Dealing with a similar problem on my YJ; I have installed a new vacuum diaphragm can (it was leaking), and rerouted the vacuum hose from the valve cover to an electric vacuum pump. Braking my YJ w/ it's 37 x12.50's is now a lot easier, and I have the added advantage of having vacuum assisted braking even when the jeep engine stalls. It will be interesting to find out what was casing your hard braking condition; look forward to your post.

94 YJ, SOA, 2-1/2 Alcans, ARB-front, Detroit-Rear, 4.56:1 gears, Oasis Trailhead compressor, 4:1 Terra Lo, 37x12.50x15 SSR's, 8000 lb Ramsey, & etc.