Sick zip line. A few suggestions for you, that I hope will help.
The pulley wheels that you get at Home Depot and other hardware stores are zinc plated and the cable will chew them up rapidly. You will actually see zinc filings raining down on your head. Find a place to get steel pulley wheels or buy them online.
Make sure the axles fit well and grease the crap out of them.
It doesn't sound like you are using a coated cable, but don't. It is much slower. Also use the thicker 1/4 inch cable.
I built my own zip line from scratch and it took me a while to get it right. These things will make your Zip Line way faster and more fun.