Z axis homing problem

I am trying to install auto leveling with a probe (like Printrbot) and after replacing the normal z endstop with it when I try to home the Z axis the z goes down about 10mm (the amount differs every time), then stops. It counts the position it stopped at as home so you cannot bring the bed back up to where it should be. I think it is something in Marlin but I have tried everything. I did everything it said to do in Thomas Sanladerer's guide. This is really annoying and would love to hear any suggestions on what to do.
Thanks

When you say "the z goes down about 10mm", I think you mean the bed goes down? The equivalent to a PrintrBot head going up?

If so, that's typically caused by the probe being continually triggered. The home process is to move down quickly until Z is triggered, then move up 10, then move down slowly until Z is triggered.

Try an M119 command when the probe is well clear of the bed and see if it says Z is triggered.

Well, I think it's doing its job just fine, did the light at the sensor light up when it stops? If you are expecting for it to act like your old switch, getting really close to the bed it won't happen, you need to set your nozzle to sensor offsets so once it finishes the G29 command it will go down the amount set in the offset and then will start printing.