340sFastback
Well-Known Member
I run 7/8 with proportioning valve set to send full pressure to rear wheels seems to be working. But maybe I swap in 13/16 at some point.
Which master cylinder bore size to use for power disc Kelsey/Hayes
- Thread starter cudajim
- Start date
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slantsixdan
=..=
But also sounds like something the matter with the front brakes. Maybe it'll clear up once you have a good master cylinder and a proper bleed job…but could also be poor choice of brake pads, brake pads improperly bedded in/glazed, one or more stuck pistons in the calipers…
As far as what master cylinder to use, Dana67 is correct. Make sure you use a disc brake master cylinder, not a drum brake one. Such as this one.
As far as what master cylinder to use, Dana67 is correct. Make sure you use a disc brake master cylinder, not a drum brake one. Such as this one.
cudajim
cudajim
I took a wheel off the rear and rechecked. I am running 3/4" bore size in the rears.
Are the primary and secondary shoes in the correct location?
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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Step on the pedal;
If it is not high and hard right away; Something is wrong
Give it three rapid strokes and immediately keep the pedal down; If it is now high and hard; then something is not adjusted right. Could be the rear shoes, but could also be that the front pads are not properly fitted.
But if it never gets hard then; one of four things is wrong;
1) there is air in the system, or
2) the front hoses are expanding, or
3) the M/C is leaking internally, or
4) the pushrod is not adjusted properly
All of these are simple to find and simple to correct.
I have those KH stoppers,
with braided hoses, and the standard 10x 2 rear shoes with 15/16 W/Cs, and a 15/16 M/C with a Booster off a mid 70s F-body; All parts were used when I got them, and rebuilt by me.
I run 295/50-15s on the back and 235/60-14s on the front with NO PROPORTIONING of any kind and without exception, these are the best A-body brakes that in 54 years of driving almost exclusively As and Fs, I have ever had.
What I'm saying is; Keep after it;
the parts and the combination thereof, are more than adequate for a street car. Randomly changing parts may cure the problem, or it may not.
BTW
Make sure your front wheel-bearings are adjusted accurately. Because on these brakes the caliper is fixed, as the rotor turns, it knocks the pistons back into their bores. This creates the clearance that prevents the pads from dragging, a good thing. But if the bearings are loose or the rotors are warped, the pistons may get knocked back too far, and the next time you step on the pedal, they come late to the party if at all.
Contrary to the factory literature, these rotors can be machined on modern brake lathes.
If it is not high and hard right away; Something is wrong
Give it three rapid strokes and immediately keep the pedal down; If it is now high and hard; then something is not adjusted right. Could be the rear shoes, but could also be that the front pads are not properly fitted.
But if it never gets hard then; one of four things is wrong;
1) there is air in the system, or
2) the front hoses are expanding, or
3) the M/C is leaking internally, or
4) the pushrod is not adjusted properly
All of these are simple to find and simple to correct.
I have those KH stoppers,
with braided hoses, and the standard 10x 2 rear shoes with 15/16 W/Cs, and a 15/16 M/C with a Booster off a mid 70s F-body; All parts were used when I got them, and rebuilt by me.
I run 295/50-15s on the back and 235/60-14s on the front with NO PROPORTIONING of any kind and without exception, these are the best A-body brakes that in 54 years of driving almost exclusively As and Fs, I have ever had.
What I'm saying is; Keep after it;
the parts and the combination thereof, are more than adequate for a street car. Randomly changing parts may cure the problem, or it may not.
BTW
Make sure your front wheel-bearings are adjusted accurately. Because on these brakes the caliper is fixed, as the rotor turns, it knocks the pistons back into their bores. This creates the clearance that prevents the pads from dragging, a good thing. But if the bearings are loose or the rotors are warped, the pistons may get knocked back too far, and the next time you step on the pedal, they come late to the party if at all.
Contrary to the factory literature, these rotors can be machined on modern brake lathes.
Is the 1 1/32 aluminum a direct bolt on or does it need any mods?
Do you have a Part number/manufacturer for it?
slantsixdan
=..=
Then your problem's elsewhere
340sFastback
Well-Known Member
Thats a nice part but if you have manual brake kelsey hayes disc brakes you will be putting both feet on brake pedal and still not stopping with that large bore master cylinder.
