Vehicle pedal went down suddenly and we expected that there were leaks somewhere. We cannot find any leaks and decided that the master cylinder was not working properly. We replaced the master and it seems that after it is bled back out, the problem still remains and fluid stops coming from the master cylinder.
Any ideas, anyone have this problem before with 4 runners?

Something we are missing?

Alicia

One thing to note "New parts does not mean good parts." It is possible that your new master cylinder is defective.
Do you have a repair manual and are you following the correct procedure for bleeding the brakes?
I don't understand the phrase "it seems that after it is bled back out, the problem still remains and fluid stops coming from the master cylinder" Are you letting the master cylinder run out of fluid while you are bleeding the brakes? Or do you mean that one of the wheel locations does squirt fluid while trying to bleed the brakes? If this is the case then the flex lines may be bad. Flex lines deteriorate internally and will show no outward signs of failure.
Did you check every inch of the brake lines for leaks? Due to vibrations, corrosion and/or road debri, brake lines can fail anywhere. Use a paper towel to wipe them down and see if there is any brake fluid leaking.

Check to make sure your master cylinder piston stopper bolt is installed properly(if equipped)
Break the lines at the master cylinder and see if you get fluid to flow at the lose fittings.
Forget about the rest of the brake system for now, if you are not getting fluid to pressurize the system.
One step at a time and you will get to the source of your problem.
Good luck.