I got an 88 YJ booster and master cylinder from the junk yard for about $25. Bought a new master cylinder ($20)using the salvage one for a core. The push rod length nor the bolt pattern are the same for your '75 (Mine's a '74), I simply drilled new mounting holes and bought an adjustable plunger rod from NAPA. You have to re-route the brake lines, I made new ones, and hook up a vacuum line, bleed the brakes and, presto, no brakes when going on that long, steep downhill when the engine dies and all your vacuum dies with it!
I ended up sticking with no booster but kept the YJ master cylinder. I also have front disc's off a '79 CJ, and that really, really helped. Some people love 'em, some people hate 'em, thats whats so great about wheelin', there's so many options and so little money!