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Vacuum Booster – Partial Brake Assist Diagnostic Tip

Vacuum Booster – Partial Brake Assist Diagnostic Tip

Problem: Poor brake assist – lack of stopping power or having to depress the brake pedal harder than usual to stop.

Solution: If you suspect the brake vacuum booster of providing only partial assist use the following steps to determine if it is a vacuum supply problem or a booster problem.


  1. Pull another vehicle of similar engine size next to the vehicle in question and connect that vehicle’s vacuum source to the suspect booster. I usually use an old piece of air line for this purpose.
  2. With the new source vehicle running apply the brake pedal in the suspect vehicle and check for a change in brake assist. If no change is felt then the booster is likely the culprit. If there is a change in the assist (pedal feels normal) then the vacuum plumbing is likely the problem.
  3. If the vacuum plumbing is the problem then closely inspect the components between the booster and the vacuum source to locate the problem.

More info: The vacuum booster has proven to be one of the most reliable parts in the brake system. This reliability has caused many technicians to overlook the booster as a potential source of brake problems. The booster is also high on the list of unpleasant parts to install. With this in mind it is a good idea to make sure the booster is the source of the problem before installing another unit.

Unfortunately this article does not allow a full explanation of the inner workings of a booster but you do need a basic idea of what is going on inside to allow accurate diagnosis. The main components inside a booster that control the assist function are the air and vacuum valves. The position of the valves determine what stage the booster is in. There are three stages of booster operation – unapplied pedal (no assist), applied pedal (assist) and hold (isolate assist). When a vehicle is running and the brake pedal is not applied the booster is in a suspended state. The vacuum valve is open and the air valve is closed causing the diaphragm to be “suspended”. When the brake is applied the vacuum valve closes and the air valve opens. Closing the vacuum valve isolates the boost chamber from the vacuum chamber. Opening the air valve allows atmospheric air to enter the boost chamber. The difference in pressures on either side of the diaphragm is what produces the assist. When the driver is at a stop light the pedal pressure is usually reduced causing the booster to go into the hold mode. During the hold mode the air valve closes and the vacuum valve remains closed. This isolates the air in the boost chamber until the driver releases the brake pedal.

Pressure Testing Brake Hydraulic System Tips

Pressure Testing Brake Hydraulic System Tips

To properly use the results of a pressure test it is best to first understand the pressures generated during different types of braking and the function of the different brake valves used in the system.

You will need to obtain a brake hydraulic pressure test kit similar to the image above

Because most manufacturers do not publish pressure specifications it is often necessary to use generic values. Typically the hydraulic pressure generated during light to normal braking will be from 300psi to 500psi. Panic braking is generally considered to start at about 600psi. The proportioning valve will start to limit pressure to the rear brakes at this time. If a full panic stop is taking place the proportioning valve will limit the rear brake pressure to about 800psi while the front brakes will climb to between 1500psi to 2000psi. The psi numbers presented here are averages based on many tests performed on different vehicles.

Two types of pressure testing are common, checking the front to rear brake pressures and checking an axleʼs side to side pressures. The uses of each of the tests is explained below:

Front to Rear pressure for a properly operating system

Checking front to rear pressure:

Performing a front to rear pressure test can be done to help diagnose the following conditions:

  • Proportioning valve malfunction – Diagnose the cause of rear wheel lockup during panic braking or possibly a proportioning valve causing a restriction (wonʼt reopen after release).
  • Premature front brake pad wear – Pressure tests are used to determine if a lack of rear braking is related to the cause of front pad wear.
  • Height sensitive control valve diagnosis or adjustment – These valves usually incorporate pressure tests in the adjustment procedure and can be diagnosed using pressure gauges.

Side to Side pressure for a malfunctioning system

Checking side to side pressure:

Performing a side to side pressure test can be done to help diagnose the following conditions:

  • Brake pull – different hydraulic pressures results in different clamping forces and can result in a pull
  • Side to side pad wear differences – if the pads on one side of an axle are wearing faster than the pads on the opposite side

