RESERVOIRS

Air & Fluid Reservoirs For Peterbilt Trucks

Reservoirs on a Peterbilt truck store the compressed air, hydraulic fluid, or coolant that various systems draw on during operation. Air reservoirs (also called air tanks) hold the compressed air supply for the brake and suspension systems, and are required to meet FMCSA volume and drain valve specifications. Hydraulic reservoirs store fluid for PTO-driven work circuits on vocational trucks. Proper reservoir maintenance — including regular draining of air tanks to remove moisture — is essential to protecting downstream components from water contamination and corrosion.

Frequently asked questions about reservoirs

How often should air tank reservoirs be drained on a Peterbilt?

Air tanks should be manually drained daily — ideally at the end of each operating day — to remove accumulated moisture from the compressed air supply. Even with an air dryer installed, some moisture passes through and collects in the tanks over time. Failure to drain air tanks allows water to accumulate, which can freeze in cold weather (blocking the drain valve or freezing in brake lines), accelerate internal tank corrosion, and contaminate brake valves and chambers with moisture-laden air. Automatic drain valves are available as an upgrade and provide continuous drainage without requiring manual operation.

Can an air reservoir be repaired if it develops a crack or leak?

FMCSA regulations prohibit welding repairs on air brake system reservoirs — a cracked or structurally compromised air tank must be replaced, not repaired. Even small cracks or pinholes in an air tank represent a structural failure point that could lead to rapid or catastrophic pressure loss. Surface rust that has not penetrated the tank wall may be acceptable during an inspection, but any tank showing active leaks, bulging, or structural corrosion should be replaced immediately. Air tank replacement is relatively inexpensive compared to the cost of a brake system failure.

How do I know what size replacement air reservoir I need for my Peterbilt?

Replacement air reservoirs are sized by internal volume (in cubic inches or liters) and port configuration (number and thread size of inlet and outlet ports, and drain valve port location). The original tank's dimensions, volume, and port layout should be matched in the replacement. For the primary supply and service reservoirs, it's important not to downsize the volume — FMCSA specifies minimum reservoir capacity relative to the brake chamber volumes on the truck. The tank will typically have its volume and working pressure stamped on the shell — use those specifications when sourcing a replacement.

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