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The rear reservoir would be for the front brakes. Unless you mean the reservoir for the rear brakes when you say rear reservoir. Neither reservoir should be pressurized. The pressure should only be created by the pistons in the MC bore. If you had moisture in the brake fluid it wouldn't be a surprise to have corrosion in the bore after 6 years. Then when you pushed the pedal to the floor you might have damaged the rubber seal inside. So, loose fitting allows reservoir ro leak dry, pedal goes to floor because of it, damaging MC seals, new MC leak looks like old fitting leak. Should have got that fitting tight the first time. Seems like it was very loose. Oh well.

It's also possible that the metal to metal seal of the brake line was damaged when you changed the MC the first time. Pretty common for the tube to twist with the fitting when the fitting is loosened. The only way to be sure is to examine the parts. A magnifying glass and bright lights help. Take a close look and you'll see where the actual sealing surface is. A spec of foreign matter can screw things up. Something to be aware of if you do change the MC. The threads don't do the sealing, they just do the clamping. You can see the cone that fits inside the end of the tube in this image from Rockauto. That's the sealing surface.

No, Pretty sure you’re MC is bad