Although nailing the "zero" position of the swash plate of a closed-loop pump is not that big of a challenge, it is important to understand that there is no such thing as a "universal adjusting technique". Steps you take to correctly adjust "the neutral" will vary greatly with the pump's control type and servo system design. You must understand in detail the function of the swash-plate positioning system as a whole, and always apply a good amount of common sense, bearing in mind that there are situations when precise null adjustment is impossible without disassembling the pump and/or replacing worn-out parts.
When you have to adjust a closed-loop pump you are not familiar with, your best strategy is to look up and study the technical info on it before performing any adjustments. Of course, if you have an unfamiliar pump on a test bench in the comfort of your shop and time isn't an issue - you can indulge yourself in tampering and back-engineering. But if you are "in the field" - tampering with the null is a big no-no! First - educate yourself about whatever it is that you intend to adjust, and only then reach for the tools.
It is important to understand that an adjustment procedure will only produce predictable results when the pump has little or no wear, which is not always the case, so a good technician must be prepared to face a pump that has significant play in the centering mechanism. In such cases, when an overhaul is not an option and the machine's operation is indisputably necessary, a technician should know that despite his best effort the null will remain drifting, and the best thing that can be achieved is a close approximation to the so wanted "zero". This is why I always recommend explaining all the "benefits" of the drifting null to the machine owner and making it clear that you won't be held responsible for the damage the machine will cause in the near future.
The keywords are always the same - knowledge and common sense!
For example, in pumps that have only the mechanical centering system (like Rexroth A4VGs with DA control), there is no need to install an external servo-pressure bypass to set the null, as the control directional vale does it for you when the solenoids have no current, but it's probably not a bad idea to check the servo-pressures to make sure the by-pass is indeed "by-passing". The same goes for pumps with no-feedback displacement controls, by the way.
Some controls keep the servo-cylinder pressurized in neutral, like this ancient MS control, and if you don't understand its operation, you may find the servo-pressure readings to be completely off.
Some mechanics state that it is imperative to set the null so that the flow is the same in both directions - the so-called "flow symmetricity" technique, which is plain wrong due to the low accuracy of a typical flow meter alone (aside from a hundred other reasons), but in some cases, you may actually need this technique to set the (hydraulic) null correctly. Take the Sauer series 40 with manual displacement control. When the control lever wears out, the neutral return spring can't center it properly, and there's no use in looking for the right position of the adjustment bracket to equal the loop leg pressures because the control lever and the attached control spool are essentially loose. In this case, one of the references you can use is the flow, and you will lock the bracket in a position, where the pump will present equal maximum flow in both directions. The excessive play may be not that big of a problem because on the actual machine the control joysticks that are attached to the displacement control levers can have their own set of centering springs and "override" whatever play the pump control may have.
As you see, closed-loop pump null adjustment embraces many distinctive techniques, but with some investigation and a little bit of sound judgment, the job becomes a hundred times easier.
I truly hope that this short series contributed to making the null adjustment matter a little bit clearer.