The booster is hard to find and I was lucky enough to find a very old for sale thread on NASIOC and contacted the seller and he still had the booster. You might have a hard time finding one but if you do, it should be pretty cheap. I couldn't find any concrete evidence for exactly which cars came with them, unfortunately. It seems they came on older GC imprezas but only a few of them. Some told me only cars with rear drums got them but others said they had rear drums and didn't get the single diaphragm booster. So, just scowl the web and the junkyards til you find one. They're easy to spot because they much thinner front to back.
Here is the Write-Up that I went off of but I made one very time saving change when I swapped mine out that I was surprised this guy didn't think of.
http://www.northursalia.com/modificatio ... oster.html
You can see the difference in the boosters in these pictures. The first picture is the single diaphragm booster. The dual diaphragm legacy booster looks like it has two compartments. The diameter steps down about halfway through the depth of it which probably houses the other diaphragm.
![Image](http://i196.photobucket.com/albums/aa166/kimokalihi/91%20Legacy%20Turbo/Brakes/Single%20Diaphragm%20Brake%20Booster/IMAG1104.jpg)
![Image](http://i196.photobucket.com/albums/aa166/kimokalihi/91%20Legacy%20Turbo/Brakes/Single%20Diaphragm%20Brake%20Booster/IMAG1115.jpg)
![Image](http://i196.photobucket.com/albums/aa166/kimokalihi/91%20Legacy%20Turbo/Brakes/Single%20Diaphragm%20Brake%20Booster/IMAG1116.jpg)
![Image](http://i196.photobucket.com/albums/aa166/kimokalihi/91%20Legacy%20Turbo/Brakes/Single%20Diaphragm%20Brake%20Booster/IMAG1117.jpg)
Start off by removing your master cylinder. This will get very messy. Suck out all the fluid with a turkey baster or similar tool. Then put some towels under it to catch brake fluid and remove all the hard lines from the master cylinder. You'll be left with something like this.
![Image](http://i196.photobucket.com/albums/aa166/kimokalihi/91%20Legacy%20Turbo/Brakes/Single%20Diaphragm%20Brake%20Booster/IMAG1110.jpg)
Then, go up under the dash and remove the four nuts holding the booster to the firewall and the cotter pin holding the brake pedal to the booster rod.
![Image](http://i196.photobucket.com/albums/aa166/kimokalihi/91%20Legacy%20Turbo/Brakes/Single%20Diaphragm%20Brake%20Booster/IMAG1111.jpg)
This is where I took one extra step that saved me a lot of hassle and time. Remove the cotter pin holding your clutch pedal to your clutch master cylinder rod.
![Image](http://i196.photobucket.com/albums/aa166/kimokalihi/91%20Legacy%20Turbo/Brakes/Single%20Diaphragm%20Brake%20Booster/IMAG1112.jpg)
Climb back out of the car and undo the two nuts holding your clutch master cylinder to the firewall and pull it out of your way. Now the booster can come right out really easily and the job becomes so much more enjoyable. I started to pull on my booster and saw that it was hitting the clutch master and thought,"I wonder why that guy never removed the clutch master? Seems like the only thing that its hitting." I was right.
![Image](http://i196.photobucket.com/albums/aa166/kimokalihi/91%20Legacy%20Turbo/Brakes/Single%20Diaphragm%20Brake%20Booster/IMAG1113.jpg)
![Image](http://i196.photobucket.com/albums/aa166/kimokalihi/91%20Legacy%20Turbo/Brakes/Single%20Diaphragm%20Brake%20Booster/IMAG1114.jpg)
![Image](http://i196.photobucket.com/albums/aa166/kimokalihi/91%20Legacy%20Turbo/Brakes/Single%20Diaphragm%20Brake%20Booster/IMAG1120.jpg)
Not to sound clitche, but the rest is the opposite of removal. Just put the single diaphragm booster in the firewall and the clutch master back in the firewall, install the nuts and cotter pins and put your brake master back on and put new fluid in and bleed everything. Then go out and enjoy your awesome pedal feel!
![Image](http://i196.photobucket.com/albums/aa166/kimokalihi/91%20Legacy%20Turbo/Brakes/Single%20Diaphragm%20Brake%20Booster/IMAG1122.jpg)
I had to contact Grimmspeed for a longer rod for my brake master cylinder brace.
![Image](http://i196.photobucket.com/albums/aa166/kimokalihi/91%20Legacy%20Turbo/Brakes/Single%20Diaphragm%20Brake%20Booster/IMAG1124.jpg)
![Image](http://i196.photobucket.com/albums/aa166/kimokalihi/91%20Legacy%20Turbo/Brakes/Single%20Diaphragm%20Brake%20Booster/IMAG1184.jpg)
![Image](http://i196.photobucket.com/albums/aa166/kimokalihi/91%20Legacy%20Turbo/Brakes/Single%20Diaphragm%20Brake%20Booster/IMAG1185.jpg)