The stock bore is 1"
This master is only 0.03125" larger in diameter.
.835 sq in
.785 sq in
It's only 6% more force.
If you press with 250 lbs of force you add 15 extra pounds. Negligible.
Not to mention, I bet all 1 inch mc that have been rebuilt once or twice are close to that size
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Inertia
Well-Known Member
I have actually changed thru all 4 m/cs in an afternoon at teh track to get a realistic comparison, there is significant difference.
We also went thru a series of different composite pads for reality.
We also went thru a series of different composite pads for reality.
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I do not see any mention of braking co-efficient. The friction co-efficient of the pads & shoes.
They are available in different materials that changes the brake feel...& the amount of force to generate braking.
They are available in different materials that changes the brake feel...& the amount of force to generate braking.
Kendog 170
Let the boy go !
Well as I said my pedal is rock hard and would take adrenalin to lock them up. I might go to the 15/16 one this summer.
340sFastback
Well-Known Member
Yeah over the decades I ran the stock size bore and some I bought prob bigger bore. Now I run a 15/16 bore with EBC reds, steel braided flex lines, 7/8 rear cylinders. If other ppl like using two feet to stop, 6 percent more force or what ever, go ahead put what ever you want on your car don't matter to me.
Inertia
Well-Known Member
All the m/c diameters worked in our tests.
The difference was in the "feel and control" over the application and release, - not how much effort.
The travel of the pedal, and the drivers control over the brake modulation.
Saying it takes n% more, - more than what, - you have to feel the difference, a number is just a number until your able to compare by feel.
The difference was in the "feel and control" over the application and release, - not how much effort.
The travel of the pedal, and the drivers control over the brake modulation.
Saying it takes n% more, - more than what, - you have to feel the difference, a number is just a number until your able to compare by feel.
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More than a 1" bore.
I agree it is feel but if you pick up a 100lb box then a 106lb box are you really going to notice it?
If on the other hand you can barely lift the 100 lb box you might not be able to lift the 106 lb box
Most braking is done with very little foot pressure.
Now the 14% less force 15/16 mc you might notice that.
I suspect your testing was on a road course?
Where heavy braking and highly controlled braking are needed.
I'm not arguing with your testing or results I just suspect that we are talking apples to oranges.
It will not hurt my feelings if you correct my assumptions
Inertia
Well-Known Member
I totally respect mosta your stuff, You have totally valid points,
I'm just old school. Lol
"show me" lol
I'm totally with you.
I would want a blind test with multiple people to do the feel testing.
junkyardhero
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i dunno man, that might scare the **** out of their seeing eye dogs...
This is the correct factory one. It’s new old stock for 67 to 70
I have them for sale in for sale section if needed
I have them for sale in for sale section if needed
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puertobob
Member
My 69 had the big master and wouldn’t stop. Called Dr Diff. He said 15/16” is. Correct. I have no more problems. Works awesome now.
I have owned K-H 4 piston brakes on my 68 FSFB for years and the only issue I have with them is the pistons stick. Change seals on the pistons and replace any pitted pistons. This is paramount to getting good braking on the front, and as other said, a proportioning value is absolutely necessary.
Ultimately, I went with the 11.81" cop rotors and '73+ spindles/brakes. These are absolutely the best brakes I have had on my Cuda, and they are 100% factory, just not from '68. In the rear I did the Jeep Liberty swap to disks. Low-cost conversion per Mopar Action article and they work great.
Ultimately, I went with the 11.81" cop rotors and '73+ spindles/brakes. These are absolutely the best brakes I have had on my Cuda, and they are 100% factory, just not from '68. In the rear I did the Jeep Liberty swap to disks. Low-cost conversion per Mopar Action article and they work great.
340sFastback
Well-Known Member
Right but ppl are gonna say only 6 percent more leg effort with larger bore master cylinder. I run 15/16 on mine too, manual brakes, and not much leg effort to stop car. The smaller bore does give pedal more travel but if brakes bled correctly not a problem
Swinger 340
Well-Known Member
I did a changeover myself a couple years ago. Did you install new flexible brake lines at the rear axle and front calipers. They soften with age and expand when braking. Your issue has nothing to do with the MC bore size. A bad seal yes, but I don't think so in your case.
340sFastback
Well-Known Member
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