Mini excavators and compact loaders are designed to work across multiple functions simultaneously — digging, grading, lifting, and driving attachments — all from a single hydraulic power source. This efficiency comes with a challenge: not every circuit in the machine requires the same pressure. Auxiliary circuits that power attachments such as hydraulic breakers, augers, grapples, or tilt buckets often operate at a lower pressure than the main drive or boom circuits. Without the right pressure management components in place, running these circuits directly off the main system can damage actuators, seals, and sensitive control components.
A pressure reducing valve with relieving is the component that solves this problem. It reduces main system pressure to a safe, preset downstream level — and critically, it also relieves any excess pressure that builds up on the downstream side, preventing pressure spikes that a standard reducing valve cannot address on its own.
This article covers how pressure reducing valves with relieving work, why construction equipment specifically depends on them, and how HFD Hydraulic’s VRPRL and VRPRL/U series are engineered to handle these demands.
Quick Overview
| Main Keyword | Pressure reducing valve with relieving |
| Application | Mini excavators, compact loaders |
| Featured Products | VRPRL, VRPRL/U |
| Key Function | Downstream pressure regulation + automatic excess pressure relief |
| Pressure Rating | Up to 210 bar / Al body — Up to 350 bar / Steel body |
| Flow Range | 20 l/min (size 38) — 50 l/min (size 12) |
| Operating Temperature | −40°C to 100°C (−40°F to 212°F) |
What Is a Pressure Reducing Valve with Relieving?
A standard pressure reducing valve is a normally open, 2-way valve that closes progressively as downstream pressure rises toward the preset limit, maintaining a stable reduced pressure in the secondary circuit regardless of fluctuations in the main circuit. Power & Motion Tech describes pressure-reducing valves as “the most practical components for maintaining secondary, lower pressure in a hydraulic system.”
The relieving variant adds a second, critical function. In certain operating conditions — particularly when a load pushes back fluid into the downstream circuit while the valve is closed — pressure on the outlet side can rise above the set point. A standard reducing valve cannot vent this trapped pressure because it is already in the closed position. A pressure reducing valve with relieving anticipates and eliminates downstream pressure rises by tying the relief port opening to the same pressure regulation move, rather than reacting only after pressure climbs above a set point.
The result is a single compact valve that performs two roles: it steps down upstream pressure to a safe downstream level, and it vents any downstream overpressure back to the tank before damage can occur.
Why Construction Equipment Needs Pressure Reducing Valves with Relieving
Multi-Circuit Pressure Management
Mini excavators and compact loaders run several hydraulic functions at different operating pressures from one pump or pump group. Main boom, arm, and travel circuits typically operate at the full system pressure. Auxiliary attachment circuits and pilot control circuits, by contrast, are rated for significantly lower pressure. In compact excavators, secondary manifolds for lower-pressure auxiliary functions benefit from pressure reduction from the main system, with the main system pressure plumbed in and reduced to the appropriate level for downstream functions.
Without a reducing valve in the auxiliary circuit, directing high-pressure flow to a lower-rated attachment or actuator risks component failure, seal damage, and shortened service life.
Pressure Spikes from Attachment Load Reversal
Attachments like hydraulic breakers, grapples, and augers generate dynamic loads. When an attachment stalls, decelerates rapidly, or reverses direction, the load can drive fluid back into the downstream side of the reducing valve. This is precisely the scenario where the relieving function becomes critical — it vents the backflow pressure safely to the tank rather than allowing it to build into a destructive spike.
Pilot Circuit Protection in Control Systems
In hydraulic excavators, pressure reducing valves output pilot pressure to directional control valves corresponding to work elements including boom cylinders, arm cylinders, and blade cylinders, with separate reducing valves for each function’s extension and retraction directions. These pilot circuits operate at far lower pressures than the main working circuits. Any pressure excursion in the pilot circuit can cause unintended valve actuation or damage sensitive pilot components. A pressure reducing valve with relieving keeps pilot circuit pressure stable in both directions of operation.
Demanding Operating Conditions
Construction equipment operates in environments that stress hydraulic components far beyond the laboratory: ambient temperatures from sub-zero winters to hot summer job sites, contaminated oil, vibration, and continuous duty cycles. A pressure reducing valve lowers system pressure in the partial circuit and keeps it constant, which protects components, increases process stability, and improves overall system efficiency.
HFD Hydraulic VRPRL and VRPRL/U: Overview
HFD Hydraulic offers two pressure reducing valve with relieving models — the VRPRL and the VRPRL/U — both engineered to the same core specification, with the /U variant optimized for unidirectional flow applications. Both are available in two port sizes and two body materials to match different machine requirements.
