First off, great thread! I just spent several hours going over everything while covering the grave shift at work for the Christmas break. I think I have the solution to your mushy pedal... well actually, you already stated it in post
#1283 . You have the brake lines at your master cylinder on backwards. The front Master Cylinder port goes to the rear brakes and the rear port goes to the front brakes. I went out to the parking garage and verified this on my 91 EX. You'll notice that the master cylinder reservoir is mounted at a slight upward angle. The rear section of the reservoir has a larger available volume of fluid (front disks w/ large pistons) and the front has a smaller available volume of fluid (rear disks/drums with smaller pistons in calipers/wheel cylinders). If you look at the link in post
#1281 the picture shows that the first piston activates and pushes the second piston via hydraulic pressure (optimal) or my mechanical contact when nearing the end of its stroke as an O-s**t safety measure. Depending on who you talk to, front brakes perform about 70% of the vehicle braking. What you are experiencing is that the rear disks are doing the primary braking. When the first piston (rear calipers) reaches the second piston (front calipers) you have used up all of the master cylinder stroke which +mushy pedal and weak brakes. This is also why you have had some minor improvement when adjusting your rear proportioning valve.