Performing a Front to Rear Pressure Test

  1. Connect the gauges using the manufacturerʼs instructions. Most pressure gauges connect to the caliper or wheel cylinderʼs bleeder screw hole (See Figure 85.4). Make sure to properly bleed each gauge hose.
  2. Place the gaugeʼs memory needles on zero.
  3. Start the vehicle. Apply the brake first using light to moderate force as if making a normal stop. Note the pressure readings, under this pedal force the front pressures should not be over 500psi in most vehicles. Compare the front and rear pressure readings. At this pedal force they should be equal or near equal. If equal continue to next step. If there is a large difference with the rears being substantially lower than the fronts there is a potential problem. The list of possible causes will depend on the system configuration. The list will include:
  • Restricted proportioning valve
  • Restricted brake hose to the wheel or axle
  • Restricted steel brake line
  • Out of adjustment or restricted height sensitive control valve
  • Restriction in ABS modulator


Preventing Damage to ABS Modulators During Brake Pad Replacement

Preventing Damage to ABS Modulators During Brake Pad Replacement

Many of us are unaware that every ABS modulator is equipped with multiple filters. These filters can be placed in the inlets, isolation and dump valves and pump or motor. The filters are designed to keep the ABS unit free of debris. Below you can see a few examples of these filters. Most filters are a very fine screen mesh while others are a more course metal mesh. These filters are unserviceable.

Because these filters are unserviceable it is very important to keep them clean. Dirty filters can lead to dirty particles entering the ABS unit. Particles can clog ports and keep actuators from properly sealing. You can inadvertently send sludge and dirty particles up into the ABS unit by depressing a caliper piston too quickly.

This is a brake master cylinder made from acrylic. You can see that it is clean and looks to be capable of performing its proper function.

This is the same acrylic master cylinder with the addition of sludge filled brake fluid. When the caliper piston is depressed too fast it stirs up sludge and quickly sends it back up through the brake system. This sludge can clog ports and malfunction the ABS unit.

There is a simple solution to help prevent sending sludge up into the ABS unit. First, if you have a line lock tool and are able to open the bleed screw then use it to block the brake hose. Then, while depressing the caliper piston, open the bleed screw to allow the brake fluid to escape. Your second option is to not open the bleed screw and don’t block the brake hose. This must be done carefully, but if you take at least 60 seconds to slowly depress the caliper piston then you normally will not stir up any sludge or debris. This will help to keep the ABS filters, ports and actuators clean.

Tech tip: When changing brake pads, take at least 60 seconds to slowly depress a caliper piston. 

Rotors, Brake Hoses and More

Rotors, Brake Hoses and More

Brake Service Basics

With everything that could go wrong, servicing brakes can sometimes feel overwhelming. Over the years we have found if we keep it simple and use tried and true service methods your brake work can be very successful.

The following pictures and text present brake service repair tips and methods learned over the years or shared with us from seasoned professionals.

Line Locks

Professional brake service people normally use line locks like the one shown. Line locks are diagnostic tools in addition to being service tools. If you are changing a caliper and don’t want the system to run dry because the hose is dripping constantly then block it off with a line lock tool. There are several types. Just don’t use locking pliers.

Valve Stem Block

If you have a “banjo” type of hose to caliper connection you can block the fluid flow while changing the caliper by inserting a valve stem into the large bolt opening of the hose.

Use Soap and Water

If you have your rotors turned wash them off with soap and water before reinstalling them. Even if they have been sprayed with a brake clean type of product it is best to wash them with soap and water. Brake cleaning products remove oil based film but it takes soap and water to really clean everything.

Rotor Coating

Some new rotors have a petroleum coating while other have a unknown coating or some even say to remove from the box and install on the car. Some coatings are supposedly ok to make contact with a new set of pads and do no harm, but many knowledgeable professionals still wash all rotors before installation. Most professionals suggest washing new rotors with a brake clean type of cleaner then a soap and water wash. This is the safe way to be sure they are really clean and ready to use.

Rotor Run Out

If you see a shiny spot on a rotor such as the one shown it’s an indication of excessive rotor run out (wobble). This problem may have been caused by either rust and scale behind the rotor hub face, the wrong method or value of torque when tightening the wheels or hub run out that caused the rotor to not track at zero run out.

Hanging Caliper

Don’t ever hand a caliper by its attaching hose. Use a wire hook, a flexible wire wrap or something to positively hold it from hanging on the attaching hose.

Excessive Heat

The separation of the hose from the crimp holding the attaching end was caused by excessive heat. In this case a torch was used to heat the end to enable the flare nut on steel brake line to be loosened. While not common, extremely high brake operating temperatures can also cause this connection to be weakened or fail.

