More disc brake questions

-
moparmat2000

moparmat2000

Well-Known Member
Joined
May 13, 2010
Messages
20,882
Reaction score
10,858
Location
Grand Tetons
I have a disc drum setup for my 67 barracuda that consists of 10" X 2.50" bbp drums on the back, and 11.75" rotors with 2.75" piston calipers on the front. (Stock 79 cordoba setup)

I purchased a couple 85 diplomat master cylinders. The bore size is 1-1/32" bore, i'd like to run one of these as a manual brake setup.

Is this enough to get a good pedal feel out of the brakes, and enough piston diameter in the master to actually get it to stop, also is the 85 diplomat master cylinder metric or SAE thread for the fittings.

These masters were closeout from rock auto and at $12 and some change, so i bought 2 of them. My other cuda a 69 will have 85 M body disc brakes so this master was a natural for it, but i was wondering if it would also work for my 67s setup with the 79 'doba brakes.

Thanks
Matt
 
I had to use the 1 1/32 bore MC with the 2.75 bore calipers. I couldn't get a pedal with the smaller bore MC. I can't answer the other questions. Try threading a standard fitting into the MC to see if it fits.
 
From Dr. Diff site:

Master Cylinder/Caliper Bore Size - Quality Body Shop Drivetrain

Master Cylinder/Caliper Bore Size

All else being equal…
  • Increasing the master cylinder bore diameter yields a shorter, harder pedal stroke with decreased clamping force at the brake pads.
  • Decreasing the master cylinder bore diameter, yields a longer, softer pedal stroke with increased clamping force at brake pads.
  • Increasing the caliper bore diameter, yields a longer, softer pedal stroke with increased clamping force at brake pads.
  • Decreasing the caliper bore diameter yields a shorter, harder pedal stroke with decreased clamping force at the brake pads.
 
I think you will be O.K. but you have to run a adjustable proportioning valve. If you fine tune the front to back braking force it should throw you through the windshield. If not down size the caliper piston.
 
The 1 1/32" master cylinder will be ok. I prefer the 15/16" masters myself, they give better feel and stopping power. I also had absolutely no problem getting a great pedal on my Challenger which runs 11.75" front disks, 2.75" bore calipers (also stock for E bodies) and 11x2.5" drums out back. The brake feel with that set up is great. Also, I had no need for an adjustable prop valve on that car, even with the larger rear brakes. That's a small block E-body though with 275/40/17's on all 4 corners so your balance and front/rear weight bias may be different.
 
72bluNblu said:
The 1 1/32" master cylinder will be ok. I prefer the 15/16" masters myself, they give better feel and stopping power. I also had absolutely no problem getting a great pedal on my Challenger which runs 11.75" front disks, 2.75" bore calipers (also stock for E bodies) and 11x2.5" drums out back. The brake feel with that set up is great. Also, I had no need for an adjustable prop valve on that car, even with the larger rear brakes. That's a small block E-body though with 275/40/17's on all 4 corners so your balance and front/rear weight bias may be different.
Click to expand...
Whats the master bore your using on the challenger,
 
moparmat2000 said:
Whats the master bore your using on the challenger,
Click to expand...

It has an aluminum 15/16" bore MC from dr diff, just like my Duster. It was power before that with a 1-1/32" master. That's a little different than an A-body though because of the linkage for the power brakes.
 
furyus2 said:
I had to use the 1 1/32 bore MC with the 2.75 bore calipers. I couldn't get a pedal with the smaller bore MC. I can't answer the other questions. Try threading a standard fitting into the MC to see if it fits.
Click to expand...

I used 1" with 2.75" callipers on a Duster and it was fine....like power brakes.
For each 1/16" increase in master cylinder bore i think you need 12% more leg.An increase of 1/8" in the master bore would require 25% more leg.
..stick with one inch or 15/16"
 
I had a AAR that I raced and when I changed the torsion bars to the .830 and 90/10's it changed the weight transfer and the rear wheels would lock up on hard braking. Installing a adjustable valve I could make it stop with out locking any wheels on straight stops and when it was set up for corners just turn the knob and it was set up for the stiff suspension.
Don't let anyone talk you out of a adjustable valve.
 
some of the 60's heavyweights with front discs did not use a prop valve...like big station wagons.
smaller cars with front disc can tune their brakes without a prop valve using different rear wheel cylinders and bigger or wider rear tires which are harder to lock up.It's trial and error but the the best is using an adjustable valve.
 
I looked up 78/79 cordoba master cylinder on rockauto. 79 shows aluminum with 1&1/32" bore, the 78 shows a cast iron one with a 1&1/32" bore as well. Both with 11.75" rotors and 2.75" piston calipers. Can i assume that this bore has something to do with power brakes which both year cordobas have as std equipment?

What if i rebuild my power booster and attach this master to it?
 
Make sure it's a manual master if you are not running a booster, the master's are different, and a power master wont hold the rod, the manual master has a deep hole that the brake push rod locks into!
 
brian6pac said:
I had a AAR that I raced and when I changed the torsion bars to the .830 and 90/10's it changed the weight transfer and the rear wheels would lock up on hard braking. Installing a adjustable valve I could make it stop with out locking any wheels on straight stops and when it was set up for corners just turn the knob and it was set up for the stiff suspension.
Don't let anyone talk you out of a adjustable valve.
Click to expand...

