Manual front Disc Brakes?
- Thread starter coalman
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Hard to say. It definitely needs more pressure than manual 9" drums, since I did that swap last year (of course, they stop much better, too). Discs always require more pressure than drums, because drum shoes are designed to be "self tightening" -- as they grab the drum, the rotation pulls them in tighter, increasing braking force without requiring as much additional pressure. Discs are strictly linear -- more pressure equals more braking. BTW, this is why disc/drum systems need a proportioning valve (it's not to "balance" front/rear braking). You can adjust the amount or pressure required by choosing a different disc brake master cylinder bore -- a smaller bore requires less pressure, by slightly increasing the pedal travel distance (like using a longer wrench to get more torque). Sizes available include 15/16, 7/8, 1, 1-1/32, 1-1/8 (not all will be listed for A-bodies). Most people like 15/16 for manual brakes -- this is "stock" for early 70s A-body *power* disc brakes. Manual brakes came with the 1-1/8 bore, which is pretty macho.
Stock A-body master cylinders were 15/16" for 9" drums, 1-1/32" for manual floating disks, 15/16" for power floating disks, and 1" for the KH disks. The 1-1/8" master cylinders were for the 70-72 B/E body single piston KH disks and weren't used on A-bodies.
I like the 15/16" bore for manual disks. It's not really any more leg than was required for the manual 10" drums, which used the 1-1/32" master cylinder. I actually think the 15/16" master cylinder improves your brake modulation capabilities quite a bit with the slightly longer travel, but it still provides the max line pressure.
From the '73 service manual, "L" is the Dart model. The 2.75" calipers were B/E body fare, pin style in '73 hence the "floating" designation, the 2.6's were A-body slider calipers.
From the '70 service manual, first the fixed caliper KH's used on A's
And the single piston floating caliper KH's used on the 70-72 B/E. You can see the wheel options on the bottom for Barracuda, Satellite and Fury models (Plymouth service manual)
I like the 15/16" bore for manual disks. It's not really any more leg than was required for the manual 10" drums, which used the 1-1/32" master cylinder. I actually think the 15/16" master cylinder improves your brake modulation capabilities quite a bit with the slightly longer travel, but it still provides the max line pressure.
From the '73 service manual, "L" is the Dart model. The 2.75" calipers were B/E body fare, pin style in '73 hence the "floating" designation, the 2.6's were A-body slider calipers.
From the '70 service manual, first the fixed caliper KH's used on A's
And the single piston floating caliper KH's used on the 70-72 B/E. You can see the wheel options on the bottom for Barracuda, Satellite and Fury models (Plymouth service manual)
Great data, though I have one quibble -- you don't want to confuse drum brake MC bores with disc brake MC bores -- they are different items.
Hmm, not sure I understand. The 10" drums and 10" disks do use the same size master cylinder bores.
The master cylinders themselves are not the same though, disk/drum masters have a larger reservoir for the disk circuit and and a different residual valve pressure for the disk circuit as well. I guess I didn't make that part clear, a 15/16" drum master cylinder isn't the same as a 15/16" disk master cylinder. Is that what you meant?
Thank you also!!
Thanks a bunch....
2 Darts
A-body Addicted
This statement is only true of the "leading shoe". On a typical drum brake, the shoe in the direction of travel becomes the "leading shoe" if the wheel cylinder is at the top of the backing plate. I did a brake job on a '55 New Yorker that had a twin leading shoe front brake; it had two single ended wheel cylinders connected by a metal tube. FWIW, the rears were single leading shoe design. When the parking brake is engaged, the rear shoe pivot point is at the top.
As far as more pressure goes, I can't say. The '73 Dart mordor I converted to disk brakes already had a DB master cylinder on it. I do not recall any increase in effort needed after I installed the single piston floating caliper units. Actually, IMO in typical stop-and-go traffic, effort with disk brakes stayed fairly constant, whereas with drums, more effort was needed to get the same stopping effect towards the end of a trip.
famous bob
mopar misfit
4 wheel discs, stock 68 barracuda non-proportioning valve from original drum brake set up, w/ 87diplomat master cyl., works excellent.
Thanks..
ir3333
Well-Known Member
the proportioning valve reduces pressure to the rear brakes and the hold off valve incorporated in the combination valve delays pressure build to the rear so the front not only apply braking first but also supply about 70% of the total braking.That's why going to rear discs is not usually a big improvement...since the rear only supplies about 30% of your stopping power.
..yes discs require much more pressure (3500 - 4000 psi) that's why the pistons are 2 5/8" or 2 3/4" in the calliper but only 1" or 15/16" in the wheel cylinder.
get the proper master cylinder and a manual disc front / drum rear system will stop effortlessly with very little leg pressure
for a manual front disc / rear drum system most like no bigger than 1" or 15/16"master.
..a 15/16" master cylinder was stock for early A body manual disc front drum rear.
Put a 1 1/8" master on a manual front disc car and a lot of guys won't be able to lock them up.
..yes discs require much more pressure (3500 - 4000 psi) that's why the pistons are 2 5/8" or 2 3/4" in the calliper but only 1" or 15/16" in the wheel cylinder.
get the proper master cylinder and a manual disc front / drum rear system will stop effortlessly with very little leg pressure
for a manual front disc / rear drum system most like no bigger than 1" or 15/16"master.
..a 15/16" master cylinder was stock for early A body manual disc front drum rear.
Put a 1 1/8" master on a manual front disc car and a lot of guys won't be able to lock them up.
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roccodart440
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BillGrissom
Well-Known Member
Good comments. One possible correction - research "front metering valve" in the combo valve, which I think is the same as "hold-off valve". I recall reading that it holds off pressure to the front disks until the rear drum shoes move to contact the drums. I think it is simply a spring-loaded "pressure relief" valve that pops open ~10 psig or so.
Lets not even get into comments in another post that rear disks are better since rear drums "are not strong enough". Nobody explained how that meshes with needing a proportioning valve to reduce pressure to rear drums so they don't lockup the rear wheels.
ir3333
Well-Known Member
you are probably correct ...delay and reduce the pressure to the rear may be just as
you suggest....been a long time since i got into this.
you suggest....been a long time since i got into this.
In my real world experience, the pedal pressure between my old 10" drums and the K-H conversion I did is actually a little easier on the foot than using the drums!! I love the manual disc, and the manual steering in my 66, and never once did the bride ever complain that either was too hard to drive with!!!
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