Bubbles Up

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.

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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.

001 This is the entire package we’re using to install a Wilwood master and brake proportioning valve
This is the entire package we’re using to install a Wilwood master and brake proportioning valve on our ’66 SS396 Chevelle. This includes a brake pedal adjustment kit to allow us to adapt the master’s pushrod to the Chevelle brake pedal.

Read More: A Turn in the Right Direction

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.

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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.

002 This is the entire package we’re using to install a Wilwood master and brake proportioning valve
We preassembled the proportioning valve bracket and valve to the master in order to make sure the fittings connected properly and so there would be no surprises after we bench bled the master.

Read More: Pushed to its Limits

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.

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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.

003 This Wilwood master also offers outlets on both sides of the master
This Wilwood master also offers outlets on both sides of the master. We capped the ones on the passenger side we were not using. Be sure to use the supplied aluminum sealing washers.

Read More: Cracking the Code

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.

004 We also ordered several bottles of Wilwood’s high temperature brake fluid to use in the car
We also ordered several bottles of Wilwood’s high-temperature brake fluid to use in the car. The Wilwood 570 is listed as a DOT 5.1 fluid and offers better high-temperature stability compared to DOT 4 fluid, while also including lubricants compatible with later-model vehicles with ABS.

Read More: A Firmer Foundation

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.

005 We bolted the master with the prop valve installed on the car
We bolted the master with the prop valve installed on the car. We did this to ensure the valve would clear the inner fender panel on our Chevelle. This also allowed us to make two brake lines from the proportioning valve to the existing brake lines front and rear.

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.

006 It’s imperative that the master cylinder be mounted in the vise as level as possible
It’s imperative that the master cylinder be mounted in the vise as level as possible. This will improve the chances of removing all the air out of the master.

Read MoreAssembly Required

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.

007 Note the severe angle of the master cylinder mounted to the brake booster
Note the severe angle of the master cylinder mounted to the brake booster on this ’70 LT1 Camaro. If you do not bench bleed the master, air will be trapped in the upper forward portion of each piston bore and no amount of on-the-car bleeding will eliminate this problem.

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.

008 Each new Wilwood master cylinder comes with this plastic bench bleeder kit
Each new Wilwood master cylinder comes with this plastic bench bleeder kit. The kit is also available separately, if needed. We installed the plastic adapters into the master and then routed the lines into each reservoir using the clip to hold them in place.

Read More: SWAPPING UP

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.

009 We use a small block of wood to help us manipulate the pushrod during bench bleeding
We use a small block of wood to help us manipulate the pushrod during bench bleeding. You could also use an old shifter ball.

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.

010 With the plastic bleeder lines in place pour enough fluid in the master to cover the lines
With the plastic bleeder lines in place, pour enough fluid in the master to cover the lines—roughly half full to start should be sufficient. Then slowly pump the plunger and air will escape from the tubes. Alternate long and short strokes to ensure all the air is pushed out of the bores. The arrow points to bubbles in the lines.

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.

011 It’s common toward the end of the bleeding process to find a few small bubbles sitting at the top of the line
It’s common toward the end of the bleeding process to find a few small bubbles sitting at the top of the line that can be easily sucked back into the master. With the pistons fully depressed, by slowly retracting the piston, these bubbles will remain in the top of the tube. Then one final stroke should force the remaining air out, completing the process.
012 Wilwood has a slick tip for long term storage of brake fluid to minimize pulling moisture from the air
Wilwood has a slick tip for long-term storage of brake fluid to minimize pulling moisture from the air by placing bottles in a ziplock bag. Larger freezer bags are heavy and will accommodate several bottles.
013 Another option that can improve brake performance is a set of stainless steel flexible brake hoses
Another option that can improve brake performance is a set of stainless steel flexible brake hoses. Each vehicle will require a specific hose so check for both length and connector style before ordering. This particular set is for a GM car with the common D52-style front disc caliper.
014 some easy street driving will be necessary to both bed in new brake pads and also to ensure the system is working correctly
Another option that can improve brake performance is a set of stainless steel flexible brake hoses. Each vehicle will require a specific hose so check for both length and connector style before ordering. This particular set is for a GM car with the common D52-style front disc caliper.

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.

acp february 2024

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