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Master Cylinder size and brake issues

thebearded1

Active Member
This may be a better question for the HAMB since it's older parts but the car isn't fully traditional so I thought i'd ask here. What bore size master cylinder is best for the following setup:

Front 1940 Ford drums, wheel cylinders have 1.00" diameter
Rear 1992 Jeep drums, wheel cylinders have .8125" diameter

I am currently using a master cylinder with 1.00" bore that was intended for a full drum setup.

I'm having issues getting decent braking. I bench bled the master cylinder, bled the full brake system, and adjusted the drums to have slight drag. Pedal ratio is 4.3:1 I also tried 5.2:1 and 6.5:1 and those did not feel any better. There are 10 psi residual valves installed as well.
 
G'Day Bearded 1,
With drum brakes all round, all of the braking effort is generated by the pedal ratio. 1" M/Cyl to 1" front cylinders equals 2-1 area ratio x 6.5 (pedal ratio) All the old cars were pretty much 7-1 ratio on the pedal. I would reduce the M/Cyl diameter to 7/8" and use the 6.5-1 pedal. I trust this is helpful.
 
Where is the master mounted? On firewall or under floor? Is it single outlet or two? Where are the residual valves and how many?
What exactly are you experiencing? Too much pedal travel before braking happens? Or just not as much braking as you would like at full pedal? Do the front or rear brakes lock up under some condition?
 
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I will try the 6.5:1 ratio again.

Up front uses braided stainless lines and the rear uses rubber. I have not replaced either. Maybe I will get some new ones for the rear.

Master cylinder is mounted under the floor. The mc has 2 outlets. It uses 2 10lb (will check again when I get home) residual valves one in the front line and one in the rear line. They are located a few feet after the mc but before the front and rear line splits.
 
Yup drums all around and the inline residual valves are 10 lbs

I think I found part of the issue and need some ideas for some solutions. The previous owner set it up with the brake pedal on the outside of the frame and master cylinder on the inside of the frame and made a pushrod with 2 bends to make things line up. The pushrod doesn't want to stay flat and I believe is starting to bend more.
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I thought about putting the brake pedal on the inside of the frame but there isn't much space between the frame and the trans and i'd have to be sure it didn't interfere with the transmission linkage. Also i'd have to move the brake line and brake light switch which runs along the inside of the frame.

My other thought was to use a keyed bolt to allow the brake pedal to be on the outside of the frame and have a short lever on the inside of the frame to connect the MC pushrod too.

I'm really at my wits end with this car but don't want to give up since i've put a lot of work into it. Had the po made a wider frame there would have been a lot more room between the frame and transmission. A wider frame would also have allowed me to use a vega box with RPM's bracket.
 
I don't have any room under the dash to put the MC up there with a hanging pedal. I'd really hate to put it on the firewall as well.
 
Here's a master cylinder mounted on the firewall. Some like it, some don't.

P2082248_re.JPG
 
Do somethink like this........This is my gear shifter going from one side of the frame to the other and the handle over on the side of the body..........
 

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A brake rod that looked like this would not bend or twist.


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You could make a bell crank to go between the pedal and the master cylinder. When you press the pedal the force is transferred to the bell crank and then the force is transferred to the master.
 
Interesting options. I like LincolnuT's setup but i'd have to figure out how to make the arm going to the mc pushrod and figure some way to get it and the pedal to stay on the shaft. I could run a bolt through the shaft and arm but i will have to make the arm out of some thick material for that. I don't think set screws will hold up to the force need for braking.

Indy, That definitely wouldn't bend! When I say twist I mean the whole thing is rotating. The bent part of the pushrod is a tube and the clevis has a threaded portion that is just inserted into the tube. So that's why the whole thing can rotate. If i was to go this route it would be best to make it all one piece and have the clevis end a long threaded portion to allow for adjustment.

Neshkoro, I'm not sure how the bell crank would work. Wouldn't that change the direction which would mean the master cylinder would need to be flipped?

Gerry, That's a pretty fancy setup but beyond the scope of what I would be able to do in my garage
 
Drill through and use pins or cotter pins....I found some more photos that may show it better. I also have the car on the lift today and will take a couple of photos. Use the same princables but do make it stronger for braking...These may give you a better idea...........
 

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Neshkoro, I'm not sure how the bell crank would work. Wouldn't that change the direction which would mean the master cylinder would need to be flipped?

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If you make the bell crank like a lever door handle (the arms facing or pointing in the same direction), they would/will push the push rod in the same direction, you can even change your pedal ratio too by having the first arm longer than the second arm.
 

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