The Short Answer
The right hydraulic reservoir size depends on more than the amount of oil required to move your cylinder.
You need enough usable fluid to operate the system, but you also need room for expansion and enough reservoir capacity for the way the equipment will actually be used.
A tank can technically hold enough oil to complete the cylinder stroke and still be too small for a hydraulic system that cycles repeatedly and builds heat.
At Primary Mover, our hydraulic power units range from compact 4-quart reservoirs to tanks holding as much as 10 gallons. There is a reason for that range.
The real question isn’t just:
“How much oil does my cylinder need?”
It’s also:
“How hard and how often will this hydraulic system be working?”
Start With the Oil Volume Your Cylinder Actually Requires
Cylinder displacement is the first place to start when choosing a hydraulic reservoir.
You need enough usable hydraulic fluid to complete the full cylinder stroke without allowing the pump to run short of oil.
But single-acting and double-acting systems handle that oil differently.
Single-Acting Hydraulic Systems
In a single-acting system, the oil used to extend the cylinder leaves the reservoir and fills the cylinder.
A telescopic dump cylinder is a common example.
If the cylinder requires approximately five gallons of hydraulic oil to reach full extension, the power unit needs enough usable fluid to supply that volume while still maintaining an appropriate fluid level and headspace in the tank.
That can require a relatively large reservoir.
Double-Acting Hydraulic Systems
A double-acting cylinder operates differently.
As oil is pumped into one side of the cylinder, oil from the opposite side returns to the reservoir. Much of the hydraulic fluid is essentially being transferred from one chamber to the other.
That does not mean a double-acting system requires no reservoir capacity.
The rod takes up space on one side of the cylinder, so the two chambers do not have equal oil volumes. You also need to account for hoses, valve passages, safe operating level, fluid expansion, and the actual duty cycle of the equipment.
Double-acting systems can often use smaller reservoirs than comparable single-acting systems—but the complete application still matters.
The Question People Often Miss: How Often Will You Use It?
Two hydraulic systems using the exact same cylinder can have very different reservoir needs.
Why?
Duty cycle.
Consider two dump trailers.
One trailer raises for 30–90 seconds and then sits for an hour.
The other belongs to a contractor cycling a heavily loaded dump bed repeatedly throughout the day.
Those systems are doing very different work.
With intermittent use, the reservoir, cylinder, hoses, and surrounding air have time to absorb and release heat between cycles.
With frequent or continuous operation, that heat can begin to accumulate.
That is when the hydraulic reservoir becomes more than a storage tank.
It becomes part of the system’s heat-management strategy.
Why Reservoir Size Can Affect Hydraulic Oil Temperature
More hydraulic oil provides more thermal mass.
In simple terms, a greater volume of oil generally takes more energy to reach the same temperature.
A physically larger reservoir may also provide more tank surface area to transfer heat to the surrounding air, depending on the tank material, placement, and airflow.
This is one reason Primary Mover may pair a gas-powered hydraulic power unit with a five-gallon reservoir even when the actuator could technically operate with much less fluid.
With a gas-powered unit, the engine can keep the hydraulic pump turning while the equipment is in service. Pumps, valves, hoses, restrictions, and repeated work all introduce losses into the system.
Over time, those losses become heat.
Can Hot Hydraulic Oil Make Your Equipment Run Slower?
It can.
As hydraulic oil heats up, its viscosity decreases—it gets thinner.
Gear pumps rely on tight internal clearances to move a predictable amount of hydraulic fluid.
If the oil becomes too thin for the pump clearances and operating pressure, more fluid can leak internally from the high-pressure side of the pump back toward the low-pressure side.
That reduces volumetric efficiency, meaning less of the pump's theoretical flow becomes usable flow leaving the pump.
The effect can become more noticeable as system pressure increases.
In the field, this can sometimes look like a dump trailer that raises normally during the first few loads of the morning but becomes noticeably slower later in the day after repeated cycling.
That doesn’t automatically mean hot oil is the problem—but oil temperature belongs on the diagnostic list.
What We Saw During Primary Mover Bench Testing
We recently saw the effect of hydraulic oil temperature while bench testing a compact hydraulic power unit.
During a hot run, the unit produced:
6.35 LPM at 500 PSI
After the system cooled, we tested it again at essentially the same voltage and current.
The cooler test produced approximately:
6.9–7.0 LPM at 500 PSI
That is roughly a 10% increase in measured flow at the same pressure test point.
This was one controlled bench comparison—not a universal rule saying every hydraulic system will gain 10% flow when cooled.
But it clearly demonstrates why oil temperature matters when evaluating hydraulic performance.
Testing one power unit while the hydraulic system is heat-soaked and another with cooler oil can distort the comparison.
Will a Bigger Hydraulic Reservoir Solve an Overheating Problem?
