ASE A5 Hydraulic Testing, Bleeding and Fluid — Questions and Answers
Question 1: A vehicle with a conventional dual-diagonal split hydraulic system is being bled, and the manufacturer gives no special sequence. Which wheel should be bled first?
- Left front, because it is closest to the driver
- The wheel closest to the master cylinder
- The wheel farthest from the master cylinder (Correct answer)
- Either rear wheel, then the fronts in any order
Correct answer: The wheel farthest from the master cylinder
Bleeding normally starts at the wheel with the longest line run from the master cylinder, which is usually the right rear. Air is then pushed out of the longest circuits first.
Unless the manufacturer gives a different sequence, bleed from the wheel farthest from the master cylinder and work toward the closest. The usual order is right rear, left rear, right front, left front. This clears the longest lines first, so air from them is not left behind or pulled back toward the master cylinder. Always follow the service information, because some vehicles with ABS or unusual line layouts require a specific order. Keep the reservoir topped up the whole time so that air is not drawn into the master cylinder.
Question 2: Why must a brake fluid container be kept tightly capped when it is not in use?
- Glycol-based brake fluid absorbs moisture from the air, which lowers its boiling point (Correct answer)
- Brake fluid evaporates quickly and the container would soon be empty
- Air contact turns the fluid into a petroleum-like substance that swells seals
- Exposure to light causes the fluid to become thicker and slower to flow
Correct answer: Glycol-based brake fluid absorbs moisture from the air, which lowers its boiling point
DOT 3, 4 and 5.1 glycol-ether fluids are hygroscopic and absorb water. Absorbed water lowers the fluid's boiling point and promotes corrosion.
Glycol-based brake fluids are hygroscopic, meaning they absorb moisture from the atmosphere, and an open container does this quickly. Water in the fluid lowers its boiling point, so heavy braking can form vapor bubbles and cause a spongy or lost pedal. Water also corrodes calipers, wheel cylinders, and ABS valve bodies. For this reason, use fluid from a freshly opened, sealed container and recap it immediately.
Question 3: A technician is told that a customer previously topped off the reservoir of a vehicle with DOT 5 silicone fluid. The system is now to be serviced with DOT 3. What is the proper procedure?
- Add DOT 3 on top of the DOT 5, since both are DOT-rated and compatible
- Add a small amount of alcohol to blend the two fluids together
- Fully flush the system with the specified fluid and replace any contaminated seals and hoses as needed (Correct answer)
- Drain only the master cylinder reservoir and refill it with DOT 3
Correct answer: Fully flush the system with the specified fluid and replace any contaminated seals and hoses as needed
Silicone DOT 5 does not mix with glycol-based DOT 3, so the system must be completely flushed. Seals that were exposed to the incompatible fluid may need replacement.
DOT 5 is silicone-based and will not mix with glycol-based DOT 3, 4 or 5.1 fluids. Mixing them leaves droplets of one fluid suspended in the other, which can trap moisture and cause corrosion, seal damage, and a spongy pedal. The correct repair is a complete flush of the lines and components with the specified fluid, followed by a thorough bleed. Any seals, hoses, or components that show swelling or damage must be replaced. DOT 5 also should not be used in ABS systems unless the manufacturer says so.
Question 4: A technician is pressure bleeding a brake system. Which of the following is the correct practice?
- Pressurize the system to about 15 psi using an adapter that fits the master cylinder reservoir (Correct answer)
- Pressurize the system to about 100 psi to speed the process
- Remove the reservoir cap and hold the pedal down with a block of wood
- Pressurize the reservoir with shop air directly, without a diaphragm tank
Correct answer: Pressurize the system to about 15 psi using an adapter that fits the master cylinder reservoir
Pressure bleeders typically run at roughly 10–25 psi through a proper reservoir adapter. Higher pressure risks damaging seals and the reservoir.
A pressure bleeder attaches to the master cylinder reservoir through a cap adapter made for that reservoir. It supplies fluid at low pressure, typically around 10 to 25 psi, so air is pushed out when each bleeder screw is opened. The tank must use a separating diaphragm so that compressed air never touches the fluid and mixes with it. Excess pressure can split the reservoir or damage the master cylinder seals. Always check the manufacturer's spec for the vehicle first.
Question 5: A technician has just installed a new master cylinder. What should be done before it is connected to the brake lines?
- Fill the reservoir with fluid and pump the pedal until firm after the lines are attached
- Bench bleed the master cylinder to remove air from its bores and outlet ports (Correct answer)
- Apply a light coat of grease to the piston bores to help the seals slide
- Back off the pushrod adjustment fully so the pistons stay retracted
Correct answer: Bench bleed the master cylinder to remove air from its bores and outlet ports
Bench bleeding removes air trapped in the master cylinder bores before installation, making the later system bleed much easier.
A new or rebuilt master cylinder contains air in its bores and outlet passages. It is mounted in a vise, and tubes are connected to the outlet ports and routed into the reservoir. The piston is then pushed in slowly with a blunt tool until no more air bubbles appear. Doing this before installation prevents large air pockets from being pushed into the lines. The wheel circuits still need to be bled after installation. Never use grease in the bores, since it contaminates the fluid and damages the seals.
