Why the Simple Act of Bench Bleeding a Master Cylinder is Important
By Jeff Smith – Photography by the Author
In the world of high-performance driving, there’s nothing more unsettling than a mushy brake pedal. Oftentimes this is the direct result of shortcuts taken when installing a new master cylinder. The most universal cause of a spongy brake pedal is air in the system. Brakes are applied using hydraulic pressure created at the master cylinder. If even a small amount of air is trapped anywhere in the system, this creates that unsteady pedal.
One of the most common causes of air in the system when replacing or repairing a master cylinder is skipping the process of bench bleeding the master. To be blunt, that shortcut just doesn’t work. Air is nearly guaranteed to be trapped inside the master cylinder when the bench procedure is ignored. This happens because master cylinders are often mounted in the vehicle at an angle, trapping air in the master that never leaves.

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So, all master cylinder manufacturers including Wilwood recommend that anytime a master cylinder is installed or if it is removed to be rebuilt, that the bench bleeding process be properly executed.
We thought we’d not only run through this process to reveal how easy it is, but also to include a couple of simple tech tips that will not only help build a firmer brake pedal but also touch on the advantages of including an adjustable brake proportioning valve.
But first, let’s get into the procedure for bench bleeding. We will be using a compact Wilwood tandem master cylinder with a 7/8-inch piston diameter since our vehicle is an early Chevelle using manual, non-power-assisted disc brakes front and rear. Wilwood offers this and other master cylinders in various piston diameters up to 1 1/8 inch to fit your requirements.

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The first step after removing the components from their shipping boxes and reading the instructions was to pre-fit the master and the bracket assembly to the car so we could determine the correct brake line orientation and make a couple of simple brake lines to complete that process.
With the brake lines fitted, we next made sure we had the proper master cylinder plunger length by using Wilwood’s pedal pushrod kit that included several pushrods and clevises that we used to make the proper connection. Be sure to leave roughly 1/16 inch of clearance when connecting to the pedal so that the master does not apply pressure with the pedal fully released. This is an easy mistake to make and often becomes worse when the brakes heat up.

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With the master pre-fitted, we could now move to the bench-bleeding process. One of the most important steps here is to ensure that when mounting the master to the vise that the master is as level as possible. If the master is positioned at an angle, there is a good chance air will be trapped inside the bore, which will make a successful bleeding process difficult.
Each Wilwood master cylinder comes with its own small plastic bleeder kit but if you are rebuilding a used master, you can purchase this bleeder kit separately using the number found in the parts list chart in this article. Install the plastic adapter fitting into the master, slip the plastic lines on the fittings, and use the black plastic clip to retain the lines in each reservoir. Then, fill both reservoirs with fresh brake fluid and make sure the fluid is high enough to always submerge the end of the plastic lines below the fluid.

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With fluid in each reservoir, use a screwdriver with sufficient length to allow pushing the master cylinder piston through its complete length. This is important. If the screwdriver or plunger is not long enough, the pistons will not travel their full length and air will remain in the master. With the master level in the vise, slowly push the plunger inward, which will displace air from both bores. You will see this as bubbles escaping from the clear plastic lines and also escaping from the fluid entry points in the reservoir floor.
Use a combination of short and long strokes to push all the air out of both reservoirs. Toward the end of this process, you may notice tiny bubbles of air at the top of the arc formed by the plastic lines. This is why Wilwood used clear plastic lines so you can see these bubbles. If the plunger is released quickly, this pulls the air bubbles back into the master cylinder.

One trick to eliminate these last bubbles is to retract the piston slowly enough to keep the bubble at or near the top of the plastic line. Once the piston is again at rest, you can eliminate this air with one easy push of the plunger.
Now with the master cylinder properly bench bled, you can reinstall it on the firewall and quickly hook up the brake lines you have previously made. It’s a good idea to have some outlet plugs for the master cylinder in place to prevent leaking brake fluid onto any painted surfaces in the engine compartment. This will save the hassle of leaking fluid ruining nice paint. A couple of towels placed under the master are further insurance.

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This might be a great place to discuss brake fluid. The accompanying chart lists both the dry and wet boiling points for the various brake fluids. The DOT lists these temperatures as the standard, but many companies, including Wilwood, offer fluid that exceeds these standards. But don’t be tempted to use race brake fluid in a street-driven car since these higher operating temperature fluids also tend to more readily absorb water. We chose Wilwood’s 570 DOT 5.1 for our streetcar.

The best recommendation is to use a fresh bottle of brake fluid with any new or rebuilt master cylinder and to fully flush the entire brake system of old fluid. All brake fluids absorb water over time so be mindful of that as one of your maintenance procedures.
The next step is to bleed the rest of the hydraulic system. We won’t go into details on how to do this except to say that it’s always best to start at the farthest wheel away from the master, which is normally the right rear, and work toward the master with the left front as the last bleed position. One tip is to turn the proportioning valve fully counterclockwise when bleeding the rear brakes as this minimizes any flow restriction to the rear brakes.

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Since we added an adjustable proportioning valve to our application, once the master cylinder is bolted in place and the entire system has been bled, it will be necessary to adjust the proportioning valve to optimize braking. Again, we won’t go into all the details, but the important aspect is to start with the valve adjusted to the minimum pressure to the rear and slowly work your way up in two turn adjustments to where the rear brakes will lock just after the front brakes.

The goal with this adjustment procedure is to prevent premature lockup of the rear brakes before the front since this can cause loss of vehicle control in an emergency stop. Do this testing with proper tire pressures, a half tank of fuel, and no passengers in the car. Always do your testing in an area where there is no traffic and preferably no obstructions near the test section of road. Keep in mind that any major change to the vehicle’s ride height, rear tire diameter, suspension, or weight distribution will affect this adjustment.

We’ve covered a little more than just bench bleeding, but it should be clear now why this simple step is so important. Take your time, give yourself a brake, do the job right, and the end result will put a smile on your face for a job well done.




Brake Fluid Boiling Points
Note that DOT 5 fluid is silicone-based and is not recommended for either race or street use. All temperatures are expressed in degrees Fahrenheit. Wet brake fluid temps are defined by the DOT as fluid containing a minimum of 4 percent water. As water percentage increases, this lowers the brake fluid’s boiling point. Wilwood’s Race Only fluid is not DOT approved.
Fluid | Dry | Wet |
DOT 3 | 401 | 284 |
DOT 4 | 446 | 311 |
DOT 5 | 500 | 356 |
DOT 5.1 | 500 | 356 |
Wilwood 570, 5.1 | 570 | 313 |
Wilwood EXP 600 | 626 | 417 |
Wilwood Race Only | 645 | 432 |
Parts List
Description | PN | Source |
Wilwood tandem master cylinder, 7/8 inch | 261-14961-BK | Summit |
Proportioning valve kit | 260-14248 | Summit |
Brake pedal pushrod kit | 330-13914 | Summit |
Wilwood fluid, 570 Hi-temp, 12 oz. | 290-0632 | Summit |
Wilwood fluid, EXP 600, 12 oz. | 290-6209 | Summit |
Wilwood race only, 15 oz. | 290-16353 | Summit |
Wilwood stainless front brake line kit | 220-11371 | Summit |
Sources:
Summit Racing
(800) 230-3030
summitracing.com
Wilwood Engineering
(805) 388-1188
wilwood.com
Click on this issue’s cover to see the enhanced digital version of Bubbles Up.