I changed the brake booster an bled the brakes. The back right brake bled fluid out when it was bled. All fluids are full an the vacuum lines are good. But when I push the brake pedal it goes all the way to the floor
I doubt the new master cylinder was bled properly.
The easiest way I have found, is to put a piece of clear tubing over the closest caliper nipple (usually left front), and extend that over the fender and submerge the other end in the master cylinder reservoir.
Be sure to cover the fender to prevent brake fluid from damaging the paint.
Crack the nipple. Slow repeated pumping should push the air through the system, with the bubbles rising to the top of the master cylinder - thus bleeding out the trapped air.
Once there are no more visible bubbles in the tubing, I usually pull out the "bleeding" fluid with a turkey baster, and replace with new. Then bleed all four wheels in a normal fashion, starting with the furthest away (probably right rear). Watch the master cylinder level so that it does not get too low.
It takes some time, but I feel this is much simpler than "bench bleeding" the master cylinder, which is traditionally how it's done.
I doubt you will have much success attempting to bleed the system properly at back right caliper/cylinder.
I would bleed this system in the following fashion, with a separate bottle / container and piece of tube.
1 fill master cylinder reservoir
2 start with closet brake to master cyl and connect tube with free end in container (container needs some fluid in so end is below the fluid surface)
3 Bleed that corner - have someone push pedal and release bleed screw, lock bleed screw before pedal is allowed to return
4 repeat until last corner (farthest) is done.
Check and if necessary repeat.
The separate container allows new fluid to replace the older existing fluid.
I have also found that releasing the connections at the master cylinder to allow the "high" bubbles out as a first step is effective.
The system hasn't been bled properly, you need to re-bleed it. I won't go into process as the other 2 answers cover that, instead I'll talk about the gotchas I know about.
The biggest gotcha is not keeping the fluid reservoir full when bleeding. Brake fluid reservoirs aren't big and it only takes a few pumps of the brake to get the level low, and then the system sucks air. When bleeding I overfill the reservoir and then check it every few pumps.
Another gotcha is allowing air back into the system from the calipers, which can happen if you unscrew the bleed nipple too far, or don't close the nipple tightly in between bleeds. The process is push the pedal, open the nipple, then close it while there's still some pressure on the pedal, if you let up pressure before you close the nipple the system will suck back in.