I believe the ball seal and base gasket come in one kit, so you'd be replacing both anyways. You could also try cleaning the underside of the seal where the ball makes contact. Just push down the pedal, and use a small brush thru the hole to try to clean it up. Does the bowl hold water? Make sure that when you use the head, you let the pedal snap up (just slide your foot off it), or you may not get a good seal.
Also check the clamp and the half circle pieces that hold the head to the base to make sure they're tight and aren't damaged.
You may also be leaking from the ball shaft. It has o-rings on it that can fail, causing a leak. If moving the pedal around changes the hiss or stops it, this could be your leak.
The pump is going to cycle, because you have a leak, and since you can hear the leak, you'll get more rapid cycling because it's a significant leak.
Welcome to the world of chasing vacuum leaks, it can be a pain, but I still think they're the best head system out there.
