What Really Caused This Hydraulic Seal Failure?

Hydraulic repairs don’t always go as planned. A technician recently shared a frustrating case involving a piston seal failure that kept happening—even after replacing the seal. The cause? Something most people wouldn’t suspect.

The piston seal was bypassing, causing a loss of hydraulic power.

After opening up the cylinder, they found that the urethane seals had deteriorated and was leaking.

The fix seemed simple—swap out the seals, reassemble, test, and send it back to the customer.

The same cylinder was back in the shop with the same failure.

The Hidden Culprit: Water and Glycol Contamination

The heat exchanger had a leak

Water and glycol from the exchanger were mixing into the hydraulic oil.

The contaminated oil, combined with system heat, was breaking down the urethane seals.

Most people think of contamination as dirt, metal shavings, or debris. But in reality, it can be air, moisture, or fluids that aren’t compatible with the hydraulic system.

Why Urethane Seals Failed So Quickly

As water content in hydraulic oil increases, the operating temperature of urethane drops significantly. That means heat from the system accelerates material breakdown—exactly what happened here.

How to Prevent Seal Failures Like This

Check for Contamination

Cloudy, milky, or foamy oil – A clear sign of water contamination.

Unusual fluid smell – Glycol-based contamination can smell sweet.

Sludge or thickened oil – Indicates chemical breakdown.

Inspect the Heat Exchanger and Cooling System

Check for drops in coolant levels—it could be leaking into the hydraulic circuit.

Look for fluid mixing—oil should never have a milky or foamy appearance.

Use the Right Seal Material

EPDM or PTFE seals handle water-based fluids better.

Always match your seal material to your hydraulic fluid.

Check Seal Compatibility 

Before you choose a seal, be sure to check the seal’s compatibility with the hydraulic fluid and operating conditions. Using the wrong material can lead to premature failure, downtime, and costly repairs.