'70 Mach I brakes

[Trey Ragsdale]

In trying to get more holding power for rear brake launches on my '70 Mach, I replaced my stock rear brakes (2" X 10") with bigger brakes (2-1/4" X 11") from Currie. The bigger brakes actually don't have quite as good static grip as the stock ones did! After 500 plus miles the drum and shoe surfaces have minimal wear, so little that some parts of the shoes linings have not broken the original surface. It's leads me to believe the rear brakes aren't getting the pressure they should - but nothing else in the brake system was changed. I even tried sanding the drum and shoe surfaces to trie to help "seat" them. I do have a whistling sound that I think is coming from the brake booster and suspect it is getting weak, but I don't know how that could affect the rear brakes. I haven't compared the wheel cylinder bore sizes between the stock and bigger brakes yet, but am curious if a size change would require more rear brake pressure bias. Speaking of that, would a brake proportioning valve on the front brakes give me more rear brake in a static situation (sitting at a stop) or do they only work in a dynamic situation (applying brakes while moving)? Thanks for your input.