I have a question for those more knowledgeable about hydraulic pressure and selecting the best bore size on brake master cylinder sizes. I have heard the smaller the bore size the more line pressure you will have to the brake cylinders. Is this correct?
The reason I'm asking is that I'm going to replace the single master cylinder with a later model Mopar aluminum with dual reservior on my 1966 Dodge Coronet with a SSBC disc brake conversion kit with 11 inch rotors and 4 piston iron calipers up front and stock 10.5x2.5 drum brakes in the back. I'll put in a proporsioning valve and remove the residual valve in the M.C. for the disc brakes, if it has one. I don't know the bore size of the front caliper pistons or the pistons in the back, sorry
What size M.C. bore size would be best in your opinion,1 1/8, 1.0, 7/8 inch I.D. or what size

Replaced front drums on my Ebody, non power brake car. 1 1/8 stock mc

After market disc brake kit from well known 3rd party supplier. Wilwood Dynalites

Horrible front brake performance. Following some advice, installed a set of pads that gave adequate performance, but were filthy dirty.. Filthy after 10 or 20 miles. Supplied by Hawk.

After extensive research, replaced mc with a 15/16 inch mc picked up from Amazon for 50.00 and installed with ceramic pads, also from Amazon. Smaller mc bore = higher brake pad pressure.

Front braking infinitely better,,,can now lock them,,,and do not have to clean wheels after each 10 miles of driving.

Then had to deal with rears, as car was once a SS/E drag race car, rear self adjusters had been removed and Bubba had replaced with pair on both wheels of some star wheel adjusters from God knows what model car,,,but both of them were marked as right side.

All well now

Your results may vary.

Depends a bit on if the car originally came with manual or power brakes.
A manual brake car usually have a higher brake pedal ratio, and can get away with a larger diameter MC.

SSBC don't seem to be bothered at all about pedal ratios with their kits which is probably why a lot of folks aren't that thrilled with their performance.

in the corners are best forum there is a calculator embedded in the post called all you ever wanted to know about braking.

But the tldr basically is, the smaller the mc bore, the longer the stroke but the more pressure you have at the brakes.

The larger the mc bore, the shorter the stroke, but you get less pressure for the same effort.

The larger bore mc generally are used with power assist from what I have seen. But you need the dual diaphram boosters to give enough assist.

with manual brakes use a 15/16" bore. power brakes try 1 1/32" bore.

I have a disc/drum combo with manual brakes. Went from a 1 1/32" MC to a 15/16" to get more pressure but just wound up with a lower/softer pedal and no noticeable braking improvement.....went back to the 1 1/32" and all is good.

Thanks to all of you for your help
I have three diffferent new bore size M.C. for projects on the shelf now, I think I'm going to put the smallest bore sizs in the 1966 Dodge first and see how that works, if I don't like it I may go up the largest (1 1/8)and see how that works so I have some real life experiences from those tests

The relationship in pressure change is proportional to the difference in piston surface area. It is no more or less complicated that this. You are simply converting pounds of force to pressure (pounds per square inch). Because area changes exponentially with diameter change, it does not take a lots to affect the pressure change.

For a given pedal force, the 15/16" bore will give a 30% increase in line pressure over a 1 1/8" bore.

Cannot say this is exactly the numbers that I got on my E Body when switching from 11/8 to 15/16 mc on non power brake car, but it was a bunch and helped very much.

Good explanation.

I've done most of the thinking for you:

https://board.moparts.org/ubbthreads/ubbt...ed-to-know.html


Here's a spreadsheet I threw together late one night. Please excuse the typos.

http://www.3gduster.com/brakes.html

Bought a 68 dart with manual brakes. Car had a 1 1/32 master cylinder on it and the brakes were uncomfortable. Had a good pedal but felt like the brakes were 50%. Changed to a 15/16 and made a world of difference. Good pedal and stops much better.

Would anyone have a part number for a 15/16" master cylinder, four bolt, for disc brake front? Thanks in advance, Bob

I just finished putting the same master on my car it's the raybestos # MC36406 master cyl which is supposed to be for a 73-75 A body with power disc brakes with the 15/16 bore.

the wagner pn for the 4 bolt, 15/16" manual disc brake master is : F80266. check the piston bore for the rubber retainer groove, as it seems lately, this part# is used for both manual and power brake applications and may, or may not not have the retainer groove in the piston. in power applications, the groove is not needed. if you use a non grooved master in a manual application, you MUST make sure there is a positive, non moving, stop installed to prevent the pedal push rod from falling out of the master cylinder piston bore. as can be imagined, if the pedal push rod falls out of the piston bore, you may experience a very bad day. i don't know if the bendix or raybestos brand masters are set up the same way, but please check to make sure you know what you have and make any necessary adjustments as needed.