brake booster question 49 coronet

I have the front original brakes and cylinders attached to a brake booster/vac*** M/C in my 49 coronet, mounted to the firewall. My question is this....what would cause the pedal to go all the way to the floor during a trip around the neighborhood and cause the black rubber gasket inside the resevoir to **** down into the chamber?

 

So did all the fluid disappear too?

 

If you have a small leak in your brake circuit (often when not driven the wheel cylinders can
leak) ,when you apply the brakes the piston in the master cylinder can get forced into the end of the bore, which may have corrosion causing it to jam.
This may not be the case, but it happened to me, had to rekit the master cylinder.

 

no, the fluid level is still the same. it was wierd, worked fine at one corner and then when I pulled in the garage the pedal went to the floor. checked the resevoir and the fluid was good, but the rubber gasket/seal was ****ed into the rear chamber (the one with the front brakes applied). I did see a few drips of fluid on the front tube that is (or I thought was) blocked off. Would air have caused that if the front resevoir ****ed air into the M/C?