School me on MC bore size (and in the process help me solve my brake problem.)

I'm sure some of you helped me out a few weeks ago on a wheel cylinder repair. I've been fighting with the brakes ever since. Short question: Curious as to the relationship that the master cylinder bore size has on brake performance. Is it possible to have too-small of a bore?

Long question: Everything in my brake system is new. I replaced a 1.25" bore master cylinder with a dual-piston 1" bore. 3/16" lines. All new wheel cylinders (1 3/8" bore.) It's a drum/drum system on a 1948 Chevy 1-ton. Even with the shoes adjusted out to "too-tight" I still can't get much pedal. Pedal goes all the way to the floor and no amount of pumping helps or changes it. There's no booster or hydrovac setup. CPP told me that 1" should be more than enough but did say that they don't work very much with 1-ton applications.

 

My 67 Impala has a 1.125 bore with 4 wheel drums it will throw your ass on the hood if you aren't careful. I think you need to look at the wheel cylinder bores, may be too big for the master!

 

This is exactly the tech info I'm looking for. How do you size the master in relation to the wheel cylinders?

 

i would say if you downsized the bore on the master and upped the bore on the wheel cylinders, your not pushing enough fluid into the cylinders, and running out of throw before the brakes work. i would think if you increased one you would to increase the other. just my thoughts.
i had some brake questions and called wilwood.really smart guy on the phone. mybe you could call and pick their brain

 

Assuming your actuating rod is adjusted properly for free play, I agree with Cayager; too little fluid being moved by the MC due to the smaller bore.

 

I have been down this road. Going up to 1.125" from 1" was the difference between shitty brakes and getting all 4 wheels to lock up. Here is a trick. Take some vise grips and clamp off the rubber lines the. Hit the brake pedal as if its right at the top your issue is most likely a lack of volume to the wheel cylinders


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Damn auto correct on an iPhone. Clamp the rubber brake lines then pump the brake pedal. If you have pressure then your issue is most likely due to the master cylinder not being able to move a late enough volume of fluid.


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1.25" to 1" is quite a change, the area was reduced by something like 34%. But still you should have some pedal if the system is leak free, properly adjusted, and bled.

Did you leave the pedal ratio as it was originally, or did you modify it to get full master cylinder travel? Usually a dual M/C has more travel than a single.

 

Pedal ratio is the same. The M/C seems to be bottoming out and right when the pedal hits the floor I can see the bracket ever so slightly move.....

Ordered a 1.125" master cylinder. $40 seems like a cheap way to test this theory out.

Also, pulled one of the lines and put a plug in it. I was able to get good hard pedal without any problem.

 

Does that trick work for both lines, or just one?

 

Unfortunately, only had a plug for 1 of the holes. The other one is a different size.

 

If you can find an adapter or a different plug and try it, then you might be able to find the problem...if one end of the truck is causing it.

 

Are you sure that the push rod is letting the master cylinder piston come back all the way. I would think that you should be able to catch up with it by pumping if all the air is out of the system.

 

Did you say the original cylinder is 1.25" or did you mean 1.125"??
If the original was 1.25" you should fit another 1.25" cylinder, If 1.125" then another 1.125" one should be used.

I have a 1" cylinder on my roadster which had a 1.0625" cylinder as stock, and if I don't keep on top of brake adjustment, the pedal can get low real quick.

Mart.

 

i fought a camaro for three days once. no pedal. everything was new front to back. took the preportioning valve apart, cleaned it. put it back on. fixed. don't know if you have one on your truck but it's a thought. only reason i took it apart was that there was nothing else left to do.

 

Original was 1.25" but I've been unable to locate a 1.25" replacement that is drum/drum.


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Do a quick check to make sure the master cylinder piston is being allowed to come ALL the way back. I recently saw a situation that was driving the shop nuts, and that was the problem. They spent a couple days fooling with the system before discovering that the piston wasn't returning all the way because there wasn't enough slack, and the piston couldn't uncover the port allowing enough brake fluid to be pumped.

 

I highly doubt that a 1.25 dual cylinder was ever made. Bigger trucks with hyd. brakes had MCs up to 1.75 but all were singles . At least in my years as a parts pusher for truck parts never saw any. A 1.125 will make a big diff over the 1" as to fluid volume moved and with a bit of luck solve your problem ,if not go back to the single or change the brakes at the wheels to something off smaller truck which may require changing the axles. Ain't no free lunch,change one part of a system it do have a effect on the rest.

