Brake pedal will go all the way to the floor

Search for the brake fluid leak the brake system is a sealed unit if going through fluid like crazy it is leaking somewhere look under at back of tires and along the top of fuel tank for fluid if not noticeable on floor also inspect the brake booster

You must have air in the brake lines. You will have to bleed the lines to get the air out. You will have to locate the bleed valve near each wheel. It is easier to take off the wheel so you can see everything. The brake line will have a drain screw near the place that the line goes into the brake caliper assembly. You will need a helper. Loosen the screw a bit then have your helper press the brake to the floor. Fluid and/or air will come out. While they hold it to the floor, tighten the screw so it won't such air back in when the brake peddle is released. With the brake pedal released, open the drain screw again and have the helper press the brake to the floor again. Repeat this until no more air comes out when the brake pedal is pressed. Then move to the next wheel and repeat. During the process make sure that the fluid in the master cylinder does not run out. The level will drop each time you bleed some of it from the bleed valve (or drain screw or whatever you want to call it). It helps to open the drain screw just enough to let a little fluid out. You won't waste as much that way. By time you get through with the last wheel the brake should not go to the floor anymore.

for the pedal to go to the floor there is still air in the brake system. Start again starting from the longest line bleeding each in turn to the shortest line . make sure you have adjusted up the rear brakes before bleeding as every time you press on the pedal the shoes expand right out and when you take your foot off the pedal the shoes go all the way back in. adjust them manually and do not rely on the self adjuster as it only adjusts on wheel movement

you need to bleed it with the car running

First, check your brake fluid levels, if it is low then that enplanes your problem. Also, when was the last time you had the brakes changed? if the are worn down, then this will also cause a low fluid level. if the brakes are fine and the fluid level is good and you still have the same issue, then your problem is probably your master cylinder. Brake boosters hardly ever go bad.

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.

For left hand drive vehicles: (If the car does not have ABS system you do not need any diagnostic equipment!)

IMPORTANT NOTE: You need using diagnostic equipment connected to the data link connector of the car in order to bleed the ABS block. Follow instructions by scantool equipment for bleeding the ABS block.

1. This bleeding procedure requires two people. Remove the brake system reservoir cap, and fill up with new brake fluid until "MAX" level is reached.
2. Lift the car, or use a channel for access under vehicle.
3. Put the gearbox in "N" - Neutral position, or in "P" - Parking position for automatic transmission.
4. Start the engine and push the brake pedal firmly for 4 - 5 times to the floor, and maintain the brake pedal pushed to the floor.
5. Begin with the rear right wheel. Another person remove the dust cap and loose the brake caliper bleeding screw until the brake pedal goes to the floor. At this moment maintain the brake pedal pushed to the floor, engine idling, and tight the brake caliper bleeding screw. Use a plastic hose: one end connected to the brake caliper bleeding screw, and another end inside a recovery bottle for used brake fluid.
6. Repeat steps no.#4 and no.#5 until no air flows through the brake caliper bleeding screw. Tight the brake caliper bleeding screw, and put the dust cap in their place.

IMPORTANT NOTE: All the time check the brake fluid level in the brake system reservoir, and fill up if necessary!

7. Push the brake pedal firmly for 4 - 5 times to the floor, and maintain the brake pedal pushed to the floor, engine idling.
8. Continue with the rear left wheel. Another person remove the dust cap and loose the brake caliper bleeding screw until the brake pedal goes to the floor. At this moment maintain the brake pedal pushed to the floor, engine idling, and tight the brake caliper bleeding screw. Use a plastic hose: one end connected to the brake caliper bleeding screw, and another end inside a recovery bottle for used brake fluid.
9. Repeat steps no.#7 and no.#8 until no air flows through the brake caliper bleeding screw. Tight the brake caliper bleeding screw, and put the dust cap in their place.

IMPORTANT NOTE: All the time check the brake fluid level in the brake system reservoir, and fill up if necessary!

10. Push the brake pedal firmly for 4 - 5 times to the floor, and maintain the brake pedal pushed to the floor, engine idling.
11. Continue with the front right wheel. Another person remove the dust cap and loose the brake caliper bleeding screw until the brake pedal goes to the floor. At this moment maintain the brake pedal pushed to the floor, engine idling, and tight the brake caliper bleeding screw. Use a plastic hose: one end connected to the brake caliper bleeding screw, and another end inside a recovery bottle for used brake fluid.
12. Repeat steps no.#10 and no.#11 until no air flows through the brake caliper bleeding screw. Tight the brake caliper bleeding screw, and put the dust cap in their place.

IMPORTANT NOTE: All the time check the brake fluid level in the brake system reservoir, and fill up if necessary!

13. Push the brake pedal firmly for 4 - 5 times to the floor, and maintain the brake pedal pushed to the floor, engine idling.
14. Continue with the front left wheel. Another person remove the dust cap and loose the brake caliper bleeding screw until the brake pedal goes to the floor. At this moment maintain the brake pedal pushed to the floor, engine idling, and tight the brake caliper bleeding screw. Use a plastic hose: one end connected to the brake caliper bleeding screw, and another end inside a recovery bottle for used brake fluid.
15. Repeat steps no.#13 and no.#14 until no air flows through the brake caliper bleeding screw. Tight the brake caliper bleeding screw, and put the dust cap in their place.
16. Now the brakes are bleeded. Check the brake fluid level in the reservoir. Fill up with new brake fluid until "MAX" level is reached. Put back the brake reservoir cap.
17. Test the car brake system. The brake pedal movement no more than 1/3 of total stroke until the floor, when brake pedal is applied. Maximum admissible is 1/2 of total stroke.

I suggest replacing the master cylinder assuming that you bled the system properly. You sound competent so it seems likely that the seals in the MC might be failing. That being said the brake lines can hold a lot of air :)

the brakes have to be bleed you wrote that when you start the car the pedal goes to the floor that means the booster is working fine take it back and tell them you want the brakes bleed

sound like you need a master cylinder