310T Hydraulic Systems — Questions and Answers
Question 1: A hydraulic lift gate on a truck drops slowly under load even though the pump motor runs continuously. What is the MOST likely cause?
- A worn or bypassing hydraulic pump (Correct answer)
- Low hydraulic fluid level
- A blocked filter restricting return flow
- An incorrectly adjusted relief valve set too high
Correct answer: A worn or bypassing hydraulic pump
When the pump runs but the cylinder cannot hold load under continuous demand, internal pump wear causes fluid to bypass internally, reducing effective flow and pressure. Low fluid would also cause slow operation but typically stalls intermittently; a blocked return filter increases back-pressure rather than causing bypass; and a relief valve set too high would allow excessive pressure, not cause slow descent.
Question 2: Which hydraulic fluid property is MOST important when selecting fluid for a truck system that operates in both extreme cold and high-temperature conditions?
- High flash point
- Wide viscosity index (VI) (Correct answer)
- High water content tolerance
- Low specific gravity
Correct answer: Wide viscosity index (VI)
Viscosity Index (VI) describes how much a fluid's viscosity changes with temperature. A high VI means the fluid stays within an acceptable viscosity range from cold start through full operating temperature, protecting pumps and seals in both extremes. Flash point relates to fire safety, not temperature range performance; water tolerance and specific gravity are not the primary selection criteria for wide-temperature operation.
Question 3: When inspecting a hydraulic cylinder that is leaking past the rod seal, a technician notices the rod surface has a spiral groove worn into it. What caused this damage?
- Over-pressurization of the cylinder
- Contaminated hydraulic fluid containing abrasive particles (Correct answer)
- Incorrect rod material for the application
- Excessive extension speed causing hydraulic hammer
Correct answer: Contaminated hydraulic fluid containing abrasive particles
A spiral groove on a hydraulic rod is a classic signature of abrasive contamination in the fluid. Dirt or metal particles caught between the rod and wiper/seal rotate as the rod travels, cutting a helical pattern. Over-pressurization causes seal extrusion, not spiral scoring; incorrect material would cause uniform corrosion or pitting; hydraulic hammer damages end caps and hoses, not the rod surface in a spiral pattern.
Question 4: A technician is bleeding a hydraulic braking system on a coach. What is the correct procedure to ensure all air is removed?
- Bleed only the wheel cylinder furthest from the master cylinder first (Correct answer)
- Start at the caliper closest to the master cylinder and work outward
- Begin at the highest point in the system and work downward, ending at the master cylinder
- Crack all bleed screws simultaneously and apply steady pressure until fluid runs clear
Correct answer: Bleed only the wheel cylinder furthest from the master cylinder first
Air rises, so the caliper farthest from the master cylinder — and typically the one with the longest fluid path — is bled first to push trapped air completely through the system. Bleeding closest to the master cylinder first risks trapping air in the long downstream lines. Opening all screws simultaneously causes uncontrolled pressure loss. Bleeding from the highest point is a principle used in cooling systems, not hydraulic brake circuits.
Question 5: What does a hydraulic system relief valve do when system pressure reaches its set point?
- It shuts off the pump motor to prevent over-pressurization
- It bypasses excess fluid back to the reservoir to limit maximum system pressure (Correct answer)
- It opens a secondary circuit to share load with another pump
- It closes the main control valve to stop cylinder movement
Correct answer: It bypasses excess fluid back to the reservoir to limit maximum system pressure
A relief valve is a pressure-limiting safety device: when system pressure hits the set point, the valve opens and diverts fluid back to the reservoir (tank), holding pressure at the maximum set value without damaging components. It does not stop the pump motor, engage a secondary circuit, or close control valves — those are functions of separate components.
Question 6: A hydraulic pump produces adequate flow at low speed but cavitates at high engine RPM. What is the MOST likely cause?
- A restricted inlet (suction) line or clogged suction strainer (Correct answer)
- A failed pressure relief valve stuck open
- Excessive pump output pressure exceeding system rating
- Air trapped in the hydraulic reservoir
Correct answer: A restricted inlet (suction) line or clogged suction strainer
Cavitation occurs when the pump cannot draw in enough fluid to fill its displacement chambers, forming vapor bubbles that collapse violently. At low RPM the demand is modest, so a partially restricted suction line is adequate; at high RPM the pump demands more fluid than the restricted line can deliver, causing cavitation. A stuck-open relief valve causes pressure loss, not cavitation. Excessive output pressure damages downstream components. Air in the reservoir contributes but the primary cause of RPM-dependent cavitation is suction restriction.
310T Hydraulic Systems
A hydraulic lift gate on a truck drops slowly under load even though the pump motor runs continuously.
What is the MOST likely cause?