ASE A5 Master Cylinder — Questions and Answers
Question 1: A vehicle's brake pedal slowly sinks toward the floor while steady pressure is held. There are no external leaks and the reservoir level has not dropped. What is the most likely cause?
- Worn or damaged piston cups inside the master cylinder bore (Correct answer)
- A restricted brake hose at the left front wheel
- A glazed brake rotor surface
- A seized rear caliper piston
Correct answer: Worn or damaged piston cups inside the master cylinder bore
A pedal that sinks with no external leak and no fluid loss points to fluid bypassing the master cylinder piston cups internally.
When the primary piston cup wears or is damaged, fluid slips past it instead of building and holding pressure. The pedal sinks under steady foot pressure even though the reservoir level stays the same. Pumping the pedal may temporarily restore a higher pedal. A restricted hose or seized caliper would cause drag or pull, not a sinking pedal. A glazed rotor would reduce friction but would not make the pedal sink.
Question 2: During a master cylinder replacement, the pushrod is adjusted too long so that it holds the primary piston forward with the pedal released. Which symptom is most likely?
- Excessive brake pedal travel before the brakes apply
- Brakes that drag and overheat because the compensating port stays covered (Correct answer)
- A hard pedal with the engine running
- A brake warning lamp that stays off during a circuit failure
Correct answer: Brakes that drag and overheat because the compensating port stays covered
A pushrod that is too long keeps the primary cup over the compensating port, so pressure cannot release and the brakes drag.
With the pedal released, the primary cup should sit clear of the compensating (bypass) port so fluid can return to the reservoir and pressure can equalize. A pushrod that is too long holds the piston slightly forward and blocks that port. Residual pressure stays trapped in the lines and the brakes drag, building heat. A pushrod that is too short causes the opposite problem: extra pedal travel and a low pedal.
Question 3: When bench bleeding a master cylinder, why are the bleed tubes or fittings routed back into the reservoir with their ends below the fluid level?
- To keep the piston seals lubricated with dry air
- To raise fluid temperature during the procedure
- To prevent air from being drawn back into the bore on the piston's return stroke (Correct answer)
- To measure how far the piston travels
Correct answer: To prevent air from being drawn back into the bore on the piston's return stroke
Keeping the tube ends submerged means that only fluid, not air, is drawn back into the bore when the piston returns.
On a bench bleed the master cylinder is held level in a vise, the outlet ports are fitted with tubes or plugs, and a blunt rod pushes the pistons in slowly. As the pistons return, they draw whatever is at the tube ends back into the bore. Submerged ends mean that fluid is drawn in rather than air. Repeat the strokes until no air bubbles appear. Only then is the cylinder installed, so air is not carried into the lines.
Question 4: A technician finds brake fluid on the firewall and inside the vacuum booster behind the master cylinder, and the reservoir level keeps falling. What is the most likely cause?
- A leaking front brake caliper piston seal
- A leaking rear (primary piston) seal in the master cylinder (Correct answer)
- A cracked brake hose at the rear axle
- A faulty brake light switch
Correct answer: A leaking rear (primary piston) seal in the master cylinder
Fluid at the booster and firewall points to the master cylinder's primary piston rear seal leaking out of the back of the bore.
The master cylinder is mounted on the front of the booster, and the primary piston's rear seal keeps fluid from escaping toward it. If that seal fails, fluid runs out of the bore onto the booster and firewall and the reservoir level drops. Fluid can also be drawn into the booster's vacuum chamber and damage its diaphragm. A leaking caliper or hose would leave fluid near the wheels instead.
Question 5: A customer complains that the brake fluid level in the reservoir is slightly lower than at the last service. The vehicle has front disc brakes, and the pads are about half worn. A thorough inspection finds no leaks. What should the technician conclude?
- The master cylinder has failed internally and must be replaced
- The level drop is normal because fluid fills the caliper pistons as the pads wear (Correct answer)
- The brake hoses are collapsing and need replacing
- The reservoir cap diaphragm has failed
Correct answer: The level drop is normal because fluid fills the caliper pistons as the pads wear
As disc pads wear, the caliper pistons extend farther and take up fluid, so the reservoir level falls gradually.
A caliper piston moves outward to follow the pad as it wears, which increases the space behind the piston that has to be filled with fluid. That fluid comes from the reservoir, so the level drops slowly over the life of the pads. If the inspection shows no leaks and the drop matches pad wear, no repair is needed. A sudden or rapid drop would point to an external leak or an internal one at the booster.
Question 6: To test a vacuum brake booster, a technician pumps the pedal several times with the engine off, holds firm pressure, and starts the engine. What should happen if the booster and vacuum supply are working properly?