Key Specifications
| Specification | VRPRL / VRPRL/U |
| Body material options | Aluminium, Steel |
| Max pressure — aluminium | 210 bar (3,050 psi) |
| Max pressure — steel | 350 bar (5,100 psi) |
| Nominal flow — size 38 | 20 l/min (5.3 US gpm) |
| Nominal flow — size 12 | 50 l/min (13.2 US gpm) |
| Operating temperature | −40°C to 100°C (−40°F to 212°F) |
| Compatible fluid | Mineral-based oil |
| Fluid viscosity range | 10 to 200 cSt |
| Max contamination level | ISO 4406: 18/16/13 |
| Port connections (size 38) | G3/8 or SAE8 |
| Port connections (size 12) | G1/2 or SAE10 |
VRPRL — Standard Bidirectional Variant
The VRPRL maintains consistent downstream pressure by adjusting hydraulic fluid flow automatically. When downstream pressure reaches the preset limit, the valve progressively closes. If pressure on the downstream side continues to rise, due to load reversal or backflow, the relieving function opens to vent excess fluid to the tank, protecting downstream components. The size 38 and size 12 variants cover flow requirements from light auxiliary circuits to higher-demand attachment circuits.
Both aluminium and steel body options are available: the aluminium body is suited to lower-pressure auxiliary and pilot circuits up to 210 bar, while the steel body supports high-pressure environments up to 350 bar, typical of primary working circuits in compact construction machines.
VRPRL/U — Unidirectional Variant
The VRPRL/U shares the same pressure ratings, flow ratings, port configurations, and temperature range as the VRPRL. Its unidirectional design is optimized for circuits where hydraulic flow moves consistently in one direction, which simplifies installation and ensures reliable, consistent pressure regulation in single-direction auxiliary circuits. Like the VRPRL, the /U variant incorporates the built-in relieving function to handle downstream pressure excursions.
The VRPRL/U is suitable for applications requiring precise pressure regulation and overload protection in circuits where reverse flow is not a design requirement — for example, auxiliary hydraulic lines supplying single-acting cylinders or unidirectional hydraulic motors driving attachments.
👉 View the VRPRL/U product page
How the VRPRL and VRPRL/U Fit Into Construction Equipment Circuits
Auxiliary Attachment Circuits — Mini Excavators
Modern mini excavators, including zero-tail-swing models increasingly used in urban construction, carry auxiliary hydraulic lines to the end of the stick to power a wide range of interchangeable attachments. The most demanding attachments are those requiring both high flow and high pressure to operate; for example, a felling head or similar high-demand attachment requires combining flow from multiple pumps to achieve both the speed and the force needed. When such attachments are not in use, and the circuit transitions to lighter tools, the downstream circuit must be regulated to prevent over-pressurization of attachment-side components.
Installing a VRPRL in the auxiliary supply line allows the operator to use attachments rated for lower operating pressures without the risk of damaging them with main circuit pressure. The relieving function handles any pressure that builds on the attachment side during tool engagement, deceleration, or changeover.
Auxiliary Hydraulic Circuits — Compact Track Loaders
Compact track loaders and skid steers use auxiliary hydraulic circuits to drive high-flow attachments including mulchers, planers, and cold planers. These machines commonly provide both standard and high-flow auxiliary circuits, with different pressure and flow characteristics. Where an attachment’s rated working pressure is below the main system pressure, a pressure reducing valve with relieving in the supply line protects the attachment’s hydraulic motor and seals while maintaining the required downstream pressure.
The VRPRL/U in particular is well suited to single-direction auxiliary hydraulic motors, where its unidirectional optimization provides clean, stable pressure regulation.
Pilot Control Circuits
The pilot valve in a pilot-operated pressure reducing valve relieves only enough fluid to position the main valve spool or poppet so that flow through the main valve equals the flow requirements of the reduced-pressure circuit. If no flow is required in the low-pressure circuit during a portion of the cycle, the main valve closes. This behavior makes the VRPRL well suited for excavator pilot control circuits, where pressure demand changes constantly as the operator switches between functions.
Selecting Between the VRPRL and VRPRL/U
| Consideration | Choose VRPRL | Choose VRPRL/U |
| Flow direction | Bidirectional circuits | Unidirectional circuits |
| Actuator type | Double-acting cylinders, reversible motors | Single-acting cylinders, unidirectional motors |
| Application examples | Boom/arm pilot circuits, grapple circuits | Single-direction auger drives, single-acting tilt circuits |
| Body material | Al (≤210 bar) or Steel (≤350 bar) | Al (≤210 bar) or Steel (≤350 bar) |
| Port size selection | Size 38 (20 l/min) or Size 12 (50 l/min) | Size 38 (20 l/min) or Size 12 (50 l/min) |
For most auxiliary circuits on mini excavators where attachments may require bidirectional hydraulic supply, the VRPRL is the appropriate choice. For cleaner, single-direction circuits such as those supplying unidirectional hydraulic motors on compact loader attachments, the VRPRL/U offers optimized performance.
Pressure management in mini excavators and compact loaders is not a single-valve problem. Multi-circuit machines require precise control at every branch of the hydraulic system, and the auxiliary circuits — often the most varied in terms of attachments and operating pressures — are where a pressure reducing valve with relieving delivers clear, measurable value.
The VRPRL and VRPRL/U from HFD Hydraulic address both sides of this requirement: they reduce main circuit pressure to a safe, stable downstream level, and they relieve any excess downstream pressure before it becomes a system problem. With operating pressure ratings up to 350 bar in steel-body configuration, a broad temperature range, and two port sizes covering the flow needs of most construction equipment auxiliary circuits, both models are engineered to perform in the conditions that compact construction machines actually operate in.
To learn more about pressure reducing valves in general, see our related articles:
