3/4" master cylinder?

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jos51700 said:
Wait, are you saying that you have the front and rear lines hooked together at that junction block?

If so, the systems would function as one. Which would include that adjustable pressure bit.....which reduces line pressure.
Click to expand...
no.... the early mopar distribution block has 2 circuits... the front in then left/right out and back in and out.. can actually just skip the back and go from the prop valve to the rears but the line was already at the distro block.. sooo
 
If the pressure testing was with the caliper installed on the knuckle, you probably have some air being compressed at the adapter. Caliper pressure can be checked off the knuckle with the rotor or block of wood between the pads and the bleeder/gauge not on top. Could also try loosening the gauge from the adapter so it leaks, then tighten. It sounds like you just need to go with a smaller bore master or a booster or a custom offset push rod (like fords) for a pedal modded to a higher ratio (raised attachment).
 
MV8 said:
If the pressure testing was with the caliper installed on the knuckle, you probably have some air being compressed at the adapter. Caliper pressure can be checked off the knuckle with the rotor or block of wood between the pads and the bleeder/gauge not on top. Could also try loosening the gauge from the adapter so it leaks, then tighten. It sounds like you just need to go with a smaller bore master or a booster or a custom offset push rod (like fords) for a pedal modded to a higher ratio (raised attachment).
Click to expand...
yup.. .the post started with me trying to find a smaller master :)

And i actually modified my pedal ratio today from 6.4:1 to 7.25:1
 
Its an odd problem because 15/16 master cylinder and 2.75 bore caliper it should have lots of stopping power. I run that combo with 11 3/4 front disc and I can stomp brake pedal lock up tires easily. And braking during normal driving very little leg effort to slow car down.

Have you asked Dr Diff for any ideas?
 
3/4" M/C? I would be very careful with that & disk brakes. A prayer to Allah would be a good start. The m/c volume is the problem; you have a LOT less with 3/4" compared to a a 1" m/c. 44% less volume to be exact....
The m/c body would need to be quite long to hold enough volume without the m/c piston bottoming out, plus a safety margin. You will also have more pedal travel, pedal may hit the floor. Some serious calculations would need to be made...
Sounds like , 'It was a good idea at the time...'
 
340sFastback said:
Its an odd problem because 15/16 master cylinder and 2.75 bore caliper it should have lots of stopping power. I run that combo with 11 3/4 front disc and I can stomp brake pedal lock up tires easily. And braking during normal driving very little leg effort to slow car down.

Have you asked Dr Diff for any ideas?
Click to expand...

This!

I've run the 2.75" calipers with 10.95" disks and 11.75" disks, and with the factory 1-1/32" master cylinder (lower line pressure) and the 15/16" master cylinder and never had any issues locking up the front brakes. With the 11.75" disks I was able to lock up 275's on both my Challenger and my Duster.

The issue is not the master cylinder bore. Those are stock calipers that worked just fine with the factory master cylinders.
 
I've driven manual MC front disc-rear drum A bodies that stop as well as newer cars, all with basic parts.
 
Kern Dog said:
I've driven manual MC front disc-rear drum A bodies that stop as well as newer cars, all with basic parts.
Click to expand...

Yeah..i have also.. my FFR Cobra was a disc/drum manual that stopped amazingly well (after fixing the pedal ratio.. thing was BAD when i got it) I am just stumped on this.. will hopefully figure it out by next spring..
 
340sFastback said:
Its an odd problem because 15/16 master cylinder and 2.75 bore caliper it should have lots of stopping power. I run that combo with 11 3/4 front disc and I can stomp brake pedal lock up tires easily. And braking during normal driving very little leg effort to slow car down.

Have you asked Dr Diff for any ideas?
Click to expand...

yeah.. emailed him.. not much to say, everything looks perfect and should just work. soo no idea.
 
i don't know how you are measuring pressure at a length of 14"? Not mentioned but i had a problem stopping with new front disc brakes..it was scary. It wasn't a pressure problem but "bedding in" issue. I ended up taking the car out on a quiet highway and hauling it down as best i could "hard" from about 50mph 3 times. then after they cooled some gentle stops from about 30mph. Brakes grabbed and it stopped nicely from that point on. You should have more than enough pressure even with a 7/8" master.
 
