Like many,. I am swapping to front discs and will maintain the rear drums. It will be manual setup. I read recently that you can get by without adding the disc/drum adj prop valve if you change to a 13/16" wheel cylinder bore. Has anyone done this swap without installing the adj prop valve or what are thoughts relative to the cyl bore on the rears? Thanks!
Rear Wheel Cylinder Bores - disc swap
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AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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I did that on a 68 Barracuda.
The rear w/c size will depend on the rear tire size as compared to the fronts. The more tire patch that you have on the rear, the more the bias can be shifted to the rear.
The factory limits the rear line pressure to about 75% of the front.
>If you are running big and little tires, then you can power up the rear pressure possibly to the same as the fronts; but you may have to limit the w/c size to prevent premature rear wheel lock up.
There are 3 sizes available 13/16, 7/8,& 15/16. Which one will work for you is anybody's guess. I suggest you start with what you got, and find an abandoned parking lot to begin testing. If the rear locks up first, there's a good chance it will try to pass you. Obviously that would not be good. Try the next smaller size;they are cheap!
>However, if you are running 4 same-size tires, then you may need to limit the rear line-pressure, with an aftermarket P-valve, or install the factory combination valve from a discbrake car. Wheel cylinder size alone may not prevent the issue.
FWIW, I run the 15/16 w/c with 295/50-15s, and no pressure-limiting; in combination with the KH 4-piston front dbs and 235/60-14s. I ran the same parts on 275 rears and it was good.
I could run a bigger w/c on the rear with the 295s,or perhaps bigger rear drums, but the car stops so nice that I just want to drive it,now.
So to recap; it depends on your rear contact patch, as it compares to the front. A long narrow patch is better than a short wide patch, size for size.
The rear w/c size will depend on the rear tire size as compared to the fronts. The more tire patch that you have on the rear, the more the bias can be shifted to the rear.
The factory limits the rear line pressure to about 75% of the front.
>If you are running big and little tires, then you can power up the rear pressure possibly to the same as the fronts; but you may have to limit the w/c size to prevent premature rear wheel lock up.
There are 3 sizes available 13/16, 7/8,& 15/16. Which one will work for you is anybody's guess. I suggest you start with what you got, and find an abandoned parking lot to begin testing. If the rear locks up first, there's a good chance it will try to pass you. Obviously that would not be good. Try the next smaller size;they are cheap!
>However, if you are running 4 same-size tires, then you may need to limit the rear line-pressure, with an aftermarket P-valve, or install the factory combination valve from a discbrake car. Wheel cylinder size alone may not prevent the issue.
FWIW, I run the 15/16 w/c with 295/50-15s, and no pressure-limiting; in combination with the KH 4-piston front dbs and 235/60-14s. I ran the same parts on 275 rears and it was good.
I could run a bigger w/c on the rear with the 295s,or perhaps bigger rear drums, but the car stops so nice that I just want to drive it,now.
So to recap; it depends on your rear contact patch, as it compares to the front. A long narrow patch is better than a short wide patch, size for size.
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good pedal quality? Any concerns or challenges experienced?
Relative to this, is changing from 9 in. rears to 10 in., in addition to the disc upgrade, worth the effort? Can you even safely put a 9 in. disc kit on front with 10. in rear drums?
Sedanman
67-9 Valiant specialist
The ones I have pulled off of disc/drum cars was 15/16 or 13/16
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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I run a dual M/C with a 15/16 bore, and the fancy teflon-lined front hoses. And as mentioned, the factory KHers, with 10x2.5s out back. Also a booster from a mid-70s Dart.
Awesome brakes! Stops like crazy with the big tires. Great modulation. Initially, I had a bigger bore in the M/C and it was harder to modulate on the edge. Yeah it had a harder pedal, and it took a bit more leg power. But braking on the edge of lock-up was trickier. And I was just 47 at the time so don't think that I was; old, weak, or ready to be put out to pasture,lol .
When I say brakes like crazy, I mean it's like throwing out a giant anchor; Hang on Lucy!
'Course I have a10.9Scr, Manual-tranny-equipped, fire-breathing,6 liter engine; with the idle speed set up so low that when I take my foot off the gas, that too tends to want to slam your nose into the dash-pad; so much so that the car is real easy on front brake pads.Compression braking is not to be taken lightly!,lol
Awesome brakes! Stops like crazy with the big tires. Great modulation. Initially, I had a bigger bore in the M/C and it was harder to modulate on the edge. Yeah it had a harder pedal, and it took a bit more leg power. But braking on the edge of lock-up was trickier. And I was just 47 at the time so don't think that I was; old, weak, or ready to be put out to pasture,lol .
When I say brakes like crazy, I mean it's like throwing out a giant anchor; Hang on Lucy!
'Course I have a10.9Scr, Manual-tranny-equipped, fire-breathing,6 liter engine; with the idle speed set up so low that when I take my foot off the gas, that too tends to want to slam your nose into the dash-pad; so much so that the car is real easy on front brake pads.Compression braking is not to be taken lightly!,lol
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