Ever since I opened the reservoir the other week to add fluid until I can get the cylinder kits in, the clutch master has been leaking from under the cover.
Is there some special trick to getting the cover seal in place?
Shouldn't be, just be careful with the alignments of the sealing surfaces with the gasket when assembling the parts. No need to crank down hard on two cap screws either.
Couple of things come to mind though...
1. You're not overfilling the reservior. If you do, it may leak due to expansion of the brake fluid when it become warm from braking.
2. What's the condition of the cap seal? It may cause leaking if it's bad.
3. When I went through my reserviors' caps and seals, I found white corrosion on sealing interfaces of both the cap and its reservior, and it was also stuck on the seal too. I cleaned it all off and re-assembled. There were no leaks in the area around the cap's sealing perimeter.
If you have done it, you may want to entertain the idea of flushing the hydraulic systems, then back filling with silicone fluid instead of DOT 3 or 4 brake fluid. The latter, will do a number on your paint and plastic over time