This may sound stupid but do you have the calipers on the correct sides with bleeders on top ?
 
Icetech said:
Lines are in the right orientation.. and am using a wilwood proportioning valve..

The main issue is the pressure drop within 14" of line.. it's just weird.. the common denominator is the calipers but it doesn't make sense... hoping drdiff gets some 2.75s in stock and i will replace these just to be sure.
Click to expand...
What is between? Do you have the distro block with the warning light? OEM Prop valve assembly?

If that is actually true and not a mistake, that is a big clue
 
67Dart273 said:
What is between? Do you have the distro block with the warning light? OEM Prop valve assembly?

If that is actually true and not a mistake, that is a big clue
Click to expand...
1756129051325.png
 
I wish something came to mind. My 67--when it was together--I had stock 73/4 Duster calipers/ rotors and semi metallic pads. I did not use the larger what, Cordoba calipers? I also have a disk brake axle out of a Versailles. I used the factory 67 drum master, and simply poked holes in the residual valves. No prop valve. I could squall the tires pretty easy at freeway speeds
 

67Dart273 said:
I wish something came to mind. My 67--when it was together--I had stock 73/4 Duster calipers/ rotors and semi metallic pads. I did not use the larger what, Cordoba calipers? I also have a disk brake axle out of a Versailles. I used the factory 67 drum master, and simply poked holes in the residual valves. No prop valve. I could squall the tires pretty easy at freeway speeds
Click to expand...

yeah.. i shouldn't have made the post honestly.. nothing anyone can do over the internet.. It feels like my cobra did when i got it and the pedal ratio was wrong.. but mine is good. Will figure it out eventually.
 
Bewy said:
3/4" M/C? I would be very careful with that & disk brakes. A prayer to Allah would be a good start. The m/c volume is the problem; you have a LOT less with 3/4" compared to a a 1" m/c. 44% less volume to be exact....
The m/c body would need to be quite long to hold enough volume without the m/c piston bottoming out, plus a safety margin. You will also have more pedal travel, pedal may hit the floor. Some serious calculations would need to be made...
Sounds like , 'It was a good idea at the time...'
Click to expand...

The volume of fluid nec to apply pressure to a fully bled system is a few drops.

You are merely applying pressure thru any size pump to a full system.

It certainly may take a few pumps more to fill a voided system, but once totally full, you could apply brakes with an eye dropper, if it'd take the pressure.

Disc brakes just "relax" by a few thou, shoes properly adjusted, pedal should be at top.

Your hydraulic press 50 tons, likely has the same size pump as a 10 ton.

The pump/mc is merely to add pressure .
 
Inertia, post #43.
Huh????????????
Allowing for movement of the pads/caliper flex, piston movement of 0.020" would generate 0.55" of movement of a 3/4" m/c piston. Then there is the travel of the rear brakes to be added to total pedal movement.
 
This one is weird. I've never seen one before:
1756190640091.png


Icetech said:
no.... the early mopar distribution block has 2 circuits... the front in then left/right out and back in and out.. can actually just skip the back and go from the prop valve to the rears but the line was already at the distro block.. sooo
Click to expand...

This is the most common 4 wheel drum distribution block that I have seen:

22 6I.JPG


These have no proportioning to them. Ma Mopar achieved proportioning through the drum widths and wheel cylinder sizes.
Now, the front disc/rear drum systems had a proportioning valve from 1973-76 that looked like this:

Texas PV.JPG
 
Bewy said:
Inertia, post #43.
Huh????????????
Allowing for movement of the pads/caliper flex, piston movement of 0.020" would generate 0.55" of movement of a 3/4" m/c piston. Then there is the travel of the rear brakes to be added to total pedal movement.
Click to expand...

Run across this many times with book theory and actual fact .
Actually test it, - get back to me .
 
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