The windmill's air pump is a time-proven design, which was later used on the K and S cylinder press series and even the GTO offset presses. Since it's essential to moving paper through the machine, it needs to be in top shape. Here are a few things I learned about when cleaning out the pump on my press.
It's not too hard to remove the pump to work at a bench. I removed the collar from the bottom pivot, and the cover from the bearing where the piston rod meets its crank. Then the whole unit slides off. Obviously the hoses are disconnected before this step. Disassembly follows, and it's pretty straightforward; just remember to keep track of which screws go where, etc. I cleaned off all of the spring steel air valves to ensure things were sealing and opening when they should. The packing on the piston (a felt-y material) was fine on my relatively low-use pump so I didn't mess with it. The most important thing I did was to clean the piston rings and their grooves. Just as in your car engine, the rings need to move freely in order to ride tightly against the cylinder bore, which enables high compression, or air pressure in this case.
Since the rings spring outward when the piston is removed from the pump, one needs to find a way to get the thing back together. As the rings are very springy, one would need three hands to do it without help. I'm sure it's been done with two hands, but I decided to make life easier and borrow an automotive tool idea. For replacing automotive pistons there is a tool which clamps around the piston to hold the rings in as it is slid back into the cylinder bore. Since the piston on the windmill's pump is far larger than that of any car, I measured its diameter and grabbed the next largest hose clamp from the hardware store. With this it was possible to clamp in one ring at a time by its top half, push the piston down until the ring's bottom half was in the cylinder, then loosen the clamp and do the next, and so on. There are three rings in all. I replaced them with the gap in each ring 120 degrees from the others, so the gaps were evenly spaced around the circumference.
Replacing the pump was the trickiest part. Since the bearings have a very precise fit (as do all on the machine) it took some doing to press the piston rod end exactly straight onto the crank.
Once done, I turned the press over slowly to ensure nothing was misaligned. I didn't notice any huge difference in function, but it was good to know that the pump's usual wheeze and snort were indeed normal and not caused by years of gunk or something else wrong.
On a related note: Anyone out there have a service manual for the Heidelberg windmill? I mean the one that Heidelberg service people would have had, with "official" procedures. How about one for the K cylinders, as long as I'm daydreaming? (I have the operator and parts manuals which came with the presses.) In the meantime, as I work on our machines I will follow the fundamental rules: don't break anything, and don't do anything you can't change back. And oil everything.