Wilwood Master Cylinder Bore Size: How to Choose

By Core Brake Parts Tech Team — Circleville, OH warehouse

Choosing a Wilwood master cylinder bore size comes down to one trade-off. A smaller bore makes more line pressure for the same push on the pedal but needs more pedal travel. A larger bore moves more fluid per inch of stroke and gives a shorter, firmer pedal, but it takes more leg effort to reach the same pressure. The right size is the one that matches your pedal ratio, whether you run a booster, and the total piston area of the calipers it has to feed.

How Bore Size Changes Pressure and Pedal Feel

A master cylinder is a piston in a bore. Line pressure equals the force on the pushrod divided by the area of that piston, so shrinking the bore concentrates the same force onto a smaller area and pressure goes up. The cost is volume: a smaller piston pushes less fluid for every inch it moves, so the pedal has to travel farther to fill the caliper pistons and bring the pads into firm contact with the rotors.

Brake engineers describe the two ways to get it wrong the same way. A bore that is undersized for the calipers gives a long, soft or spongy pedal. An oversized bore gives a rock-hard pedal that still does not stop the car hard, because the driver cannot push hard enough to build pressure. Put simply: decreasing bore size decreases pedal effort and increases pedal travel, and increasing bore size does the opposite, with everything else in the system unchanged.

Wilwood's Pressure Chart by Bore Size

Wilwood publishes a chart of estimated line pressure for a 100-pound push on the pedal pad. The math behind it is 100 times the pedal ratio, divided by the bore area in square inches. Here are the common bore sizes at a 6:1 pedal ratio and at 7:1, straight from that chart. The figures apply to single master cylinder systems, not balance bar setups.

Read the table two ways. Going down a column, a 1-1/8 inch bore makes a little over 60 percent of the pressure a 7/8 inch bore makes for the same foot. Going across a row, adding pedal ratio raises pressure without touching the master cylinder. The bore area column also tells you volume: a 1-1/8 inch piston displaces 0.99 cubic inches per inch of stroke against 0.60 for a 7/8 inch piston, about 65 percent more fluid for the same pushrod movement.

Step 1: Measure Your Pedal Ratio

Wilwood defines pedal ratio as the distance from the pedal pivot to the middle of the foot pad, divided by the distance from the pivot to the master cylinder pushrod attachment. A tape measure is accurate enough. Wilwood's guidance for typical ranges:

• Manual (non-boosted) brakes: about 6:1 to 7:1.
• Power-assisted brakes: about 4.5:1 to 5:1, because the booster supplies the extra force.
• Wilwood pedal assemblies: offered in 5.1:1, 6:1, 7:1 and 10:1, with the 10:1 pedal built for off-road race vehicles running non-boosted brakes and large-piston calipers.

If you are converting a car from power to manual brakes and keep a low power-brake pedal ratio, no bore size will make the pedal feel right. Fix the leverage first, then pick the bore.

Step 2: Add Up Caliper Piston Area

Pressure alone does not stop the car. Clamp force is line pressure multiplied by piston area, so the calipers decide how much fluid the master cylinder has to move and how much pressure it needs to make. Wilwood calculates a caliper's effective piston area from one side of the caliper only, which also holds true for a single-piston caliper. The formula on its instruction sheet is area = bore x bore x 0.785 for each piston, then add the pistons on that side together.

Wilwood's own example is the Forged Superlite 6 with 1.12, 1.12 and 1.62 inch pistons: 0.99 + 0.99 + 2.06 = 4.04 square inches. For comparison, a single 2.75 inch piston works out to 5.94 square inches. At 1,000 psi, that 4.04 square inch caliper puts roughly 4,040 pounds of piston force on each pad.

Total the front calipers and the rear calipers separately. The end of the car with more total piston area needs more fluid volume, which matters for both the bore choice and the plumbing. If you are still choosing hardware, browse our brake calipers with the piston sizes in hand.

Step 3: Match the Bore and Plumb It Correctly

With pedal ratio and piston area known, use the chart to land in a sensible pressure range, then let the caliper volume steer you. Large multi-piston calipers at all four corners push you toward a bigger bore so the pedal does not run long. Smaller calipers, or a manual system where leg effort is the complaint, push you toward a smaller bore. When you fall between two sizes, the smaller bore gives lighter effort and more travel, and the larger gives heavier effort and less travel. Race cars with dual master cylinders on a balance bar are sized one circuit at a time, since each master cylinder feeds only the front or only the rear and the bar shifts bias mechanically. Shop master cylinders once you have your size.