Unless you don't need one. 60k+ miles on my Challenger with that brake set up, and with 1.12" torsion bars up front, 275's all the way around and even the 11x2.5" drum brakes out back it would lock the front's slightly before the rears. No adjustable prop valve can change that (they can only reduce the rear pressure), none needed. You only need one if you need to reduce the pressure to the rear brakes. And if you're not constantly swapping suspensions or tires, you only need to set it up once.

ir3333 said:
some of the 60's heavyweights with front discs did not use a prop valve...like big station wagons.
smaller cars with front disc can tune their brakes without a prop valve using different rear wheel cylinders and bigger or wider rear tires which are harder to lock up.It's trial and error but the the best is using an adjustable valve.
Click to expand...

Exactly. There are a lot of factors that determine the brake bias. Tire size, brake size, wheel cylinder and caliper bore size, front/rear weight balance, spring rates, etc. There are a few different rear wheel cylinder bores out there for these cars, but if I needed to back the rears off I would be more apt to install an adjustable prop valve, just makes it easier to tune.

moparmat2000 said:
I looked up 78/79 cordoba master cylinder on rockauto. 79 shows aluminum with 1&1/32" bore, the 78 shows a cast iron one with a 1&1/32" bore as well. Both with 11.75" rotors and 2.75" piston calipers. Can i assume that this bore has something to do with power brakes which both year cordobas have as std equipment?

What if i rebuild my power booster and attach this master to it?
Click to expand...

Yes the power brakes would make a difference, the linkage for the power brakes actually has an effect on the pedal ratio. A and E bodies didn't even run the same master cylinder bores with power brakes because of that, the A's were actually backward, they ran a 15/16" master with power and 1 1/32" with manual. E's were the other way around, and it sounds like the Cordoba's used the 1-1/32" with power as well.

As for running the power brake master with manual brakes, you will have to check to see if they have the provisions for the manual rod and retainer, they probably don't. The aluminum ones don't, that's one of the things Cass does to modify the later aluminum master cylinders for use with manual brakes.
 
I havent gotten the master cylinders yet, however since they are remanned units i am keeping my fingers crossed that there will be a pushrod retainer groove in the end.

Anybody make metric to SAE adaptors if these happen to be metric?
 
moparmat2000 said:
I havent gotten the master cylinders yet, however since they are remanned units i am keeping my fingers crossed that there will be a pushrod retainer groove in the end.

Anybody make metric to SAE adaptors if these happen to be metric?
Click to expand...
any parts store has them NAPA for sure
 
Cool thanks. I will be getting these in the mail tuesday. Will keep y'all updated on if they have pushrod grooves or not.
 
72bluNblu said:
Unless you don't need one. 60k+ miles on my Challenger with that brake set up, and with 1.12" torsion bars up front, 275's all the way around and even the 11x2.5" drum brakes out back it would lock the front's slightly before the rears. No adjustable prop valve can change that (they can only reduce the rear pressure), none needed. You only need one if you need to reduce the pressure to the rear brakes. And if you're not constantly swapping suspensions or tires, you only need to set it up once.



Exactly. There are a lot of factors that determine the brake bias. Tire size, brake size, wheel cylinder and caliper bore size, front/rear weight balance, spring rates, etc. There are a few different rear wheel cylinder bores out there for these cars, but if I needed to back the rears off I would be more apt to install an adjustable prop valve, just makes it easier to tune.



Yes the power brakes would make a difference, the linkage for the power brakes actually has an effect on the pedal ratio. A and E bodies didn't even run the same master cylinder bores with power brakes because of that, the A's were actually backward, they ran a 15/16" master with power and 1 1/32" with manual. E's were the other way around, and it sounds like the Cordoba's used the 1-1/32" with power as well.

As for running the power brake master with manual brakes, you will have to check to see if they have the provisions for the manual rod and retainer, they probably don't. The aluminum ones don't, that's one of the things Cass does to modify the later aluminum master cylinders for use with manual brakes.
Click to expand...

O.K. 72bluNblu Just a FYI you said that you changed to 1.12 Tbars and the front would lock before the rear, Yes now you don't have the weight transfer to the front so you need to put the valve in the front line to reduce and balance the braking force!! It's not just for the rear. It's when some guy doesn't fully understand or care how it works, that's when he gets out of control and crashes into someones car that does.
 
I think you want the front to lock up first.
..isn't that why the prop valve goes in the rear,to reduce the rear pressure?If the rear
locks first you will spin.
 
You don't want ether end to lock first, all 4 at the same time!! Front locks first and it understeers, pushes, plows,
wont turn. Rear locks first oversteer, rear comes around, spins out, but you can control it if you know how to drive. Understeer is very hard to get under control, once you get off the brakes the car is still sliding if you are in corner usually it's to late to recover.
 
I talked to my machinest at work, explained that these dippy masters may not be machined fully for a manual brake pushrod. He said if they are not, to dissassemble and bring in the new master cylinders pistons, along with an old manual brake piston and he could machine them out for a reciever groove for the O ring. I am hoping they do have this feature, as sometimes he is overwhelmed and it takes time for him to do stuff for me.
 
well if they are not useable i wouldn't bother machining them,just return them.
buy new from Master Power Brake.They have the correct 1" master for our manual disc mopars and the quality is very high.
Some buy re-man but they occasionally leak.
 
How about this for a pushrod setup.

Screenshot_2017-01-29-15-13-28.jpg
 
Adaptor. Easily made, or buy one from DrDiff for $38 all day long off evilbay

Screenshot_2017-01-29-15-37-58.png
 
Last edited:
You must log in or register to reply here.
-

Back
Top