slack in parking brake cable
I had the same problem('84 with 31"BFGs) with having to stand on the brake. The first time I tried to launch my 18' boat, the CJ parking brake wouldn't hold the jeep on the ramp. I took it home, and checked the amount of takeup or slack in the parking brake cable, and I was able to tighten the cable almost 3/4" before the rear brakes started to catch. When I took it down from the lift, not only was the parking brake fine ( holds great on the ramp with only 2 clicks of the pedal), but I can lock up the brakes with only moderate pressure-not like power brakes, but still a whole lot safer than using all my weight! This was after I took the jeep to a brake shop to have them give the system a once-over for general safety and wear...
I'm still planning to switch out my old lines for extended stainless braided brake lines ( added a 4" Tuff Country lift), which should improve things a little more.
"84 CJ7, 258-I6,4spd man. Tuff Country 4"lift, Herc'd tub, Painless kit, 4.88s,detroit rear, 35X12.5 Cooper ATs