While driving yesterday the brake pedal unexpectedly went down to the floor when pressure was applied. I could get the brakes to work poorly if I vigorously pumped the pedal. I managed to limp home. Where should I look first? The reservoir is completely full. I checked the vacuum hoses where I could access them from above and all look ok. I would like to get the brakes working even a little bit to drive to the mechanic and save the tow fees. Any suggestions?