Hydraulic Pressure Release on a Mustang 940

[MikePhua]

The Mustang Brand and the 940 Series
Mustang Manufacturing, founded in 1865 and later integrated into the Manitou Group, has long been recognized for its compact construction equipment. The Mustang 940 skid steer loader, produced during the 1990s, was part of a generation that emphasized mechanical simplicity, hydraulic versatility, and affordability. With thousands of units sold across North America and Europe, the 940 became a popular choice for small contractors, landscapers, and agricultural users. Its robust frame and straightforward hydraulic system made it a favorite among operators who preferred machines that could be serviced without proprietary diagnostics.
Core Specifications and Hydraulic Layout
The Mustang 940 typically features:

• Engine: Yanmar 4TNE84-MS diesel engine
  
• Operating weight: Approximately 2,700 kg
  
• Rated operating capacity: Around 700 kg
  
• Hydraulic pressure: Factory spec of 2,400 psi for auxiliary lines
  
• Hydraulic fluid type: ISO VG 46 or equivalent
  
• Control system: Mechanical levers with auxiliary hydraulic toggle
  

Terminology Annotation

• Auxiliary Hydraulics: Additional hydraulic circuits used to power attachments like augers, grapples, or trenchers.
  
• Charge Pump: A low-pressure pump that feeds fluid into the main hydraulic system, ensuring consistent pressure and flow.
  
• Cold Start Bypass Valve: A valve that redirects fluid during cold conditions to prevent overloading the filter and pump.
  

Symptoms of Hydraulic Pressure Retention
Operators often encounter difficulty when attempting to disconnect hydraulic attachments due to residual pressure in the lines. This pressure can prevent quick couplers from releasing, leading to frustration and potential damage. Common signs include:

• Couplers refusing to disconnect
  
• Hydraulic fluid spurting upon loosening fittings
  
• Attachments remaining pressurized after shutdown
  

In one case, a snow removal contractor in Wisconsin reported that his Mustang 940 retained pressure in the auxiliary lines even after the engine was off. He had to manually relieve the pressure using a wrench and drain pan, risking fluid spray and injury.
Safe Pressure Release Procedure
To safely release hydraulic pressure on a Mustang 940, follow these steps:

• Turn off the engine and remove the key
  
• Lower all attachments fully to the ground
  
• Wiggle the control levers to bleed residual pressure
  
• Locate the auxiliary couplers and slowly loosen them with a wrench
  
• Wear gloves and safety glasses to protect against fluid spray
  
• Allow fluid to drain into a pan before fully disconnecting
  

Terminology Annotation

• Quick Coupler: A hydraulic fitting that allows fast connection and disconnection of hoses without tools.
  
• Residual Pressure: Trapped hydraulic pressure remaining in the system after shutdown.
  

Hydraulic Test Port Diagnostics
The Mustang 940 includes two hydraulic test ports:

• One located near the charge pump feeding into the valve body
  
• Another positioned before the charge filter at the cold start bypass
  

Testing these ports with a pressure gauge can reveal system health. A healthy auxiliary circuit should hold steady at 2,400 psi at full RPM. If readings fall below 1,700 psi, as reported in some cases, the engine may be underpowered or the pump worn. The cold start bypass should show around 250 psi; a near-zero reading suggests valve malfunction or blockage.
Engine and Hydraulic Interplay
Hydraulic pressure issues often correlate with engine performance. If the engine stalls under load or fails to start without ether, it may lack the horsepower to sustain hydraulic demand. Diesel engines can idle smoothly but falter when asked to deliver torque under pressure. Causes include:

• Clogged fuel injectors
  
• Weak lift pump
  
• Air leaks in fuel lines
  
• Dirty air filters
  

A technician in Ontario replaced all four injectors and bypassed the glass fuel bowl to eliminate leaks. After these changes, the engine regained RPM stability and hydraulic response improved.
Terminology Annotation

• Lift Pump: A low-pressure pump that delivers fuel from the tank to the injection pump.
  
• Ether Start: A method of cold-starting diesel engines using volatile spray to ignite combustion.
  

Preventive Maintenance Recommendations
To maintain optimal hydraulic performance:

• Replace hydraulic filters every 500 hours
  
• Bleed air from lines after fluid changes
  
• Inspect hoses and couplers monthly for wear
  
• Use high-quality hydraulic fluid with anti-foaming additives
  
• Monitor pressure readings during operation and compare to factory specs
  

Operator Tips and Field Anecdotes
Operators should avoid sudden directional changes while using attachments, as this can spike pressure and strain the system. In muddy conditions, the Mustang 940 may stall if the bucket is curled and lowered simultaneously. Feathering the controls—applying gradual input—helps prevent engine overload.
A landscaper in Pennsylvania shared that he trained his crew to always cycle the auxiliary toggle after shutdown to relieve pressure. This simple habit reduced coupler damage and improved attachment changeover speed.
Conclusion
The Mustang 940 skid steer is a durable and capable machine, but hydraulic pressure retention can pose challenges if not properly managed. Understanding the relationship between engine output, hydraulic demand, and pressure release procedures is essential for safe and efficient operation. With regular maintenance and informed troubleshooting, operators can keep their machines running smoothly and extend their service life well beyond expectations. Whether grading driveways or trenching for irrigation, the Mustang 940 remains a reliable partner when treated with care.