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Pedal Pushers: How to Figure Out Pedal Ratio & Master Cylinder Bore Size

Pedal 1
Pedal 2
Pedal 3
Pedal 4

This manual master cylinder is one of the most common you’ll find at a drag strip and it’s also perfect for many hot rod applications. It is a Mopar master cylinder, often found on applications such as Dodge mini vans and Chrysler Cordoba’s (Summit Racing offers them through Strange Engineering, Mopar Performance, Wilwood, Cardone, Dorman and others). These master cylinders are available in at least two different bore sizes: 1-1/32-inch and 1-1/8-inch. They’re manufactured with an aluminum body along with a relatively large capacity plastic reservoir with dual outlet bores (which correctly face the driver side fender when mounted on the firewall).

The Mopar-style master cylinder has one shortcoming: the size of the outlet fittings. The front fitting is a 9/16-20-inch inverted flare while the rear is a 1/2-20-inch inverted flare. What’s so difficult about that? You’ll have trouble finding the right fittings. Both Lamb Components and Strange Engineering manufacture special adapters specifically for these master cylinders, which allow an easy hook up to -3 AN fittings. See the Summit Racing catalog for Strange master cylinder fittings.

In this case, the brake master cylinder is adapted to the car with a billet piece from the folks at TRZ Motorsports. If you have a Mopar, Summit Racing offers several special adapters from Mopar Performance that enable you to mount the later model tandem master cylinder to your car.

The typical Detroit pedal assembly looks like this. This vintage Chevy hanging arrangement is designed to accept the brake pedal assembly, and if equipped with a clutch, that too. If you take close look at this pedal, you can see two different master cylinder pushrod mount holes--one is for a booster-equipped application, while the other is for a non-boosted brake arrangement. For a late model, non-boosted manual application, many fabrication shops modify the pedal assembly by creating a mount that is higher (up the pedal) than the original. By moving the mount position higher, the pedal ratio is improved.

When it comes to reworking the brakes on your car, two things seldom come into consideration: your master cylinder and pedal ratio. And those are two factors that are absolutely critical to your brake system performance. The brake pedal acts as a lever to increase the force the driver applies to the master cylinder. The master cylinder, in turn, forces fluid to the disc brake caliper pistons or drum brake wheel cylinders.

Take a close look at a brake pedal. You’ll find that the pivot point (where the pedal swivels) and the mounting point for the master cylinder pushrod are often rather different. If the length of the pedal or the distance between the pushrod mount–or a combination of both–is changed, then there is a change in the amount of (leg) force required to energize the brake master cylinder. This is the mechanical advantage of the lever or in simpler terms, the “pedal ratio.” There is a mathematical equation that helps you figure it out:

Input Force X Pedal Ratio / Brake Piston Area   =  PSI

The formula essentially figures out the amount of force exerted by you through your leg times the pedal ratio divided by the area of the brake piston(s). The force is measured as pounds per square inch (PSI), and we’re told that the average adult male can exert roughly 300 pounds of force (maximum) with one leg. That’s a whole bunch! Something in the order of one-third or one-half that figure is certainly more relaxed, even in a bad-to-the bone hot rod.

Let’s look at how all this applies to activating your master cylinder. A typical manual master cylinder usually requires something in the range of 600 to 1000 PSI to function properly. You have to figure out how to transform the 100 or so pounds of relaxed leg pressure (roughly one-third of the 300 PSI as we mentioned above) into pretty big PSI figures. This is accomplished by way of a lever (the brake pedal). In order to arrive at the pressure required, changing the overall length of the pedal is possible, although it’s probably a bunch easier and usually far more practical to simply shorten the distance between the pivot point and the master cylinder pushrod mount location. That’s how fab shops modify brake pedals.

But we’re not done yet.

There’s something else to think about: brake line pressure. This pressure is a totally different thing than the force you apply to the pedal. That force acts in one direction and is addressed in pounds. Pressure, on the other hand, acts in all directions and is addressed in PSI (pounds per square inch). As we pointed out above, varied (pivot length) brake pedals can be used to change the force. Inside the hydraulic system, the surface area of the piston is what is affected by pressure. Decreasing the bore size of the master cylinder increases the pressure it can build.