Wilwood's aluminum tandem master cylinder instructions add rules that decide whether the bore you chose works in the car:

• Full stroke is mandatory. The pushrod must bottom out in the bore before the pedal hits the floorboard, including insulation and carpet. On the 7/8 and 15/16 inch tandem units the stroke is 1.10 inches. If one circuit fails, the pedal may travel 50 to 80 percent of that stroke before the other circuit builds pressure, so a pedal that runs out of room removes the safety margin.
• Primary outlet to the bigger end. Connect the primary outlet to the brakes at the end with the greater total effective piston area, usually the front.
• Residual pressure valves. All drum brake circuits need an inline 10 pound residual valve. Disc brakes with the reservoir mounted lower than the caliper bleed screws may need a 2 pound valve to stop fluid drain-back and excess pedal travel. Four-wheel disc systems with the reservoir above the bleeders usually do not need one.
• Bias adjustment. Use an adjustable proportioning valve to set front-to-rear balance.
• Lines and fittings. Use double-flared lines only, no sealant on the fittings, and hard steel lines along the chassis with flexible lines only between chassis and suspension. See our brake lines and brake hoses.

Wilwood also states these tandem units are built for custom, performance and racing brake systems and are not intended as a direct replacement for any OEM application.

Bench Bleeding and Testing After Install

Bench bleed the master cylinder before attaching the lines: fill the reservoirs with fresh fluid from a sealed container, keep the cylinder level, and cycle the piston with bleeder tubes routed back into the reservoir until no air comes out. Wilwood specifies 20 to 25 foot-pounds for the outlet fittings and plugs with their aluminum crush washers. Then bleed the whole system and check for leaks.

Wilwood's minimum test procedure: hold firm pedal pressure for several minutes and confirm the pedal does not sink; make several hard stops at 2 to 5 mph while turning lock to lock, then pull the wheels and check for rubbing or leaks; finally make moderate and hard stops at 15 to 20 mph. Do not drive the car if the pedal sinks or can be pushed to the floor. Wilwood tandem master cylinders are not user serviceable and go back to the factory for rebuild.

Frequently Asked Questions

Does a smaller master cylinder bore give a firmer pedal?

No. A smaller bore makes more pressure with less effort, but the pedal travels farther, which usually feels softer. A larger bore shortens travel and firms up the pedal but takes more force to stop the car.

What bore size should I use with manual disc brakes?

Start with your pedal ratio, ideally 6:1 to 7:1 for manual brakes, and your total caliper piston area, then use the pressure chart. There is no universal answer; the right bore is the one that reaches useful pressure with the full stroke still available before the pedal hits the floor.

Can I fix a hard pedal without changing the master cylinder?

Sometimes. Raising the pedal ratio increases pressure for the same bore, and a booster adds force. If the pedal ratio is already right and the pedal is hard with weak stops, the bore is likely too large for the calipers.

Do I need a residual pressure valve with a Wilwood master cylinder?

Drum brake circuits need a 10 pound valve. Disc brakes need a 2 pound valve only when the master cylinder reservoir sits lower than the caliper bleed screws.

Which outlet goes to the front brakes?

The primary outlet goes to whichever end of the car has more total caliper piston area, which is the front on most vehicles.

Why is my new master cylinder pedal going to the floor?

Air in the system, a leak, a pushrod adjustment that stops the piston from returning fully, or fluid drain-back without a residual valve are the usual causes. Re-bleed and check for leaks and pushrod adjustment before driving.

Sources

1. Wilwood Master Cylinder and Pedal Ratio Chart (FL162) - pressure by bore and pedal ratio at 100 lb effort, bore areas, caliper piston area method and Superlite 6 example
2. Wilwood Aluminum Tandem Master Cylinder Instructions (DS-969) - 1.10 in stroke, circuit failure travel, outlet plumbing, residual valves, fitting torque, area formula, test procedure
3. Wilwood: Pedal Ratios and How to Find Them - pedal ratio definition and manual versus power brake ranges
4. Upgrade Motorsport: Guide to Master Cylinder Sizing and Brake Bias - smaller bore gives longer, softer travel and larger bore shorter, firmer travel; dual master cylinders on a balance bar
5. TBM Brakes: Master Cylinder Sizing - pressure equals force over area, symptoms of undersized and oversized bores