Question 6: During a hydraulic test, the brake pedal slowly sinks toward the floor under steady foot pressure. No external leaks are found at the wheels, lines, or master cylinder, and the fluid level in the reservoir does not drop. What is the most likely cause?
- A restricted brake hose collapsing internally
- Worn or leaking seals inside the master cylinder letting fluid bypass the piston (Correct answer)
- A stuck residual check valve
- Excessive pushrod free play at the brake pedal
Correct answer: Worn or leaking seals inside the master cylinder letting fluid bypass the piston
A pedal that sinks slowly with no external leak and no loss of fluid points to internal bypass at the master cylinder piston seals.
If the pedal slowly sinks while holding steady pressure and there are no external leaks, the fluid is leaking past the primary piston seals inside the master cylinder. The fluid returns to the reservoir, so the level does not drop. The technician should confirm by checking for a sinking pedal with the engine off and then replace or rebuild the master cylinder. A restricted hose would cause dragging brakes rather than a sinking pedal. Too much pushrod free play would cause a low pedal, but the pedal would not sink under steady load.
Question 7: What is the purpose of a residual pressure check valve in the master cylinder outlet for a drum brake circuit?
- To limit pressure to the rear brakes during hard stops
- To release trapped air to the atmosphere through the reservoir cap
- To prevent fluid from flowing back to the master cylinder when the pedal is applied
- To hold slight pressure in the line so the wheel cylinder cups stay sealed and air is kept from entering (Correct answer)
Correct answer: To hold slight pressure in the line so the wheel cylinder cups stay sealed and air is kept from entering
Residual check valves hold roughly 6–18 psi in drum brake lines, keeping the cup seals tight against the wheel cylinder bores and reducing air entry.
Residual pressure valves are located in the outlet of the master cylinder or in the line fittings for drum brake circuits. They hold a small amount of pressure, commonly around 10 psi, in the lines after the pedal is released. This keeps the lip seals in the wheel cylinders pressed against the bore walls, so that air is not drawn in and fluid does not leak out. They are not used on disc brake circuits, because the caliper has to retract fully, and a residual valve there could cause drag. A faulty one can cause dragging drums or a soft pedal.
Question 8: On a vehicle with ABS, air remains in the hydraulic control unit after the wheel circuits have been conventionally bled, and the pedal is still soft. What is the best next step?
- Replace the ABS wheel speed sensors
- Use a scan tool to run the ABS automated bleed procedure, which cycles the valves and pump, and then bleed the wheels again (Correct answer)
- Disconnect the ABS fuse and bleed the wheels again with the ignition on
- Remove the hydraulic unit and bleed it on the bench
Correct answer: Use a scan tool to run the ABS automated bleed procedure, which cycles the valves and pump, and then bleed the wheels again
Many ABS units hold air in internal passages that conventional bleeding cannot reach. A scan tool bleed cycles the valves and pump so the air can be pushed out.
The hydraulic control unit has internal valves, accumulators, and passages that can trap air. Conventional bleeding at the wheels may not move it. Most manufacturers provide a scan-tool-driven procedure that cycles the solenoids and pump motor while the technician opens each bleeder in turn. After this the system should be bled again at the wheels to clear the air the unit released. Follow the service information, because the required steps and bleeding order differ between manufacturers.
Question 9: Which of the following is a safe fluid to use for cleaning hydraulic brake components such as a master cylinder bore or caliper pistons during service?
- Mineral spirits
- Denatured alcohol or clean brake fluid (Correct answer)
- Engine oil
- Penetrating oil spray
Correct answer: Denatured alcohol or clean brake fluid
Denatured alcohol and clean brake fluid will not harm rubber seals. Petroleum products cause the rubber to swell and fail.
Brake system seals and hoses are made from rubber compounds like EPDM that are formulated for glycol-based brake fluid. Petroleum products, including mineral spirits, engine oil, and penetrating oil, make this rubber swell, soften, and fail. Even a small amount of contamination can cause a sticking caliper or master cylinder failure. Parts should only be cleaned with denatured alcohol or fresh brake fluid, and hands and tools should be free of grease or oil. Contaminated systems must be flushed and all rubber parts replaced.
Question 10: A technician uses a brake fluid tester and finds that the fluid's boiling point is well below the minimum for its DOT rating. What does this indicate?
- The fluid contains too much moisture and should be replaced (Correct answer)
- The fluid has been contaminated with silicone and should be topped off
- The fluid has too high a viscosity for the system
- The master cylinder has failed internally
Correct answer: The fluid contains too much moisture and should be replaced
Water absorbed by glycol-based fluid lowers the boiling point. A boiling-point reading below the rated minimum means the fluid should be replaced.
Boiling-point testers heat a sample and measure when it boils. This is a way to estimate how much water the fluid has absorbed over time. As the water content rises, the boiling point drops, which means the fluid can form vapor in the calipers during heavy braking, causing a soft pedal and fade. If the reading is below the minimum for its DOT rating, flush the system and refill with fresh fluid of the correct type. Test strips for copper content are also used to detect corrosion inhibitors that have broken down.
ASE A5 Hydraulic Testing, Bleeding and Fluid
A vehicle with a conventional dual-diagonal split hydraulic system is being bled, and the manufacturer gives no special sequence.
Which wheel should be bled first?
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