UCA Nut at Drivers Side Firewall Removal/Re-installation
Hi I ran into the same problem. Here's my solution and it worked like a charm! You'll definitely want to do this to make re-installation easier.
Edit: After separating the master cylinder from the booster I ran into some trouble reconnecting the two units.
In order to reconnect the two I had to bleed the left front brake to relieve the pressure on the line, this allowed me to reconnect the units. I then secured the bleeder and let the car sit overnight. The brakes worked fine in the morning.
In order to prevent this issue.
With the car in park and the engine turned off pump the brake pedal until it builds pressure (According to the Chilton's manual at least 20 times). This exhausts the vacuum in the system. After exhausting the system vacuum you can then proceed to separate the master cylinder from the booster.
Loosen the two inner brake lines from their retainer with a pick. There is also a plastic retainer clip securing the brake lines that run to the rear of the car on the inside of the fender well that needs to be loosened prior to separating the two. Just follow the brake lines you'll see it. Then use a pick to disconnect the two wire harnesses from the master cylinder.
Then disconnect the Brake master cylinder from the booster. There are only two MM bolts securing the two together. DO NOT force the brake lines, they should bend just enough to get the two apart. If you feel excessive resistance check to see if there are any retainer clips that you missed. The only two that you should need to disconnect are the two locations listed above.
Once the brake lines are loose carefully separate the the brake master cylinder from the booster. Pull the master cylinder back about six inches until it clears the booster and move aside.
This will provide you with excellent access to the UCA nut at the drivers side firewall. You should be able to easily fit an MM ratcheting wrench on the nut and get enough leverage from the top to break it free and remove it. Just be careful not to damage the sensor on the brake booster.
There is no brake fluid between the master cylinder and the booster. The booster works on a vacuum so make sure the surface is clean and the gasket is in place prior to reassembly.