poor braking

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changed from stock front drum brakes to lightweight front disc brakes and used master cylinder required 1-1/8 bore brake pedal is hard as a rock car does not slow down checked the ratio on the brake pedal measures perfect the car is a 72 dart any help is appreciated

[Dan Fahey]

Originally Posted by back for more

changed from stock front drum brakes to lightweight front disc brakes and used master cylinder required 1-1/8 bore brake pedal is hard as a rock car does not slow down checked the ratio on the brake pedal measures perfect the car is a 72 dart any help is appreciated

Have a used once Wilwood Master cylinder for sale.

Dan

[Larry Hill]

On our 72 charger when we changed breaks from power to manual the linkage under the dash had to be changed to a different hole to give us more of a mechanical advantage. I want to say the bore in the master cylinder was smaller making it easy to build psi.
Good luck.

[Rory McNeil]

Like Larry said, at least on Fords, the manual brake cars came with a different brake pedal, with a pivot for the master cylinder pushrod closer to the pivot for the pedal itself, for more mechanical advantage. Again, with my Fords, manual master cylinders a re usually a smaller bore, like 7/8" or 15/16" diameter, compared to over 1" for a power brake application.

[Tom Goldman]

As was mentioned pedal ratio is critical .If the car originally had power brakes the ratio was only around 4 to 1 ,a ratio of 6 to 1 is considered ideal . also ,If you are only using discs on the front and retained the drums on the rear ,a master cylinder with a smaller bore will help.
On a disc /drum car I usually use a 1 3/32 bore master cylinder which wit the 6 to 1 pedal usually will yield around 1000 psi with reasonable pedal pressure.

[Gmirza]

On my ‘71 Duster, I used an aluminum master cylinder that came on the Volare/Aspens. I have Strange fronts and stock 10” rear drums and factory proportioning valve also from Volare/Aspen. Never a problem stopping or holding the car. You could have a bad brake hose or a stuck caliper. Make sure too use the braided front brake hoses.

[rebelrouser]

Change the master cylinder bore to a smaller diameter, I usually use one inch. The smaller the bore of the master cylinder the more force at the wheels. Pascal's law is used to calculate a well balanced brake system.


14.3 Pascal’s Principle and Hydraulics | University Physics Volume 1 (lumenlearning.com)

[Dave Gantz]

Don't forget the hose between the frame and the rear axle.

[Eman]

If you really want to diagnose then you need a pressure gauge. As someone mentioned smaller bore MC means increased brake pressure but also less fluid volume per stroke. Larger bore less pressure but more volume. Rock hard pedal is good but you might have other problems. What condition are the back brakes, most assume everything is good if they aren't leaking but wheel cylinders freeze up, good pedal just no braking action.

[Greg Reimer 7376]

Many moons ago when I worked on cars for the general public, a customer brought in an early '70's Dodge van with a leaking master cylinder. We ordered out a new master, the parts house delivered it, we bench bled it and installed it, and all was fine except that you couldn't stop it without the brakes grabbing so hard it wanted to put you through the windshield. Calling the parts store and re ordering it and returning the first one revealed that the first one was for manual brakes, not power. It had a smaller bore and that caused there to be much more line pressure than the larger bore unit intended for the power unit.It may have been packaged wrong, who knows now. There is a decided difference between manual brakes and power assisted brakes as to the bore diameter.