Mine have never leaked. Top panniers IMO Dead easy to use, about the right volume, the bigger you get, the more you'll fill 'em, i presume yours leak at the mount point?
I have a set of the Zega boxes and use an Oring instead of the spring washer originally used. (Lost the spring washer actually). Never leaked before or after so I do not know if it helps. Another tip to stop leakage is to silicon the donut to the box, bugger to get off but why do you need to except as in my case where I needed to ship them overseas and thought it was a good idea to remove them .
But wait there is more as they say on the telesales.If you have the 15mm donuts and 18mm racks or have not placed the donuts close enough to the corners of the racks use the 15mm donuts on the 18mm bars and crimp the bars exactly where the retainers sit. This stops them moving as much. (or if you are a scrooge like myself saves you from buying another set of donuts)