I experience a lot of knockback as well, but this is coming from a guy who has floating calipers. I use C5 calipers with a 7/8 Wilwood MC.
Around Buttonwillow & Thunderhill, and the El Toro track and autocross, I can do a few laps and know reliably which braking zones I will experience knockback; some corners are completely trouble free, and some corners the pedal is undoubtedly soft in the same zone lap after lap..."gotta pump it up for turns 3,5,8...etc"
Deflection is definitely part of the problem; 28 spline axles and the original 8.2" rear aren't what I would call stiff.
But there is also another nagging problem and that's axle play (inboard/outboard motion of the axle). I'm guessing I'm not the only one who has 3/16" of axle freedom, and if you go around a combination of corners where one or both axles moves, there's another source of pads spreading. This became very clear to me in autocross; there is much more severe and abrupt lateral weight transfer and consequentially I must pump the brakes around the whole track to keep the pedal up. Also very prevalent after hitting track curbing on road courses, where you get a lot of impact that can move
and deflect the braking system.
I am about to install a 5lb knockback spring under the piston (or maybe a pair under each for 10lbs) to see if it helps.
The residual valve we all know does the same thing. I bought one but haven't installed it yet. I hate messing with brake lines so I want to try the springs first. Running a quick number on residual valves: 10psi residual pressure on a 40mm diameter (C5) piston = 19lbs. So it will be pushing against the piston almost 4x harder than the spring I'm installing.
If you keep the caliper pins lubricated and free from binding, the springs or valve should be able to do their job. I don't know if 5lbs, 10lbs, or 19lbs of force is enough, but I am going to start with 5lbs and work my way up.
I will be sure to post the spring install and results after RTTC.
