Master Cylinder Bore Sizing

Mason · Mar 12, 2020

Hey everybody, I am currently working on choosing a master cylinder to put into my car as the recent one I put in gave up. I'm going to get a Wilwood master cylinder but am stuck on the bore size. I have a 70 Coronet with drum brakes all around without a booster. If I'm correct I had a 1 inch bore size on mine before but I'm seeing some on here that are 7/8 inch as well as 15/16 inch for sale and not sure if it would make much of a difference. Just trying to make a choice on a master cylinder, but I'm not ready to spend $500+ to convert to power haha. Any help is appreciated!

493 Mike · Mar 12, 2020

It will depend on how strong your legs are! Smaller bores require less apply pressure but, longer stroke to move a similar volume of fluid. You will need to keep your shoes adjusted up if they are not self-adjusting.
Mike

Mopars & Missiles · Mar 12, 2020

Mason said:
Hey everybody, I am currently working on choosing a master cylinder to put into my car as the recent one I put in gave up. I'm going to get a Wilwood master cylinder but am stuck on the bore size. I have a 70 Coronet with drum brakes all around without a booster. If I'm correct I had a 1 inch bore size on mine before but I'm seeing some on here that are 7/8 inch as well as 15/16 inch for sale and not sure if it would make much of a difference. Just trying to make a choice on a master cylinder, but I'm not ready to spend $500+ to convert to power haha. Any help is appreciated!

Check your FSM for the original M/C size. 1" for 4 wheel drum on a '69 B body is the factory size, so that might be factory correct for your '70 too, but you should check your FSM to be sure.

As far as the different size masters, there is one constant to keep in mind. Larger piston bore size = MORE fluid volume, but LESS line pressure. Smaller piston bore size = LESS fluid volume, but MORE line pressure. Other things come into play as well, but if your wheel cylinders are still factory original size, then the factory M/C size should work just fine.

Mason · Mar 12, 2020

493 Mike said:
It will depend on how strong your legs are! Smaller bores require less apply pressure but, longer stroke to move a similar volume of fluid. You will need to keep your shoes adjusted up if they are not self-adjusting.
Mike

Yeah haha! I've been researching my tail off today on what would be best for my car and have just been running in circles. I emailed Cass from Doctor Diff and he told me 15/16 is probably my best bet at the expense of more pedal travel...couldnt imagine how much pedal travel would be added but I'm sure it wouldn't hurt.

Mason · Mar 12, 2020

Mopars & Missiles said:
Check your FSM for the original M/C size. 1" for 4 wheel drum on a '69 B body is the factory size, so that might be factory correct for your '70 too, but you should check your FSM to be sure.

As far as the different size masters, there is one constant to keep in mind. Larger piston bore size = MORE fluid volume, but LESS line pressure. Smaller piston bore size = LESS fluid volume, but MORE line pressure. Other things come into play as well, but if your wheel cylinders are still factory original size, then the factory M/C size should work just fine.

I'm sure it would work fine for mine as well, the b body is almost the same car from 69 in regards to interchangeability. Cass recommended me to put a 15/16 in but I'm not sure how much of a difference it really would make from 1 inch.

MW 413 · Mar 12, 2020

Going from a 1" bore to a 15/16" bore your pedal travel will increase by about 1" but will take a little less effort with non-power brakes.

khryslerkid · Mar 12, 2020

I'm useing a 15/16" with drums on four corners. I'm good with it. Sometimes I'll pump the pedal twice just to have a full pedal but it stops just like I remember back in the day. Useing new linings, the more they "break in" the better they get. They aren't going to act like the new vehicles

Mason · Mar 12, 2020

MW 413 said:
Going from a 1" bore to a 15/16" bore your pedal travel will increase by about 1" but will take a little less effort with non-power brakes.

Okay that's good to know, I think I'll go ahead and get that sized master cylinder if it means driving it is less of a workout haha, thanks for the help.

Mason · Mar 12, 2020

khryslerkid said:
I'm useing a 15/16" with drums on four corners. I'm good with it. Sometimes I'll pump the pedal twice just to have a full pedal but it stops just like I remember back in the day. Useing new linings, the more they "break in" the better they get. They aren't going to act like the new vehicles

Yeah I'm glad I got someone to reference from that's actually had experience with it. I may go ahead and get the 15/16 inch then. Have you ever considered having a 1 inch in your car or do you have no complaints?

khryslerkid · Mar 12, 2020

Mason said:
Yeah I'm glad I got someone to reference from that's actually had experience with it. I may go ahead and get the 15/16 inch then. Have you ever considered having a 1 inch in your car or do you have no complaints?

