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Discussion Starter #1
So I finally converted the old nag from 1.5" to 2.25" wide rear brakes last year. I used the wheel cylinders that were on it already, they were in pretty good shape. If you check the specs for the mythical 6 cyl convertible rear brakes, there's a slightly different diameter cylinder listed there, though not in parts catalogs. I've also noticed that for the parts places that list 6 cyl rear wheel cylinder diameters in their description, they often disagree with one another.There is a small change in spec from early 65 to later, maybe that gets them confused. Next time I buy any I'm gonna measure the bore and see if it's as advertised. Anyway, there are a few slightly different diameter cylinders out there. I didn't drive it much after the conversion because I'm usually working on something these days. I didn't even insure it until the end of summer last year. So the brakes aren't broke in enough to make any serious judgments yet. I didn't notice anything other than they worked fine when I did drive them. But someday I'll be able to slide around a little and see if there's a problem with the front/rear brake bias. Yes, I could put in a proportioning valve, but couldn't I achieve the same thing tweeking the rear cylinder diameter? Please save me from finding my old physics book and looking up hydraulics formulas. Although with the wheel cylinders and the master cylinder (Look out Felix!) having similar diameters, that shouldn't be a huge factor. It seems because the brake fluid displaced by the master cylinder has less volume to fill in a smaller wheel cylinder, it would want to move out farther. Seems like that would make it brake harder. Or maybe the hydraulic laws say different, and bigger is better. I've been impressed with the depth of technical expertise some of the people on here have shown. I appreciatively await your wisdom.
 

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You can fine-tune brake balance with different size bore wheel cylinders. A bigger bore will put more pressure on the brake. A smaller bore will put less force on the brake. I tiny change in bore may change the rear bias quite a bit. If the bore is too big the rear brakes will lock early. When converting to Granada disks I tried using the Granada rear wheel cylinders. My 70 Cougar specifies 29/32 rear wheel cylinders and Granada spec is 15/16. The result with the 1/32" bigger wheel cylinder was the rear wheels easily locked up at speeds below 30 MPH. I put the 29/32 wheel cylinders in and all was well with brake balance. The bigger wheel cylinders did seem to work well when braking at high speeds. But in city driving or rain the car was not safe.
 

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Discussion Starter #3
Thanks cougar70. Seems like my Mustang went through brake shoes faster than my other cars. The six cylinder convertibles are heavier than an eight cylinder coupe, yet they still get the wimpy little brakes. Hopefully changing to the wider back brakes will help a little. I'm hoping using the same wheel cylinders will be okay. I never used to have much for a Mustang manual, I've scrounged a couple factory manuals that help me some now, one for 1962 Falcons, and a four volume set for all 1969 models. The Falcon factory specs say the same cylinder is used for the sedans with 1.5" brakes and the wagons with 2.25", similar to my current situation. The 1969 manual shows some models get bigger back cylinders, but not always wider shoes, and some front brakes are changed too. I need to set up a spreadsheet or something to see if there's a discernible pattern. Looking back through previous posts, you can find arguments both ways for smaller or bigger cylinders braking harder. Real world experiences like yours are something to heed.
 

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Since you've converted brake parts, all bets are off for getting what you want in braking. From now on, it'll be trial and error. The braking system worked perfectly when the engineers built these cars years ago and they'll do it again (if everything is the way it was designed). Hopefully, you'll get something put together that works great. If you do, be sure to record/share what you end up with so that others here could use it on their cars (if they want). That's what this forum is all about.
 

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Discussion Starter #6
Have you worked with engineers, and the corporate bean counters who control them? Perfection is seldom very high on the goals list. Profit always is.
 

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Discussion Starter #7
Well here's an easier question for you all. Although Ford evidently abandoned its project to put the Falcon wagon rear drum brakes on the 6 cyl Mustangs, they must have gotten pretty close to pulling the trigger. The specs for the change made it to parts stores, you still see some catalogs with the wider front drums also specified for the convertible rears. I learned long ago not to mention my car was a convertible or they'd try to give me the wrong parts, because my car had the standard back drums. It's been a long time since I've seen them also listed, but I have a dim memory that the convertible wheel cylinders were a different size too. But I could be wrong there. Anyway, anyone have an old 1966 factory manual, or some other kind of specs document that lists brake shoe and drum sizes, and especially the wheel cylinder bore sizes? What's listed for the 6 cyl convertible rears? I'd be happy to give the wheel cylinders Ford originally intended for my back brakes a try, if I knew what they were. Until then, an empirical determination is all I've got. I haven't even heard from anybody else who swapped their back brakes too. I find that surprising, as there's a lot of six cylinder convertibles out there. And large parts of this forum are by and for people radically re-engineering every single system on their cars. I've mentioned this topic in a number of posts, nothing but crickets There's gotta be other people that have done this.
 

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RockAuto lists drums and pads, fitting both front and rear convertible.

There are wheel cylinders listed 16/32 for rear hardtop and rear wheel cylinders listed 27/32 that do not specify hardtop or convertible, these are for left or right side.
 
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