Pad Knockback Issue

[Sleeper68]

Hi everyone,

I have been chasing a pad knockback issue on my 68 Camaro for awhile now. I just blew up the old M21 muncie and now have some time to focus on some things such as this knockback issue.

The reason I believe I have a knockback issue is the car will get a "low" pedal after I take a series of hard corners. The more corners I take without hitting the brakes, the lower the pedal gets. It is very stressful especially when tearing up the mountain roads where I live. The pedal gets so bad that I have actually learned to tap the brakes coming out of corners to bring the pads back up to the top. Once you tap or pump the brakes the low pedal goes away, until the next series of corners. This system works ok on the street but is very hard when autocrossing because the corners come up so fast. I have not yet learned to left foot brake properly (threshold) so I stll RF brake when racing.

Here are my braking system specs:

Master Cylinder, Input system, and pedal:
Wilwood, 7/8" Bore, tandem
No booster
6:1 factory pedal
SSBC proportioning valve
All flexlines are braided SS jacket, DOT approved.



Front
13" J55 corvette rotors
GM short spindles
factory 1st gen camaro drum brake hubs
SSBC D52 Floating Calipers, 2 piston, 43mm bores

Rear:
Wilwood kit WIL-140-12570-R
12.2" rotor with integrated parking brake drum
Wilwood D154 floating caliper, 2 piston, 1.125" bores
semifloat axle by Moser
Factory style axle bearings
2psi residual pressure valve (RPV)

One day I attempted to determine which end or corner of the car the knockback was coming from. I did this by taking a series of hard corners then slowing down using the engine only. There was a pull of up hill so I used the hill to slow the rest of the way. Once stopped I turned the car off and left it in gear. I then walked around the car and grabbed each of the calipers and attempted to move them, both inboard/outboard and fore/aft. The fron calipers were pretty dang tight and did not move much. The rears however had slop to them inboard/outboard. I then got back in the car, pumped the brakes 2 times until they felt fully firm and the pedal "up" and repeated the walkaround. The rear calipers were now tight like the fronts. At the time of the test, the prop valve was set 2 turns from full pressure to rear brakes.

This all leads me to believe the source of the knockback is the rear even though it is the end with the single RPV. THE RPV is plumbed just before the "tee" fitting on the axle.

My question is: What now?

I have a 4psi and 10 psi RPV. I also have another 2psi RPV. Thinking about trying the 10psi valve first, then if it works moving to the 4 psi valve.

Please let me know what your thoughts are. Thank you.

Ryan