Chrysler Repair: Brake pedal goes to floor, repeatedly, vacuum hose, brake shoes

Question
I have had my 2001 Chrysler T&C van in at least 10 times for brakes that continually go to the floor.  I have replaced the master cylinder, calipers, pads, rotors.  No one seems to be able to find out what the problem is.  After I have the brakes bled, they will hold for approximately 4 weeks max.  After that, they will go to the floor again.  Any suggestions appreciated as I'm taking it back in on Tues. to the third garage because no can seem to fix it.  Thanks in advance.

Answer
Hi Diana,
If there is no warning light on the dash when this happens, then one possibility is that the vacuum hose connecting the power brake canister and the intake manifold has a loose fitting at either end or a crack in the hose wall. The booster run out point (point of maximum boost) is often mistaken for the pedal 'hitting the floor', especially if this happens when the van is not moving. That theory can be checked by opening a bleeder valve at a wheel to allow the pedal to move further.
An internal leak in the master cylinder or contaminated brake fluid could also cause this behavior, so although the master was replaced it might be defective. That should be considered if the vacuum hose is not the answer.
Finally, contaminated or wrong fluid used in the brake system could cause this.
Roland

PS Thanks for the kind rating and nomination. I might mention in light of the double replacement of the master that if the mechanic didn't properly bleed the master cylinder at the time of its installation, that could be a reason for the problem. Inquire about that possibility, and ask for the specifics of the technique that he used.  
The rear brake shoes should be adjusted before doing the system bleeding as well.
So if one mechanic has done all this work, I would ask that he get this working properly or refund the labor so you can take it to another shop.