I dont think holding the clutch will hold it, it more than likely releases it. As soon as you brake on a hill, the hill holder ball valve should already be holding the car by keeping brake fluid from scavenging. Let go the brake, you should be stuck on the hill. Depressing the clutch moves the ball valve and then brake fluid scavenges to let the car roll back.

I had to figure this out myself when I had a bad valve....the fix, I removed that SOB completely. Now I roll back alot like Wal-Mart.

Jay, don't clutch-hold on hills. Otherwise you'll be forced to buy a bigger turbo, injectors, and EM to go with your new, upgraded clutch. Haha!!