Low Hydraulic Fluid Symptoms: What You Need to Know

Why Hydraulic Fluid Level Matters

Hydraulic fluid is the “lifeblood” of a hydraulic system — it transmits force, lubricates parts, cools components, and helps seal internal gaps. If fluid levels drop too low, the system can’t work properly. You may see slower operation, “strange” noises (additional noise/stress during operations), or even damage to components. Additionally, low hydraulic fluid levels lead to increased heat in the system and oil reservoir, resulting in a drop in viscosity which in turn requires more energy to perform the same function.

One common cause of low fluid is leaks. Even a small, slow leak over time can reduce fluid levels to a point where the system’s performance suffers. You may notice fluid puddles or wet spots under the machine near hoses, fittings, or seals.

What Happens When Fluid Is Too Low (What You’ll Notice)

When the hydraulic fluid level drops, several symptoms may appear. Below are key ones and what causes them:

Symptom	What It Looks/ Sounds Like	What’s Going On Inside	Why That’s Bad
Whining, moaning, or squealing from pump	A whining or whining-moan noise especially under load	The pump may be drawing in air (aerating) or cavitating because there’s not enough fluid for the pump inlet.	Air bubbles reduce pressure, increase noise, cause vibration, and can damage pump internals
Gurgling or “swishing” sounds in reservoir or lines	You hear a bubbling, sloshing, or gurgling noise, especially near the reservoir or control valves	Air is mixing into the fluid or the reservoir is partially drawing air.	This leads to poor control, erratic movement, and also can overheat the fluid
Slow or jerky actuator (cylinder) motion	Movements are sluggish, delayed, or uneven (jerky)	The system cannot maintain stable pressure because there’s insufficient fluid to supply control valves and actuators consistently.	Loss of precision, risk of stalling under load, possible damage to seals or valves
Overheating of the fluid or system	You may feel high temperatures in hoses, or see fluid discoloration	Low fluid means higher flow cycles and less cooling capacity. Also, aeration or cavitation generates extra heat.	Excess heat accelerates fluid breakdown (oxidation), increases wear, and shortens component life
Erratic or inconsistent machine behavior	Controls behave unpredictably, pressure spikes or drops, sluggish under load	Because of air ingestion or insufficient fluid, pressure may fluctuate or drop unexpectedly.	Dangerous in precise operations; may damage valves or actuators
Visible fluid leaks or drops in reservoir level	Fluid level gauge is below minimum; you see fluid leaking out	The simplest indicator: fluid is leaving the system through leaks or worn seals.	If the root leak isn’t fixed, topping up fluid won’t solve the underlying problem

If you hear pump noise like whining or see gurgling sounds, or your cylinders respond sluggishly, you should shut down the system immediately to avoid serious damage to the pump or other parts.

How to Properly Add (Top Off) Hydraulic Fluid

Here are guidelines and best practices when adding fluid:

• Use the correct hydraulic fluid
  Check your machine’s manual for the recommended fluid type (viscosity grade, additive package, synthetic vs. mineral, etc.). Using the wrong fluid can cause poor performance, damage seals, or void warranties.
• Avoid contamination
  Before opening the fill cap, clean the area around it (with a lint-free cloth) to prevent dirt from entering. Don’t use unlabelled containers. Use containers that were just opened (not long-stored half-full) to reduce risk of contamination.
• Add fluid slowly and in small amounts
  Overfilling can cause problems (foaming, overflow, pressure issues). After adding some fluid, cycle the system (move actuators) to let trapped air escape, then recheck level.
• Inspect the filler screen and breather
  Many reservoirs have screens or filters at the filler cap or breather vents. Make sure they are clean and functioning so debris or dust doesn’t enter.
• Diagnose and fix the leak
  If fluid must be replenished frequently, it’s a warning. Find and repair leaks (hoses, fittings, seals) or faulty components. Just “topping off” won’t prevent damage.

Checking Hydraulic Fluid Levels in Heavy Equipment

Different machines have different designs, but the general principles are consistent. Below is how to do it safely and correctly for typical heavy machines.

General Rules Before You Check

• Park the machine on level ground so that fluid settles evenly.
• Turn off the engine and let the hydraulics cool or settle (for example, 10–15 minutes) so that fluid isn’t expanded or agitated.
• If the machine has extendable cylinders or attachments, the positions may affect fluid readings—sometimes you need to extend or retract parts to get an accurate reading.

Checking Methods by Equipment Type

Equipment	What to Do	Notes
Backhoes	Use a sight glass (if present) or a dipstick; refer to manufacturer guidelines for the “normal” level	Check that it is above the minimum mark
Dump Trucks / Truck Hydraulics	Use the correct labeled fluid, then check via dipstick or sight glass. Check several times to avoid overfilling or trapping air	Be careful not to introduce air by topping off too quickly
Excavators	Park level, possibly extend the bucket cylinder if that affects the fluid reading, then check the reservoir’s min/max marks	Make sure fluid is between min and max lines
Forklifts	Park level, lower the forks, then use the correct dipstick or sight gauge to check fluid	If fluid is low, top off with the correct grade as specified by manufacturer
Garbage Trucks / Refuse Trucks	Place on flat ground, identify hydraulic cylinders or reservoir, and check via dipstick or sight glass	If low, add fluid, then cycle the hydraulics to purge air and recheck

Important: Overfilling is harmful too — too much fluid may cause foaming, erratic behavior, or pressure issues.

After filling, run the hydraulic system through its cycles (raise/lower, extend/retract) to help purge any trapped air, then recheck fluid level and top off if needed.