Leaking Brake Fluid Reservoir - Can It Be Fixed?

I have recently replaced my clutch slave cylinder on my 85 QV 3.0 cabriolet Image Unavailable, Please Login and when I tried to bleed the system with a power bleeder it started to leak at the gasket in the top of the fluid reservoir. It showed prior signs of leakage in that area, but now it won't let me pressurize the clutch line to get all the air out in order to operate my new slave cylinder. I get to partial pressure then it bleeds out again with my clutch pedal going to the floor. Any suggestions on how to tighten or replace this gasket as shown with blue arrow in photo? Or do I need to replace the entire fluid resevoir?

 

The instructions on the power bleeder said to put 13-15 psi into the tank. I've done it before when I replaced the master cylinder a year ago and had no issues when going to 15 psi - the system held integrity no problem. The gasket starts to leak right away once I start to pressurize, and generally after 10psi it slows down or stops.

The sending unit seems to be able to freely rotate, and not sure if it was supposed to do that or not. I'm wondering if there is a way to tighten it, or if the gasket can be replaced. Worst case I can replace the whole reservoir, but wanted to also confirm if this would be the reason I cannot get the lines bled of air completely, or if there could be something else going on....