310T Hydraulic Systems

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?

Accepted Answer

A worn or bypassing hydraulic pump

Answer

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?

Accepted Answer

Wide viscosity index (VI)

Answer

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?

Accepted Answer

Contaminated hydraulic fluid containing abrasive particles

Answer

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?

Accepted Answer

Bleed only the wheel cylinder furthest from the master cylinder first

Answer

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?

Accepted Answer

It bypasses excess fluid back to the reservoir to limit maximum system pressure

Answer

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.

(310t) Truck and Coach Technician Certification Practice Test

(310t) Truck and Coach Technician Certification Practice Test

310T Hydraulic Systems