They were originally plumbed like this:
Not a good layout; when the trigger was pulled/released the water flow in the head changed directions and the pump would cavitate for an instant. I had to replace the first head from washout in less than two years. As I replaced heads I replumbed them like this:
They run smoother, and none of the replacement heads have washed out (The oldest is seven years). It's also a lot easier to service with just two unions to break loose and free up all the plumbing, and I can keep a rebuilt head with a gauge already on it on the shelf to swap out in just a few minutes.
There was no change in the
soap/wax draw with the change in plumbing.