- The pedal should push back up against the foot
- The pedal should not move at all
- The pedal should drop slightly under steady pressure (Correct answer)
- The pedal should pulsate rapidly
Correct answer: The pedal should drop slightly under steady pressure
When engine vacuum reaches the booster, the pedal drops a little as the booster provides assist.
Pumping the pedal with the engine off uses up the stored vacuum, leaving a hard pedal. When the engine starts, vacuum enters the booster and helps push the master cylinder piston, so the pedal drops slightly under the same foot pressure. If the pedal stays hard, suspect a vacuum supply problem, a leaking hose or check valve, or a failed booster. This is a key check for a hard pedal and long stopping distances, since the master cylinder itself may be fine.
Question 7: A technician finds that a master cylinder's rubber cups are swollen and the fluid looks contaminated with petroleum-based oil. What is the correct repair?
- Replace the master cylinder only and refill with fresh fluid
- Add DOT 5 fluid to dilute the contamination
- Flush the system and replace the master cylinder, all rubber hoses and seals, and other contaminated components (Correct answer)
- Bleed the system at each wheel until the fluid looks clear
Correct answer: Flush the system and replace the master cylinder, all rubber hoses and seals, and other contaminated components
Petroleum oil swells rubber seals and hoses throughout the hydraulic system, so all affected rubber parts must be replaced and the system flushed.
Brake systems use rubber parts designed for glycol-based or silicone fluid, and mineral or petroleum oil makes them swell and soften. Because contaminated fluid circulates through the entire system, replacing only the master cylinder leaves damaged seals in the calipers, wheel cylinders and hoses. The correct repair is to replace the swollen components, flush the system with approved fluid, and then bleed it. Diluting or simply bleeding contaminated fluid does not undo the damage.
Question 8: A brake pushrod gauge is used when installing a master cylinder on a power booster. What does the technician compare with this tool?
- Booster pushrod protrusion against the depth of the master cylinder piston recess (Correct answer)
- Brake pedal free play against pedal reserve distance
- Rotor thickness against the minimum specification
- Reservoir fluid volume against the maximum fill line
Correct answer: Booster pushrod protrusion against the depth of the master cylinder piston recess
The gauge checks that the booster pushrod length matches the master cylinder piston depth so the piston ends up with the correct clearance.
The gauge is set to the pushrod protrusion from the booster, then turned over and placed on the master cylinder mounting surface to see the piston recess depth. If the pushrod is too long, it holds the piston forward and can block the compensating port, which causes dragging brakes. If it is too short, it causes a low pedal and lost motion. Adjust the pushrod, if adjustable, to the specified clearance before installing the master cylinder.
Question 9: Why should brake fluid from a container that has been left open for a long time not be used when refilling the reservoir?
- It becomes thicker and cannot enter the reservoir
- It absorbs moisture, which lowers its boiling point and promotes corrosion (Correct answer)
- It turns into a petroleum-based fluid
- It loses its dye, which makes leaks harder to find
Correct answer: It absorbs moisture, which lowers its boiling point and promotes corrosion
Glycol-based brake fluid is hygroscopic, so open containers take on moisture that lowers the boiling point and causes corrosion.
DOT 3 and DOT 4 fluids are glycol-based and absorb water from the air over time. Water lowers the fluid's boiling point, so under heavy braking vapor bubbles can form and cause a spongy or sinking pedal. It also corrodes internal parts such as bores and pistons. For this reason, use fluid from a fresh, sealed container and close it again promptly.
Question 10: All four wheels drag after the pedal is released. When the technician loosens the master cylinder outlet lines, the pressure releases and the wheels spin freely. Which is the most likely cause?
- Seized caliper slide pins at all four wheels
- A plugged compensating port or a swollen cup in the master cylinder (Correct answer)
- A leaking rear wheel cylinder
- Worn front brake pads
Correct answer: A plugged compensating port or a swollen cup in the master cylinder
If dragging stops when the lines are loosened at the master cylinder, pressure is being trapped upstream, so the master cylinder or its pushrod is at fault.
Pressure that cannot return to the reservoir stays trapped in the lines and holds the brakes applied. A plugged compensating port, swollen cups from contamination, or an overlong pushrod can all cause this. It is unlikely that all four wheels would have independent mechanical faults at once. Loosening a line at the master cylinder and watching the wheels release confirms that the trapped pressure originates there.
ASE A5 Master Cylinder
A vehicle's brake pedal slowly sinks toward the floor while steady pressure is held.
There are no external leaks and the reservoir level has not dropped.
What is the most likely cause?
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