In rev.1.3, the pumps correspond with the same side as their reservoir. The left front reservoir uses the left rear pump. In previous versions, these were swapped so the left front reservoir used the right rear pump. We flipped them to make things a little easier.

If you're only using one pump, I'd say use P1 (side without the bleed pipe). But every system is unique, and some people have an easier time bleeding P2.

Tim