Hydraulic Testing, Bleeding and Fluid Flashcards

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?

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.

Why must a brake fluid container be kept tightly capped when it is not in use?

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.

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?

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.

A technician is pressure bleeding a brake system. Which of the following is the correct practice?

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 technician has just installed a new master cylinder. What should be done before it is connected to the brake lines?

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.

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?

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.

What is the purpose of a residual pressure check valve in the master cylinder outlet for a drum brake circuit?

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.

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?

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.

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?

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.

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?

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.