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Ok....

Im doing a complete frame off on my 67 GTO but im stumped on the rear brake drum. Here is the situation when I took the rear drum cap off there was a wheel cylinder that was a 15/16 bore. When I go to look up replacement parts for the 67 the only one that pops up is a wheel cylinder with a 7/8 bore. I went ahead and ordered the one with a 7/8 bore and put it on the backing plate now the problem is when I put the pistons on the wheel cylinder it pushes the brake pads all the way out and there is no way that I can push the cylinder in any more. Im starting to wonder if maybe once in the car life time if the rear end was switched out with a 66 or maybe im just not getting it right. When I look at the other side with the 15/16 bore everything is sitting in place correctly. If I was to order the 15/16 bore and place it back on there would it effect anything? Will my brake lines still fit? Just to give you a run down on what I have going on. I did a disc brake conversion on the front and the portioning valve is a Disc/Drum set up now. Also, the I have also added a brake booster for power assist. Thanks in advance.
 

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'64-72 Pontiac A body station wagons typically had 1/16" larger diam wheel cylinders. Olds and Buick A body's used wider drums and wider sealed axle bearing axles, often different rear backing plates, and the oddball wheel bearings used on the early Buick and Olds a body SW's are a PITA to source replacements for.

45+ years of brake jobs...Over the years, wheel cylinders were often replaced. Occasionally, entire loaded backing plates have also been swapped out, not just rears, so not that surprising. Substituting 1/16" larger bore wheel cylinders has been a fairly popular mod for decades, typically it's combined with a slightly larger bore master cylinder. I have a couple pair of A body SW wheel cylinders, will be doing a comprehensive comparison braking test with, combined with special matrix shoes, stiffer return springs, and cryoed drums. the special matrixshoes and springs are from Muscle Car Brakes.
 
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