My first question is about the components used.
What is the master cylinder diameter?
What are the caliper piston diameters?

There are some kits available that have mismatched master cylinders. I know SSBC was one of the big culprits there, as well as some others who used the Corvette style masters.

I've never heard a groan from the hydraulic system on brakes. Boosters can leak vacuum and make noise and of course the friction materials do it, too. Is it possible you're hearing a problem with the bottom of the brake booster diaphragm?

We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon