I just got the car on the road and my brakes suck. I have a vacuum booster with a stock master cylinder. Corvette calipers up from and SSBC calipers in the rear. It is like I have no vacuum assist at all, but the vacuum can checks out ok.
The stock master is likely not moving enough fluid to work with your new calipers. Get the right master to match what the calipers need and you should be all set.
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Eric
69 Camaro - LT1 w/ 700R4, DSE springs, swaybar and Konis. Still rockin' it old school with 15" wheels with 60 series tires.