The brake pedal goes to the floor on my 2005 jeep wrangler. this only happen in 4wd mode while off roading.

Because when you start it the brake booster will start working, start bleed on wheel farthest from master cylinder and work closer, one at a time pump up and bleed each line at least once after you don't see air anymore...

to bleed brakes is a relatively easy job for two people although it can be done by one without a pressure bleeder.NOTE keep brake fluid levels above minimum at all times during bleeding ( forgot to do it my self on more than one occasion and had to start over, trying to do too much at once) To do by your self you will need the correct ring spanner for bleed nipple, a clear tube that is a tight fit on bleed nipple ( long enough to reach down to floor) and a container with about an inch of brake fluid in it (glass jar will do it, is easier to see fluid level), Starting with the cylinder/caliper at rear furtherts from master cyl (longest brake line from tee piece) place spanner on bleeder then fit tube, placing other end of tube into container below level of fluid,. release master cyl cap/lid (no need to remove completely) undo bleeder half a turn, watch for fluid in clear tube. this should start to flow within 10 to 15 seconds (if bleeder isn't blocked with road grime clean if required), if fluid doesn't flow by it self (it should but) tighten bleeder then release again quarter of a turn, apply foot pressure to brake pedal, hold pressure on pedal till you feel pedal dropping towards the floor (half inch of pedal travel should be enough) ( fluid in container will stop air being drawn into brake lines when you release brake pedal) check clear tube for fluid/bubbles, open bleeder further quarter turn, if fluid has filled clear tube allow fluid to fill container a further eight of an inch, then close off bleeder (make sure no bubbles appear in tube whilst this is happening other wise drain more fluid till only brake fluid is visible in tube ) repeat process on opposite wheel cyl/caliper, then repeat process on front caliper furthest from master cyl , then the caliper closest to master cyl last. If two people are doing the bleeding the same clear tube and container are used,as is the order in which cyl/calipers are bled. One sits in the car and pumps the pedal until it has some resistance under foot (Whilst the pedal is held down) second person releases bleeder, first person keeps pressure on pedal whilst bleeder is released (pedal will travel towards the floor) when pedal has been depressed as far as it will travel second person closes bleeder before, the first person releases pedal and pumps the pedal again repeating the process until no air bubbles appear in clear tube at this cyl/caliper before moving to next cyl/caliper. good communication must be maintained to make this process hassle free

yes may need to bleed lines at master cylnder

Try "gravity bleeding"...to do that you attach a small diameter clear plastic howe to the bleeder valve (I use fish tank air supply hose...if it doesn't fit you stick the end in hot water and just shove it on) Open the bleed valve and put the free ind of the hose into a glass jar with a bit of fluid in it (deep hose end submerged) Open the master cover and let the fluid drain into the jar, till the master has been re-filled twice (don't lot it run dry though) Close the bleed valfe and repeat in the pattern you used before.
Unless the fluid comes out really horrible looking you can keep re-using what you drained to re-fill. If you do this and still don't have anything, I'd return the master as it may be defective.

You changed the master cylinder and vacuum booster and have no leaks anywhere, but did you bleed the entire brake system after you did this work? A little bit of air in any of the lines will make the brakes ineffective.

1. Fill master cylinder reservoir with DOT 3 brake fluid. Raise and support vehicle. Attach bleeder hose to bleeder screw on bleed line of slave cylinder. See Fig. 1 (attached in link) . Place other end of hose in glass container 1/2 full of brake fluid. Loosen bleeder screw while holding bleeder screw fitting.

2. Have an assistant depress clutch pedal to the floor. Tighten bleeder screw and release clutch pedal. Repeat bleeding procedure until fluid entering container is free of bubbles. DO NOT allow reservoir to run out of fluid during bleeding. Refill clutch master cylinder reservoir.

CAUTION: DO NOT allow bleed line to bend or flex when loosening
bleeder screw.

FIG1: http://anubiscycle.com/fixya/91wranglerslaveassy.png

Get the clutch slave cylinder and master cylinder looked at.One of the 2 has failed.If the pedal goes to the floor or is very loose,this is very likely the cause.Not an expensive fix.Hope this helps

pump up clutch pedal really fast about ten times....leave it held to floor....open bleeder on slave cyl... repeat untill you have good pedal....may need to adjust master cyl. rod on clutch pedal to get more travel....good luck....Lee

YOUR CLUTCH IS NOT DISENGAGING. THERE COULD BE A PROBLEM WITH THE CLUTCH MASTER/SLAVE. CHECK FOR ACTION AT THE SLAVE. ALSO, A BROKEN PRESSURE PLATE SPRING OR THROW-OUT BEARING CAN KEEP THE CLUTCH ENGAGED ALL THE TIME.

did you bench bleed master cyl. before you installed it? its either you did not get all the air out, or you got a defective master cyl. if you did everything right and it will not hold pressure, than i would say its defective. believe me you do get bad ones, expecialy if they are remans.