As a general rule, I advise most folks to not try camera/lens repair themselves. However, if you are in the business you have occasion to make quickie repairs to keep the wheel rolling along so I have done some in my career.
Anyway one that most could or can do is replace to rear cover on your EF 85mm f1.2L. They seem to have a problem with getting a sticky coating that makes them almost unusable. You can still get the rear cover part but supply is getting less and less so if you need one get it sooner rather than later. The Rebel T4 has a similar problem don't know if it related. Anyway beside the new part all you need is the proper Philips style screwdriver. It is tiny and a larger one will not work you will simply damage the screws.. Tweezers can be helpful as these screws are tiny. They are tightly affirmed in the lens so careful pressure is required. There are six screws.
