it could be the slave cylinder because its hydraulically actuated not the the old cable type ( just like the breaks low on fluid due to leak they become spongy because air has got in ).

now one way to check if the clutch is bad is if the revs go up and the car moves slowly ( better tested at 50 mph then deck the throttle pedal ) if it still pulls hard then its a leak or the slave cylinder is bad

 

^^Really GOOD Advice^^
Damn! didn't even think to double check the slave cylander. I remember that was a way I fixed the problem I had with my VW.