Transmission Fluid Maintenance for the 1970 Case 680CK Backhoe

[MikePhua]

The 680CK and Its Mechanical Heritage
The Case 680CK was introduced in the late 1960s as part of Case Corporation’s expansion into full-sized construction backhoes. The “CK” designation stood for “Construction King,” a branding that emphasized ruggedness and versatility. Powered by a diesel engine and equipped with a mechanical shuttle transmission, the 680CK became a staple in municipal fleets, farm operations, and small contractors’ yards. Its popularity stemmed from a balance of power, simplicity, and serviceability—qualities that still make it a candidate for restoration and continued use today.
Transmission and Rear Axle Fluid System
The transmission and rear axle in the 680CK share a common fluid reservoir, often referred to as the transaxle. This unit houses the gear train, differential, and shuttle clutch pack. Proper lubrication is essential to prevent gear wear, clutch slippage, and bearing failure. The system holds approximately 28 quarts of 135-H EP gear oil, a high-pressure lubricant designed for hypoid gears and wet clutch applications.
Terminology and component notes:
- Transaxle: A combined transmission and differential unit, common in backhoe loaders.
- Shuttle Transmission: A hydraulic clutch system allowing forward and reverse without gear shifting.
- Breather Cap: A vented plug that allows pressure equalization in the transaxle housing.
- Dipstick Plug: A threaded plug with an integrated dipstick used to check fluid level.
- 135-H EP Gear Oil: A high-viscosity extreme pressure lubricant suitable for heavy-duty gear systems.
Checking Fluid Level and Identifying the Dipstick
On early 680CK models, the transmission fluid is checked via a dipstick threaded into the transaxle housing, typically located behind the gear shifter. This dipstick may be integrated into the breather cap or mounted separately. If the dipstick is missing, some operators have improvised by inserting a flexible probe or fish tape to gauge fluid depth—though this method lacks precision.
Recommendations:

• Locate the breather cap behind the gear shifter and inspect for a threaded dipstick
  
• Check fluid level with the machine cold and parked on level ground
  
• If unsure of dipstick length, consult a parts manual or measure from the plug seat to the fluid surface
  
• Avoid overfilling, which can cause foaming and pressure buildup
  

One operator shared that his 680CK had sat through a wet winter, and upon inspection, the fluid appeared milky. This raised concerns about water ingress, likely through condensation or a compromised seal.
Milky Fluid and Water Contamination
Milky transmission or hydraulic fluid is a common sign of water contamination. Water can enter the system through:

• Condensation during temperature swings
  
• Leaky breather caps or seals
  
• Improper storage in wet environments
  

Water-contaminated gear oil loses its lubricating properties and can lead to accelerated wear. In severe cases, rust may form on internal components, compromising clutch plates and bearings.
Solutions:

• Drain the transaxle completely and inspect the fluid for water separation
  
• Refill with fresh 135-H EP gear oil and monitor for recurrence
  
• Replace breather caps and seals if signs of leakage are present
  
• Store the machine under cover or use a tarp to prevent moisture intrusion
  

A technician in North Carolina reported that after draining milky fluid from a 680CK, he found rust on the clutch drum and pitting on the ring gear. The machine had been parked near a creek and exposed to humidity for months.
Warm vs Cold Fluid Checks
While some machines require fluid checks at operating temperature, the 680CK’s transaxle can be checked cold. However, fluid expands when warm, so readings may vary slightly. For diagnostic purposes, a cold check is sufficient to detect water contamination or low fluid levels.
Tips:

• If checking hot, operate the machine for 15–20 minutes under load
  
• Raise the rear wheels and rotate them to circulate fluid before checking
  
• Use consistent conditions for future checks to track fluid behavior
  

Preventative Maintenance and Long-Term Care
To maintain transmission health:

• Change fluid every 1,000 hours or annually, whichever comes first
  
• Inspect breather caps and dipstick seals quarterly
  
• Use only high-quality 135-H EP gear oil from reputable suppliers
  
• Avoid mixing oil types or brands without compatibility verification
  
• Monitor for signs of clutch slippage, gear noise, or shifting hesitation
  

For machines in seasonal use, consider draining fluid before winter storage and refilling in spring. This prevents condensation and protects internal components.
Conclusion
The Case 680CK’s transmission system is simple but sensitive to fluid quality and contamination. By understanding its layout, checking fluid correctly, and responding to signs of water ingress, operators can extend the life of this classic backhoe. In vintage equipment, lubrication is more than routine—it’s the lifeline that keeps iron moving. And for the 680CK, a clean transaxle means a machine that still earns its name as a Construction King.