With the research that I did, most were going to 15/16". Like stated it might be a longer stroke but the smaller bore has more power/pressure. I think keeping your brake shoes adjusted properly keeps the pedal up where it belongs.

I might add that I'm useing the Dot 5 brake fluid which suppose to have a spongy feel to it. In my opinion it might be a little soft halfway through the stroke but under full pressure it's quite solid.

Mason · Mar 12, 2020

khryslerkid said:
With the research that I did, most were going to 15/16". Like stated it might be a longer stroke but the smaller bore has more power/pressure. I think keeping your brake shoes adjusted properly keeps the pedal up where it belongs.

I might add that I'm useing the Dot 5 brake fluid which suppose to have a spongy feel to it. In my opinion it might be a little soft halfway through the stroke but under full pressure it's quite solid.

Okay that's good to know, thank you for helping me out!

65hemi · Mar 13, 2020

I have 1" on my Cuda and 15/16" on my Belvedere. Both cars have the same SSBC disc brakes. The Belvedere stops way better than the Cuda and better pedal feel. The Cuda will get switched to 15/16".

Glenwood · Mar 13, 2020

65hemi said:
I have 1" on my Cuda and 15/16" on my Belvedere. Both cars have the same SSBC disc brakes. The Belvedere stops way better than the Cuda and better pedal feel. The Cuda will get switched to 15/16".

I just installed 4 wheel discs o the 64 with 15/16 bore. I haven't tried it out yet but the pedal seems soft and too much travel to me with DOT4 compared to the drums with DOT5. I'm hoping this is normal. The mc was bled first and then the entire system.

hunt2elk · Mar 13, 2020

I just did a front drum to disc manual conversion on my Super Bee project. Went with the recommendation by Cass to use my first 15/16" master. Haven't fired up the car yet, but do notice a definite difference in pedal travel compared to the 1 1/32" mc's in my other cars. Pedal travel is about 3 inches on my Bee.

Mason · Mar 13, 2020

65hemi said:
I have 1" on my Cuda and 15/16" on my Belvedere. Both cars have the same SSBC disc brakes. The Belvedere stops way better than the Cuda and better pedal feel. The Cuda will get switched to 15/16".

Thats good to know, I really wish I had just bought all disk for my car before completely redoing my brakes haha, I'll have to do the conversion in the future.

Mason · Mar 13, 2020

hunt2elk said:
I just did a front drum to disc conversion on my Super Bee project. Went with the recommendation by Cass to use my first 15/16" master. Haven't fired up the car yet, but do notice a definite difference in pedal travel compared to the 1 1/32" mc's in my other cars. Pedal travel is about 3 inches on my Bee.

I've heard of most systems with drums are recommended to not go anymore than 1" on their MC bore size but I guess it could vary. Cass seems to recommend that size for most brake systems...came across an old forum having the same debate. Hopefully yours will work out!

beanhead · Mar 14, 2020

I prefer 15/16" for manual brakes. My wife can even comfortably work the brakes...manual front disc/rear drum setup. I've only used larger bore MCs on power setups.

Mason · Mar 16, 2020

beanhead said:
I prefer 15/16" for manual brakes. My wife can even comfortably work the brakes...manual front disc/rear drum setup. I've only used larger bore MCs on power setups.

I got a 15/16" wilwood master on the way! Hoping this will work out well.

Cranky · Mar 16, 2020

Years ago I put a smaller bore MC in my Belvedere just because it was here and it was new. My brakes were easier but had a bit more travel but if you keep your shoes adjusted close, it's not a big deal. Btw, I always remove the self adjusters because I don't like my brakes dragging all the time but I did some research on hydraulics before doing it....

Joe66satellite · Jun 1, 2020

65hemi said:
I have 1" on my Cuda and 15/16" on my Belvedere. Both cars have the same SSBC disc brakes. The Belvedere stops way better than the Cuda and better pedal feel. The Cuda will get switched to 15/16".

Hope it's not too late for a response on your 15/16 master in your Belvedere I'm still struggling with the 15/16 master in my 66 satellite with the SSB disc brake kit up front how are you running yours power non power distribution block non distribution block proportioning valve non proportioning valve hope it's not too late still struggling thank you, I'm trying to run mine as just a manual disc drum setup can't get the fronts to engage well I put an adjustable prop valve in the rear line ahead of what looks like a post 66 Texas shaped distribution block

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