Hi everyone!
I have an EG5 Honda Civic from 1993 with no ABS from the factory that has a very weird brake problem. Mainly when doing highway speeds for a while, it will pull hardly to the right even on light brake pressure (I can't surely tell if it pulls from the front or goes left with the rear). Eventually, air will create in the system and the problem will reduce to a slight pulling, but the pedal will obviously get soft. The problem has largely increased since I installed new Hawk HP+ pads in the front.
Things I have done so far:
- replaced master cylinder (upgraded to a 15/16")
- replaced proportioning valve (was leaking from the rubber plug, replaced with a used one)
- replaced front calipers (upgraded to 262mm from the original 242mm)
- replaced all 4 brake lines with HEL Performance braided
- replaced brake fluid with RBF600
- checked and adjusted pedal free play and pushrod to master cylinder play
- endless bleeding of the brakes, in cross pattern (RR first, FL, RL, FR)
- front and rear alignment
- handbrake lever adjustment
All the caliper pins and sliders have been cleaned, all the pistons move freely, all brake discs are at the same temperature after applying the brakes (obviously rear are colder but equal between sides). None of the calipers are sticking and all the wheels turn freely.
Last weekend I disassembled one of the rear calipers to inspect it, and now I can't get a firm pedal anymore even after hours of bleeding. It has a soft zone at the beginning, in which you can feel the car slowing down, but it won't "bite" hard until half travel.
This whole situation is driving me mad. Does anyone have any ideas? Thank you!

 

By the sounds, I tend to think you did not do a thorough enough bench bleed of the master cylinder when you replaced it.

Also if you don't have the special pushrod depth tool from Honda it is very difficult to match the push rod to the brake booster. This can greatly effect brakes. Either not letting the rod to fully disengage or not fully engaging it.