Not necessarily.
A larger reservoir can help by:
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Providing more hydraulic oil and therefore a larger thermal buffer
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Potentially providing more tank surface area for heat rejection
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Increasing fluid residence time in a properly designed reservoir
But simply installing a bigger tank will not fix every source of excessive heat.
For example, hydraulic temperature problems could also involve:
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A relief valve bypassing excessively
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Restrictive valves or plumbing
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Incorrect hydraulic oil
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A pump operating outside its intended range
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Excessive system load
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Insufficient cooling
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Other hydraulic or electrical problems
Reservoir capacity should be treated as one part of thermal management, not a substitute for diagnosing the source of excessive heat.
Example: The Dump Trailer That Gets Slower Throughout the Day
Imagine a dump trailer that works perfectly in the morning.
The bed raises normally for the first few loads.
Then the operator spends several hours making short trips and cycling the bed repeatedly.
By afternoon, the same load seems to raise more slowly.
Hot hydraulic oil is one possible explanation.
As oil temperature increases and viscosity decreases, internal pump leakage may increase and usable flow may decrease—especially at higher operating pressures.
A smaller reservoir will also typically reach its final operating temperature faster than a larger volume of oil under otherwise similar conditions.
But heat isn't the only possibility.
You should also consider:
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Battery voltage
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Cable size
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Motor condition
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Hydraulic restrictions
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Filters
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Relief-valve settings
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Hydraulic fluid level
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Mechanical geometry
One useful clue is whether performance returns after the hydraulic system is allowed to cool.
How Do You Choose the Right Hydraulic Reservoir Size?
There isn't one reservoir-size rule that works for every compact hydraulic system.
Start with the amount of oil required by the cylinder, then evaluate the complete application.
Ask:
How much usable oil is required for the full cylinder stroke?
Is the cylinder single-acting or double-acting?
How much headspace is needed for fluid expansion and safe operating level?
Will the power unit run for seconds at a time—or cycle repeatedly for hours?
Does the pump shut off between cycles, or will a gas engine keep it running?
What temperatures will the equipment operate in?
What viscosity and operating-temperature range does the pump manufacturer specify?
Does the reservoir have enough surface area and airflow to reject the expected heat?
And if the system is already running hot:
Is the reservoir really the problem, or is something else creating excessive heat?
The Primary Mover Rule of Thumb: Size for the Job, Then Size for the Duty Cycle
When sizing a hydraulic power unit, the first question should still be:
How much oil does the cylinder require?
But the second question should be:
How often is this equipment going to run?
For intermittent-duty equipment, compact packaging may be more useful than carrying several extra gallons of hydraulic oil.
For high-cycle or continuous-duty equipment, additional reservoir capacity can help slow heat buildup and maintain more consistent performance.
The best hydraulic reservoir isn't automatically the biggest tank available.
And it isn't automatically the smallest tank capable of completing the cylinder stroke.
It's the reservoir that supports the cylinder, pump, and real-world duty cycle of the equipment.
Hydraulic Reservoir Size FAQs
Can hot hydraulic oil make a dump trailer raise slower?
Yes. As hydraulic oil gets hotter and thinner, internal leakage inside the pump can increase, reducing effective pump flow. Heat is only one possible cause, though, so battery voltage, restrictions, relief settings, fluid level, and mechanical load should also be checked.
Does a larger hydraulic reservoir create more pressure?
No. Reservoir size does not increase hydraulic pressure or change the relief-valve setting. A larger reservoir can provide more usable fluid and greater thermal capacity, which may help maintain performance during repeated use.
Do double-acting cylinders need smaller reservoirs than single-acting cylinders?
Often, yes. A double-acting cylinder returns oil from one chamber while oil enters the other. A single-acting cylinder may require the reservoir to supply nearly the entire working cylinder volume during extension. Double-acting systems still need adequate capacity for volume differences, plumbing, headspace, expansion, and heat management.
How much hydraulic oil does a dump trailer need?
It depends primarily on the cylinder or hoist oil volume rather than simply the physical size of the trailer. Calculate the fluid required for the complete working stroke, then account for safe operating level, plumbing, expansion, and duty cycle.
How hot is too hot for hydraulic oil?
There is no universal temperature limit that applies to every hydraulic system. Acceptable oil temperature depends on the hydraulic fluid, pump, seals, valves, and system design. Follow the operating limits specified by the manufacturers of your pump and hydraulic fluid.
Can I use the smallest reservoir that will stroke my cylinder?
Sometimes. For equipment used intermittently, the minimum practical reservoir may work well. But if the equipment cycles frequently, a larger reservoir may slow temperature rise and help maintain more consistent hydraulic performance.
In hydraulics, proper sizing isn’t about more—it’s about matching the system to the job.