Pistons in master cylinders are specified by bore size, but there’s a catch. The area of a circle (or bore) is Pi–R-Squared. The area of the piston surface increases or decreases as the square of the bore size or diameter. The area of something like a common 1-1/8-inch master cylinder is approximately 0.994-inch. The area of an equally common 1.00-inch bore master cylinder is approximately 0.785-inch. Switching from the larger master cylinder to the smaller version will increase the line pressure approximately 26.5 percent assuming that pedal ratio hasn’t changed.

When the pedal force is increased or the pedal ratio is increased (or perhaps both at once), the stroke of the master cylinder is shortened (brake line pressure is unaffected). When the master cylinder piston size is increased, the output pressure of the master cylinder is decreased. A master cylinder with a smaller piston will exert more line pressure with the same amount of force (pedal ratio) than a master cylinder piston with a larger piston area. Because the brake line fluid pressure is working against the surface of the wheel cylinder (or disc brake piston), increasing the area of the cylinder will increase brake torque.

So, if your car needs some help in the stopping power department, or if there’s a need to reduce the pedal effort, you have a few options: A) decrease the master cylinder bore size; B) increase the pedal ratio; or C) increase the wheel cylinder bore size. If the pedal ratio is increased, there will be more travel at the master cylinder piston. If the master cylinder bore size is decreased, the piston has to travel further to move the same amount of fluid. Typically, a master cylinder has approximately 1-1/2-inch to 1-3/4-inch of stroke (travel). What you have to do is coordinate the pedal ratio with the bore size to arrive at approximately half of the stroke (roughly 1-inch) in order to make the brakes feel comfortable and stop the car safely.

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12 Comments

  1. Perry Stapleton says:

    I have been told, by so called experts, that the larger the master cylinder bore size would give more stopping power with less leg pressure on the peddle. If I understand this artical, my information is incorrect and a smaller bore master cylinder will give more stopping power with less leg pressure. Is this correct? Thanks

  2. Pingback: Mailbag: Solutions for Poor Brake Performance and Excessive Fade - OnAllCylinders

  3. Tracy Ridley says:

    I have 15/16 bore MC. I have converted to 4 wheel disc brakes (67 barracuda) right stuff on the rear wilwood on the front. Cannot get a decent pedal. I feel I have a volume issue & intend to go to a 1″1/8 bore MC. Am I on the right track. If isolate front or rear lines pedal comes up good.

    • If volume was a thing you would have huge lines similar to fuel lines. Hydraulics is pressure. If you go to a bigger bore you lose pressure (according to the article). I too have the 15/16 MC. But I want to add a vacuum power booster to assist in my efforts.

  4. Greg Riutzel says:

    This article is spot on!!! It has good understanding of basic hydraulics. Going on the comments, don’t people get taught anything anymore? I had this in 7th grade science.

  5. I have a 1938 chevy pickup with 1 1/4 & 1 3/16 wheel cylinders. I converted to a dual resivor 1 ” bore master cylinder. The problem is the pedal travels to the floor before engaging the brakes. I have the shoes adjusted up far enough you can hear them drag. Do I need a larger bore master to displace more fluid ?

  6. Mark Charbonneau says:

    I am building a 4 door 32 plymouth hotrod. 560 hp want it to stop… All steel, heavier than most rods with disc brakes all around. Conversion disc brakes using 69-77 gm calipers on 11″ discs. Would like to go with manual brakes but vac assisted a possibility. Deciding on master cylinder- 1 or 1-1/8″? Pedal I have is 4 to 1 (power brake set up) Can change ratio if need be. Any input would be appreciated THANKS

  7. Follow up question- how does a power booster affect the 600- 1000 PSI required to function properly?

  8. I have 67 nova on grand national chassis g body every thing new stock every thing drake related rear drum new dual booster 78 blazer m/c won’t slide a tire 1/4 rear line Help. I can’t drive my 35000.00car.

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