Rust Build-Up

Just because a hose looks ok doesn’t mean it is. This hose has an internal restriction caused by rust and scale building up on the inside of the wrap around clamp holding the flexible hose in place. As the rust formed it caused inward pressure on the hose until it squeezed the hose enough to block the small fluid opening inside. This caused a brake pull to the other side of the vehicle. Appearances can be deceptive.

Locking Plier Problems

This hose also appears to be ok. There are no external cracks yet the vehicle has a pull to the opposite side. What has happened is someone at one time used locking pliers instead of a line lock on the hose. This cause the internal layers of the hose to break down and a piece of the internal liner has broken free and is now blocking the fluid from reaching he caliper.

We hope you learned something that can make your work day a little easier. Please let us know if there are any specific repairs or tips you want us to explore in the future.

For questions or comments you can email me at

Vacuum Bench Bleeding

Vacuum Bench Bleeding

Problem: Less effective bench bleeding causes air to enter hydraulic system.


Cause: Air left in the no-pressure areas of a master cylinder migrates to pressure areas once master cylinder is installed.


Solution: Apply unconventional vacuum bench bleeding technique to insure all air is removed during bench bleed process.

Step 1. Mount the master cylinder in a vise with the outlets at the highest point. Install vacuum bench bleeding kit or brake lines at outlets and depress the master cylinder piston about 1/8″. Hold the piston in the depressed position. Make sure to grip against the snap ring and not the cylinder bore. This step changes the fluid path.

Step 2. Use a vacuum bleeder to apply suction at the secondary outlet while holding the outlet hose in an upright position. If we apply vacuum with the piston in this position the vacuum will collapse the primary cup seal and suck fluid through the replenishing port into the low pressure chamber. Once it fills up the fluid will pass over the cup seal and fill the pressure chamber and then out the line fitting.

Step 3. Repeat vacuum bench bleeding process on the primary outlet port.

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Bench Bleeding the GM QTU Master Cylinder

Bench Bleeding the GM QTU Master Cylinder

Problem: Ineffective bench bleeding causing air to enter hydraulic system

Cause: Air left in the no-pressure areas of master cylinder migrates to pressure areas once master cylinder is installed.

Solution: QTU master cylinders can be bench bled using a combination of pressure or vacuum bleeding on the secondary end and RFI on the primary side. Conventional bench bleeding requires a 15 second pause between each single stroke of the master cylinder. using the methods described here the process can be done in under 5 minutes. Here are the steps involved:

1. Place master cylinder in vise so that the master cylinder body is in a level position.

2. Install bench bleeding kit supplied with master cylinder. Route rubber hose from secondary master outlet to master reservoir.

3. For primary side use RFI injector hose assembly (adapter, connector, tubing, in-line coupler) instead of bleeder kit hose.

4. Prepare the RFI injector for pressure bleeding. Install universal port adapter on output side of gun. Attach injector input hose to new fluid supply (prime gun).

5. Fill secondary half of reservoir to 1/2 full level with new brake fluid making sure to keep secondary master output hose submerged.

6. Use needle nose vise grips to push master cylinder piston in 1/8 inch and hold in that position making sure to clamp against the snap ring. (See figure 82.1)

screen-shot-2016-10-24-at-3-16-50-pm Figure 82.1

7. Place injector inside secondary reservoir against vent and replenishing ports. (See Figure 82.2)

screen-shot-2016-10-24-at-3-18-47-pm Figure 82.2

8. Push fluid through master cylinder using slow steady strokes until only fluid passes from secondary outlet (approximately 4 strokes).

9. Release vise grips allowing master to return to rest position. Move master cylinder to an angle position (as it would sit on vehicle / top of reservoir level).

10. Use UPA (Universal Port Adapter) to perform RFI bleeding on primary outlet of master cylinder. (See Figure 82.3)

screen-shot-2016-10-24-at-3-21-34-pm Figure 82.3

11. Push fluid into master cylinder’s primary outlet using slow steady strokes until only fluid escapes from quick take up valve (approximately 8 – 15 strokes). Be aware of geyser from quick take up valve.

12. Remove hose from master cylinder primary outlet and let gravity bleed until only fluid passes from master cylinder primary outlet.

13. Attach bleeder kit hose to primary outlet and route to reservoir. Master cylinder is now ready for installation.

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