STIFF BRAKE PEDAL

DRIVING ON THE HIGHWAY, I NOTICED THE HANDBRAKE LIGHT ON CONSTANTLY, ALTHOUGH THE CAR MOVED WELL, THE BRAKE PEDAL WAS STIFF AS ROCK, I HAD NO BRAKES AND CALLED IN FOR A TOW-TRUCK.
I OWN A HILUX SURF, MODEL N0:Q-LN130W-GKMQT.ENGINE NO.:2L-TE.
MY MECHANIC SAID ITS THE ALTERNATOR PUMP, AND REPAIRED IT, BUT IS THIS REALLY THE CAUSE??????????