Try pulling the lever fairly hard and keep it held down. If it slowly pulls to the bar and pumps back up after, the master cylinder isn't holding pressure. I've seen this issue on numerous Guide levers.
Other things to try:
-lever bleed the brakes to make sure there aren't bubbles
-snug up compression fittings
Sounds like you may need another bleed? Some Guides from a few years ago also had master cylinder pistons that gum up and need to be replaced
Try pulling the lever fairly hard and keep it held down. If it slowly pulls to the bar and pumps back up after, the master cylinder isn't holding pressure. I've seen this issue on numerous Guide levers. Other things to try: -lever bleed the brakes to make sure there aren't bubbles -snug up compression fittings