Vintage....are you running a stock 89 combo valve? With or without the o-ring?
I have yet to do my 96 dual booster, master and 4wheel disc ZJ combo valve swap, need to get it done cause I'm tired of moving the parts around in my garage........I'm running stock brakes up front with an SVO (Explorer, same as ZJ) disc kit in the rear. I do have the O-ring pulled from combo valve and the brakes are better than stock, but could be better still.
Wonder if the bigger M/C would be enough..........definitely be less work, but I'm going for the best ROI overall.