C5 brakes/ master cylinder issues

[vette427-sbc]

I just finished fabbing up some C5 front brakes and LS1 rear brakes on my fathers '55 Chevy. We used a C5 master cylinder and a wilwood adjustable proportioning valve. Also using an 8" dual diaphragm booster.
Heres the issue:
The car stops, but not great. The pedal goes down pretty far, then gets very hard. Doesnt feel like there is air in the lines either. It just doesnt stop like I would think it should. The pads are Hawk HPS front and Napa premium rear pads. Rotors are new as well. We made a few gradually increasing hard stops to bed the brakes and it helped a little bit, but they still arent "biting". Are most brake pedal ratios the same or at least similar? I would have thought the C5 master would be perfect with the C5 fronts and LS1 rears. Do I need more break in time for the pads to bed into the rotors?

Heres a few pics:
https://static1.pt-content.com/images/pt/2011/08/c55-1.jpg
https://static1.pt-content.com/images/pt/2011/08/c59-1.jpg
https://static1.pt-content.com/images/pt/2011/08/c52-1.jpg

[CFster]

I know what I've got is different (C6 ZO6 brakes) but going to a 1" master and 9" dual diaphragm booster just about put me through the windshield. The difference was huge from a 8" booster.

[BulldawgMusclecars]

I used essentially the same combo recently on a g- body, and I used the stock G- bodY master. The C5 master should be fine though. I'd definitely look at the pedal ratio.

[Apogee]

Your description of the pedal feel really sounds like you have air in the rear circuit. I'd start troubleshooting by plugging both ports on the MC, cracking and bleeding each port, then checking the pedal. It should be high and firm, so if it is not, then you may need to re-bench bleed the MC. If that checks out, reattach the line to the front brakes (port closest to the firewall) and test the pedal again. If it feels the same, attach the rear brakes and test again...at this point you should at least know where the problem is if not exactly what it is.

The factory C5/C6 MC/booster combo is a 1" bore with a dual-9" diaphragm booster. The dual-9" booster provides about 30% more gain than the dual-8" booster you have, but even the dual-8" should stop better than you describe if it's getting adequate vacuum supply. What is your vacuum level at idle?

Pedal ratio could certainly play a part. Most power brake sytems are in the mid-3 to 4:1 range, give or take. What's your pedal ratio?

The Hawk HPS pads won't change much during break-in unless the rotors and/or pads were not sufficiently cleaned prior to and/or during installation. Althoguh some small gains can be had, properly bedding-in lower CoF pads like the HPS is typically more a function of maximizing life and performance of both the pads and rotors, not really increasing brake torque.

Tobin
KORE3