Brake System Info and FAQs from Mark Williams Enterprises

(More tech articles on the Tech, Tests & Installs page HERE)

As racers, we're always looking to go fast, but slowing safely is actually much more important. Additionally, for the footbrakers among us, it's the braking system that holds the car at the starting line as we're way up on the converter. That's why when I saw this technical piece about brake systems on the website of Mark Williams Enterprises, I knew I had to share it here with you. The article discusses Pedal Ratio & Master Cylinder, Master Cylinder Selection, Caliper Alignment Clearance and Position, Proportioning Valves, Brake Lines and Fluid, and Troubleshooting.

Pedal Ratio & Master Cylinder
Mounting the master cylinder to a frame rail or roll bar is recommended to ensure a solid mount. With the correct master cylinder in place, the pedal ratio must be great enough to produce 1200 psi system pressure under severe braking conditions. A pedal ratio versus line pressure calculator is available here. We recommend using a pressure gauge connected to the system to verify the maximum available pressure before running the car. If the desired pressure can not be easily attained the pedal ratio must be increased until the minimum pressure of 1000 psi is easily reached.

Master Cylinder Selection
A common problem in many applications is a spongy pedal. Brake systems inherently have slack in the system that must be taken up before work can be done by the pads. Slack comes from two factors: retracted pistons must be moved back out, and misalignment caused by deflection in the axle and housing mis-alignment. One of the most common misconceptions is that a larger master cylinder will create more pressure. While a larger master cylinder creates a larger displacement, it takes more force to create the same pressure as a smaller bore. While a larger master cylinder will take up system slack with less pedal stroke, it will take more force to create the same system pressure. The result after adding the larger master cylinder is a harder pedal which needs much more pedal pressure to create the same amount of braking force. For instance, moving from a 3/4" master cylinder to a 1" requires 77.7% more force on the push rod.

The goal then is to balance the entire system. Pedal force, system pressure and lever travel all need to be taken into account. A balanced system includes the following things:

1) A lever that moves the entire stroke of the master cylinder
2) A Lever that moves comfortably in the confines of a car
3) Uses a reasonable amount of force to reach the desired system pressure
4) Is aligned well enough to minimize lever travel

Use the calculator to help you find the correct ratios. First find the correct caliper combination from the chart, then find the correct ratio for your master cylinder size.

Caliper Alignment Clearance and Position
Axle stand out controls the alignment of the brake system and as a result is extremely critical. To check stand out, first verify that the housing ends are square to the tube and axles. Install the axles and check axle standout (face of axle flange to face of housing end) as accurately as possible. See the chart below for stand out dimensions for MW brake kits. Stand out should be + .015 of the dimension listed. Shims are available to correct the alignment moving the disc outward .015” (MW catalog page 62 or online P/N 71009). Misalignment can cause caliper mount deflection which is one of the causes of a “spongy” pedal. Install your wheels to make sure the caliper to wheel clearance is adequate both radially and from outer face of the caliper to the inner face of the wheel. To bleed the system, the calipers must be positioned at 3 or 9 o’clock. This allows the bleeder to be highest point, ensuring that all air is removed from system.

Common Standouts

  • Symmetrical Ends - 2.834"
  • Olds Ends - 2.834"
  • Mopar Ends - 2.500"
  • Large Ford Ends - 2.500"
  • Small Ford Ends - 2.500"
  • GM 10-12 bolt Ends - 2.812"

Proportioning (Pressure Reducing) Valve
The front brake kits with the integral hub and adaptor are designed for Drag Race applications. The main advantage is the reduction of weight compared to the stock braking system they are designed to replace. Several considerations must be taken into account when installing these kits. When any four piston caliper is used with drum brakes on the rear, a pressure reducing valve must be installed for front pressure control. Because of the small front tire contact area compared to the rear large slick contact area, the rear brakes must absorb more of the stopping energy than the front (contrary to a normal street car). A starting point would be 70% of the rear line pressure to the front brakes. This is especially important when using drum brakes on the rear. With discs on both the front and rear the percentage could be higher depending on the weight distribution and tire size but should still use the pressure reducing valve. We have a pressure reducing valve, P/N 260-2200, and we recommend its installation with front brakes. The pressure bias should be adjusted with gauges, MW P/N 81105, in the front and rear to confirm the pressure differential and then do a stop test. When doing a stop test, for a drag race car, the front tires should skid equally or slightly after the rear tires. This test can be simulated at a very low speed (approximately 5 mph) by using a wet surface or gravel and observing the lock up sequence.

Brake Lines and Fluid
Aircraft AN-3 brake lines and fittings are recommended. Only stainless steel braided teflon hose, stainless or seamless steel tubing (3/16” x .028”) should be used for brake lines. MW has available swaged Teflon Stainless flexible braided line that can be fabricated on a same day basis. The are available online P/N 0300-X-X. Lines should be secured to chassis rails to resist vibration and routed in such a way to avoid possible contact with wheels, tires and other moving parts. Joining hard line and braided line or “T”s should be done using a bulkhead fitting and a small tab welded to the chassis. Long runs should be done with hard tubing to avoid expansion of flexible line. The amount of flexible braided hose in the system should be kept to a minimum. See page 63 or the online store for AN -3 fittings and brake line. Use of DOT 4 or 5.1 fluid with a high boiling point and lubrication for seals and pistons is recommended. Do not use (DOT 5) silicone fluids .

Troubleshooting Your Brakes

Spongy Pedal:
1) Air in system. Bleed brakes, making sure that the bleed valve is the highest point.
2) Disc warped (saucer shaped). Replace or surface rotors
3) Calipers not square with disc. Check housing end alignment, both concentricity and squareness.
4) Linings worn on taper. Make sure that caliper is centered over the rotor and the caliper bracket is not deflecting.
5) Master cylinder bore too small creating excessive high line pressure. Match master cylinder to system.
6) Master cylinder deflection. Stiffen master cylinder mounts

Brakes are locked up after run:
The piston in the master cylinder is not being allowed to return to the full retract location, when released. This condition will maintains line pressure and prevents the car from moving. Re-adjust the linkage so that the piston completely returns to the retract location.

Excessive pad wear, disc shows excess heat:
1) Lining too thin causing rapid heat transfer to calipers, boiling the fluid. Install new linings and re-bleed system.
2) Fluid contaminated with moisture causing boiling of fluid in caliper. Flush old fluid and replace with DOT 5.1 Fluid re-bleed system.

Annual disassembly and cleaning of brake system recommended. Carefully inspect pistons and piston bores. Re-assemble with new “O” rings.

Mark Williams Enterprises

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