Description
Williams: WM‑778‑A refers to a pressure protection valve used in heavy‑duty truck air brake systems.
A normally closed air valve (part of the WM‑778 series by Williams Controls) [industrial…wright.com]
Designed for commercial vehicle / truck air systems
What it does
Protects the primary air brake system from pressure loss
Opens when system pressure reaches about 70 PSI
Automatically closes if pressure drops, isolating auxiliary systems (like seats, horns, etc.)
Prevents air loss or reverse flow during system failure
Common details
Typical ports: 1/4″ NPT (varies slightly by version)
Max pressure: around 150 PSI
Often cross‑references to:
118181 (no filter version)
118588 / WM‑778‑A1
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