I've never been able to get a new dust cap onto the drum, so whenever I had to remove the cap (such as to replace the rear wheel bearings or brake shoes), I'd re-use the original cap. This worked fine until recently the cap started to leak grease because it no longer sealed tightly enough.

I removed the old cap and tried unsuccessfully as I had in the past to use a rubber mallet to hammer the new dust cap into the drum. The fit is very tight, and the cap always gets cockeyed, with one side seated fully and the other side not at all. It occurred to me that what I needed was some sort of circular device that would make contact around the full circumference of the lip on the cap and press on it evenly with each tap of the mallet. So, off to Google to search for a dust cap seating tool.

As far as I can tell, no such tool exists. I did find mention of someone using a short length of 2" ID copper tubing to rig up such a tool for use on their Chevy/Pontiac/whatever it was. As it turns out the diameter for the dust caps the Jetta uses are 1.25". I went to Home Despot and found me a 1.25" ID PVC end-cap. It fit snugly around the dust cap, and would allow me to press evenly against the lip.

I tackled the installation of the new dust cap this evening. There was one small wrinkle that I had not anticipated. Namely, the PVC is softer than the metal used in the dust cap. So with vigorous tapping the dust cap would slip and become lodged inside the PVC end-cap. I took a shop towel and folded it into thirds along its long side. I then folded it in half along the short length. I placed the folded shop towel between the dust cap and my PVC pressing "tool". The added thickness of the paper shop towel prevented the dust cap from slipping into the end-cap and further helped to evenly distribute the force of the blows from the mallet.

Result: Within about half a dozen smackers the new dust cap was seated firmly in the brake drum holding my precious grease in where it belongs.