The only major dynamics complaint I had about my Ultralite was the rear lockup issue under hard braking. The car comes with a very nice set of Wilwood brakes (front and rear) with separate master cylinders for the the front and rear so it definitely wasn’t a component quality problem.
The issue I was having is that under hard braking at autocross the rears would immediately lock up and cause a lot of oversteer. This was even worse at the track where braking at the end of straights would lock the rears well before the fronts leading to bad porpoising at the rear. The whole issue was being caused by having equal braking capability front to rear which meant that once the weight transferred forward under braking, the rear wheels had very little weight on them and were prone to locking up while the fronts were still spinning. You could avoid this by braking earlier but this would mean that the front brakes would not be used to their full ability and would lead to longer stopping distances. The real solution would be to move the brake bias forward which I first attempted to do using the balance bar in the pedal box/master cylinder setup. This made no noticeable difference to the brake bias which meant that the bias issue was worse that I suspected.
Step two was to install an inline proportioning valve(below) on the rear circuit which could reduce the rear line pressure by up to 57%. While this did make the autocross situation much better I still had the problem of rear porpoising at the track albeit much better than before.
After consulting with some of the folks on the various forums I came up with two options: 1) replace the brake caliper with a smaller caliper and 2) replace the rear master cylinder with a larger piston. Since the piston was significantly cheaper than getting 2 brake calipers I decided to replace the MC with a 7/8th inch piston (originally 3/4 inch) which has been a great solution so far and has allowed me to run a much more balanced car at the track.
If you are building a car from scratch I would recommend going for a smaller set of calipers in the rear and/or smaller brake rotors both of which will save you some unsprung weight as well. I believe that the later Ultralite owners went to using stock 12-inch Subaru rotors in the rear which sounds like an economical solution as well. If you have a car with the original setup I can say that this solution of larger MC and bias valve has worked well for me.