Probably a failed brake booster, get a mechanic to check this for you.

THIS IS RIGHT BEHIND YOUR BRAKE MASTER CYLINDER, ON THE FIREWALL

http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product_remanufactured-power-brake-booster-cardone_18690489-p?searchTerm=brake+booster

IT OPERATES OFF OF VACUUM

IS THE VACUUM LINE DISCONNECTED ON EITHER END?

IS IT CRIMPED?

THE UNIT ITSELF COULD BE BAD

WHILE IT'S RUNNING, SNATCH THE LINE LOOSE AND SEE IF IT, INDEED, HAS VACUUM

I FEEL SURE OTHER THINGS MAY CAUSE THIS---LIKE PUTTING THE WRONG FLUIDS IN THE MASTER CYLINDER IS ONE

BE CAREFUL---MAYBE NO DRIVEY TILL IT'S FIXED?

LET ME KNOW IF YOU CAN INSTALL THIS---

LET ME KNOW IF YOU DECIDE TO BUY THIS OR OTHER EXPENSIVE STUFF---I MAY BE ABLE TO AID YOU IN OBTAINING IT AT A BARGAIN PRICE

THE MEDIC