Drum brake pressure???

[rhtopdown]

What amount of pressure should my 64 Comet have on each drum brake and single master cylinder? Thanks

[Lou's Comet]

What amount of pressure should my 64 Comet have on each drum brake and single master cylinder? Thanks

That would depend on pedal pressure, pedal ratio, and master cyl bore size. The pressure range could be 400psi to 1,400psi or more.

For example, both calculations with 100 pounds of pedal pressure, and no power booster.

A 5 to 1 pedal ratio with a master cyl bore of 1" the pressure would be around 500psi

A 7 to 1 pedal ratio with a master cyl bore of 3/4" the pressure would be around 933psi (almost twice as much)

You really need to know what your master cyl bore size and pedal ratio are before figuring out where your pressure should be.

Power brake and manual brakes have different pedal ratios and different pedals. These cars are over 50 years old and things get changed over the years. Make sure you have the correct pedal. I have the original brake pedal from my comet with the power brakes. Also have a manual brake pedal that I am going to use. They have different ratio's, (push rod pin location)
Been awhile since I looked at them but pretty sure they have both pin location's drilled.

Here are a couple links/pictures to show the difference. These are for a 68 Mustang

Manual brake pedal
https://www.cjponyparts.com/cj-classics ... 68/p/PDA2/

Power brake pedal
https://www.cjponyparts.com/brake-pedal ... 969/p/PDA/

If you compare the two pictures you will see how much closer the push rod pin is to the pivot point on the manual brake pedal, compared to the distance on the power brake pedal. Comet brake pedals also have a difference between the power and manual.

Power brakes should have a pedal ratio somewhere around 4:1 to 5:1, manual brakes should have a pedal ratio somewhere in the range of 5.1:1 to 7:1.

Master cyl' same thing. Power brakes 1" bore or bigger, manual brake less than 1"bore.

You really need to figure out your bore size and pedal ratio to start figuring out which way to go to make your brakes better for yourself.

My parts are in boxes but if I can find time this week to look for them I will figure out the ratio differences for ya.

Here is a site that has some calculators for ya
http://www.wallaceracing.com/brake-master-cyl-calc1.php


Lou

[Lou's Comet]

Had to run to my shop today, I got out my brake pedals and took some pictures to show what I am talking about and the differences. Both have two holes one upper hole for the manual brake position, and one lower hole for the power brake position



Power brake pedal at the bottom, manual brake pedal above it.



Power brake pedal on the right, manual brake pedal beside it on left.

The manual brake pedal has a ratio of roughly 5.64:1, and the power brake pedal has a ratio of 3:1

Looked at Mac's and their master cyl. for stock manual 4 wheel drum setup has a 1" bore.

So everything the same. 100lbs pedal pressure, and 1" bore.

The power brake pedal with a 3:1 ratio would have a psi around 300
The manual brake pedal with 5.64:1 ratio would have a psi around 564

Almost twice as much!!

So before I got too involved with changing your brake system around. If you haven't already, I would check to make sure the pedal is correct,,,, if only to remove it from the suspect list..

Lou

[rhtopdown]

UP DATE: Bled the brakes, front bled great and pedal went to floor. Rear Right fluid just trickled out and hard pedal. Right rear nothing. No air, No fluid and pedal hard. Now hope to find something stopped up.

[lavron]

Now hope to find something stopped up.

How is the soft line from the body to the rear end? If it is old it could be the problem, I guess you could crack it loose and see what the pedal does.

See Ya,
Mike

[rhtopdown]

thanks