 

And clamping brake hoses NO WAY JOSE!! bad enough doing nice soft heater hoses. A very good way to damage the reinforcement of the hose. But when the hose fails you get to try out your up graded dual system!!

 

My first thought was the brakes aren't bled correctly. You have to be able to get the original master cylinder and put it back in. Or rebuild it. You say you have no peddle and it goes to the floor. But if you plug off a line it's hard. Pretty much points to too small bore like the guys said..

 

This sort of shit pisses me off! I wonder where this “expert” based his calculations.


There are many calculations needed for correct brakes, Wheel radius, Disc and Drum radius [ for wheel torque ], Front and rear vehicle weights, Wheelbase, Friction material Co-efficient of friction ,Deceleration rate, Calliper piston size and / or Slave cylinder size ,master cylinder size ,and pedal ratio.
Most of these factors are needed to calculate the correct clamping pressures at the maximum deceleration rate.[ allowing for weight transfer etc ]


Assuming that most of the brake components on your vehicle are compatible with each other [ or still stock specs ], then YOU HAVE A SWEPT VOLUME ISSUE and nothing else!

On a Road Race Car [ my background of experience ] we calculate 5 thou compression for each disc pad [ 10 thou per calliper ], Soft road going friction materials require more .

This friction material compression is taken up by master cylinder volume [ I am not referring to pressures here but volume only ]


Your old master cylinder with 1.25” diameter has an area of 1.227 sq inches [ to get 1 cubic inch of fluid displacement requires 0.81 inches of piston travel ]

Your new master cylinder with 1” diameter has an area of 0.7854 sq inches [ to get 1 cubic inch of fluid displacement requires 1.27 inches of piston travel ]


You now need 1.56 times the piston travel to get the same volume of fluid displacement at the master cylinder, now multiply this by the pedal ratio and you’ll wonder why the pedal goes to the floor.


NEVER listen to parts counter Jockeys making assumptions, it’s your life [and others] at stake here

 

If it were me I'd just fit a new original type.

But the following search in google showed up quite a few 1.25" master cylinders of the modern type.

master cylinder "1.25" bore" site:rockauto.com

Mart.

 

Mart -- I've looked all over. Plenty of 1.25" bores but they are all for disc/drum systems. Thanks for checking it out, though!

The 1.125" master was only $40 so I figured it was a cheap thing to try and I can always try to return it if it doesn't solve the problem. The original 1.25" master is $100+ and I'd have to throw out my trick $85 CPP bracket.

In hindsight.... should have just stuck with the original single. Just one of those things that the first error is just piling up....

 

No reason you can't use a disc/drum master on a drum/drum system, just make sure you have internal/external residuals to your vintage drum brakes.

 

You should be able to use the disc drum master cylinder if you rebuilt your brake cylinders.

Previous to 1975 drum brake master cylinders had a residual pressure valve to keep the cups pressed tight to the cylinders and prevent seepage. After 1975 they changed the cup design to incorporate a spring and did away with the valve.

At least this goes for Chrysler products. You could check if your wheel cylinders have the improved cups.

 

The wheel cylinders are all new, but how can I tell if they are the "improved" or the "old" style? When I compared them to the old ones they appeared exactly the same!

 

NAPA MBI RH01K is $55. Is that right?!

 

You can use that residual, but they're cheaper from Wilwood, Summit, etc.

Residuals keep a vacuum from forming at the wheel cylinder cups during fast release, allowing air to enter. Cup expanders, along with newer designs, made them obsolete by the middle '70s, but they still can be used with later drums brakes with good results. Just check for internal valves in the master you use before you add any to the system.

 

CPP gave me the same bullshit line about 1" being plenty. It wasn't...

 

""Your old master cylinder with 1.25” diameter has an area of 1.227 sq inches [ to get 1 cubic inch of fluid displacement requires 0.81 inches of piston travel ]

Your new master cylinder with 1” diameter has an area of 0.7854 sq inches [ to get 1 cubic inch of fluid displacement requires 1.27 inches of piston travel ]


You now need 1.56 times the piston travel to get the same volume of fluid displacement at the master cylinder, now multiply this by the pedal ratio and you’ll wonder why the pedal goes to the floor""

If you went from a SINGLE piston 1.125 bore MC to a TANDEM [dual] piston 1.00 bore MC the end result is the stroke ends up at slightly less..(1.27/2=.635)..but with less pedal effort..I think you have a air issue; did you bench bleed the 1.00 MC? If you put in a 1.125 bore tandem MC then pedal travel will be cut in half and pedal effort will be twice as much..I would try gravity bleeding..