maybe air within in the system in the Master Cylinder. You can bleed the Master Cylinder by removing the brake lines, adding more fluid, pumping the brakes, bleeding the air out of the system.
Are you losing Brake Fluid and can't find a noticeable leak? Then there is a good posibility that the rear seal in the Master Cylinder is leaking and the Fluid is going into the Brake Booster - which is the large wheel shaped drum located on the fire wall. In this case both the Brake Booster and the Master Cylinder will have to be removed and replaced. The Booster contains seals that Brake Fluid is very caustic to and will ruin these seals- causing future failures.
If no Brake Fluid is lost, no leak found: Check the Brake Booster Vacuum Advance. This should be connected to the outside of the Booster, attached to a hose, with the hose going to the engine. Check to see if this is working properly. Replace if needed.

It's very possible to be the master cylinder, the o-rings on the piston could be worn, letting the fluid seep back through instead of getting full pressure through the lines. Could also be the brake booster.
Try changing the master cylinder first, which is cheaper and easier, and see if there is a difference, you'll need to bleed the brakes again.
To check the booster,
With the engine off, pump the brakes until the pedal is hard, then hold pressure on the brake pedal and listen for air escaping, or the pedal goes slowly to the floor, Do that after the master cylinder has been changed, at least that will be eliminated.

bad power booster

When you bleed the brakes are you starting with brake furthest from the master cylinder? Also was the master cylinder bench bled? If everything there is okay and you are not getting any air in fluid stream, you could try bleeding the brakes by having some one slowly pump the pedal till it is firm. Then have them let off open first bleeder valve and slowly depress to floor and hold there untol the valve is closed. repeat this at all bleeders. You may want to have car running to do this. Try this and let me know.

My vote would be the Master Cylinder.
I had a truck sit for 4 years. After getting running again, it did the same thing you're describing. No brakes. I ended up replacing the Master Cylinder to fix the problem. Just make sure you bleed the system real good.