ASE A5 Practice Test — Brakes

A brake pedal that slowly sinks under steady pressure with no visible leaks is the classic symptom of an internal master cylinder failure. The piston seals inside the master cylinder have worn or deteriorated, allowing fluid to bypass the piston internally — fluid leaks from the high-pressure side to the low-pressure side within the master cylinder bore. No fluid leaves the system, so you see no external leaks. But the pressure drops as the fluid bypasses, and the pedal slowly moves to the floor. Worn pads (A) cause a lower pedal height but the pedal stays firm once applied. A bad booster check valve (C) affects pedal effort, not pedal fade. Air in the lines (D) causes a spongy pedal that compresses immediately — it does not slowly sink under sustained pressure. The slow sink under steady pressure is master cylinder until proven otherwise.

After installing new brake pads, the caliper pistons are pushed all the way back into the caliper bores to make room for the thicker new pads. The first pedal press pushes the pistons back out to contact the new pads — this takes up a large amount of pedal travel. The second press finds the pistons already in contact with the pads and feels normal. This is completely normal and expected. It is critical to pump the brake pedal several times before moving the vehicle after any pad replacement. If you forget, the first press of the brake pedal on the road will go to the floor with little braking force — that is a dangerous situation. Air in the system (A) would cause a consistently spongy pedal, not one that fixes itself after one pump. The master cylinder (D) would show a slow sink, not a one-time low pedal.

Both technicians are correct. The ABS hydraulic modulator (Technician A) contains internal valves and passages that can trap air during line replacement. Standard four-corner bleeding may not push air through the modulator. Many ABS systems require a scan tool to cycle the ABS modulator valves during the bleed procedure to purge trapped air — this is called an ABS bleed cycle or automated bleed. Without it, air stays trapped and the pedal remains spongy. The bleeder sequence (Technician B) matters because each manufacturer specifies the order — typically starting from the wheel farthest from the master cylinder and working toward the closest. An incorrect sequence can push air into passages instead of out of them, leaving bubbles trapped. After any brake line work on an ABS-equipped vehicle, always perform a scan-tool-assisted ABS bleed in addition to the manual bleed.

A restricted brake hose acts like a check valve — it allows pressure to push fluid to the caliper when you press the pedal, but the internal restriction prevents the fluid from returning when you release the pedal. The caliper stays partially applied, creating constant friction and heat on that one wheel. This is a common failure on older vehicles where brake hoses deteriorate internally. The inner liner collapses and creates a flap that blocks return flow. The other side releases normally, so you get one hot brake and one normal brake. Different pad brands (A) would not cause a significant temperature difference. A larger rotor (C) would actually run cooler due to more thermal mass. The proportioning valve (D) balances front-to-rear bias, not side-to-side. When one brake drags and the others do not, inspect the hose for internal restriction — replace both sides as a set.

A sticking caliper slide pin causes uneven pad wear — the inner pad wears faster than the outer, or vice versa — but it does not cause a low pedal. The caliper still applies and the piston still contacts the rotor. You get reduced braking performance and uneven wear, but pedal height remains normal. Excessive rotor runout (A) pushes the pads back each revolution, so the next time you press the pedal it has to travel further to bring the pads back to the rotor — low pedal. Rear drums that are out of adjustment (B) mean the shoes are too far from the drum, requiring extra pedal travel to close the gap. A brake fluid leak (D) reduces system fluid volume, which directly lowers pedal height because the master cylinder piston has to travel further to build pressure. Low pedal means excessive travel — something is creating a larger-than-normal gap or losing fluid volume.

Both technicians are correct. A contaminated or faulty wheel speed sensor (Technician A) can send an erratic signal to the ABS module. If the sensor intermittently drops out or reads a slower speed than the wheel is actually turning, the ABS module interprets this as wheel lockup and activates the ABS — pulsing the brakes when it should not. An incorrect tire size on one wheel (Technician B) causes the same problem from a different angle. The ABS module compares all four wheel speeds. If one wheel has a smaller tire, it rotates faster than the others for the same vehicle speed. During braking, the speed difference can exceed the ABS threshold, making the module think that wheel is locking up. This is why mismatched tire sizes — even spare tires — can trigger false ABS activation. Always check tire sizes and sensor signals when diagnosing inappropriate ABS activation.

The residual pressure check valve maintains approximately 8-10 PSI of pressure in the drum brake lines when the brakes are released. This small residual pressure keeps the wheel cylinder cups (seals) pressed firmly against the cylinder bore walls, preventing air from being sucked past the seals when the brake pedal is released. Without this residual pressure, the cups could unseat slightly, allowing air to enter the system, which would cause a spongy pedal. Disc brake circuits do not need residual pressure because the caliper square-cut seals retract the pistons slightly and maintain seal contact by design. This is why mixing up residual valves during master cylinder installation can cause problems — putting a 10 PSI valve in a disc brake circuit causes pad drag because the pads cannot fully retract. Know which valve goes to which circuit.

A hard brake pedal with poor stopping power is the textbook symptom of a failed brake power booster or loss of vacuum supply to the booster. The booster uses engine vacuum to multiply the force you apply to the brake pedal — typically providing a 3:1 or 4:1 assist ratio. When the booster fails or loses its vacuum supply (cracked hose, failed check valve, engine vacuum leak), you lose that force multiplication. You have to press much harder for much less braking — the pedal feels like a rock. The engine runs normally, so vacuum is being produced, but it may not be reaching the booster. Check the vacuum hose and check valve first. Glazed pads (A) cause reduced friction but the pedal feel is normal. Moisture in fluid (C) causes a soft pedal under heavy braking (vapor lock). Thin rotors (D) cause pedal pulsation and reduced thermal capacity.

Technician A is correct. Bench bleeding removes air from inside the master cylinder before it is installed on the vehicle. The internal passages and compensating ports in a master cylinder are complex — air trapped in these areas is extremely difficult to remove through normal four-wheel bleeding because the air pockets sit in areas that do not flow directly to the outlets. Technician B is wrong — bench bleeding is NOT optional. If you install a master cylinder full of air and try to bleed it on the vehicle, you can spend hours and never get a firm pedal because the air is trapped in the internal bypass circuits. Bench bleeding takes five minutes — clamp the master cylinder in a vise, install the bleed tubes that loop back into the reservoir, and slowly push the pistons in and out until no bubbles appear. It is a mandatory step, not a shortcut.

Minor surface scoring less than 0.015 inches deep is within the acceptable range for most rotors and does NOT require replacement — it can typically be cleaned up with a light resurfacing cut, or in some cases the new pads will wear into the existing surface and the scoring will disappear. All of the other conditions require replacement. A rotor below minimum thickness after machining (A) is structurally unsafe — it will overheat, warp, and potentially crack. Heat cracks (B) are stress fractures from repeated heating and cooling cycles — they weaken the rotor and can propagate under braking. Hard spots (D) are metallurgical changes in the rotor surface from excessive heat — they create raised areas that machining may remove temporarily but often return because the hardened material extends below the surface. Know the difference between cosmetic imperfections (scoring) and structural failures (cracks, hard spots, thickness violations).