Type 20, 24, 30 — what the numbers mean, how to read them off your old chamber, and the warning signs of a failing diaphragm every driver should know.
What the type number means
Air brake chamber "types" describe the effective diaphragm area in square inches: a Type 30 has a 30 in² diaphragm, a Type 24 has 24 in², and so on. Bigger area = more clamping force for the same air pressure. The type is usually stamped or cast right on the chamber body or its ID tag.
Drive axles on most highway tractors run Type 30 spring-brake (double-diaphragm) chambers; steer axles typically run smaller service-only chambers. Always replace like with like — the type, the stroke (standard vs long-stroke) and the mounting must all match.
Service chamber vs spring brake
- Service chamber (single): applies the brakes when you press the pedal. Found on steer axles.
- Spring brake (double / piggyback): adds a powerful internal spring that applies the parking/emergency brake when air is released. Found on drive and trailer axles.
- CAUTION: the spring section stores enormous energy. Never disassemble a spring brake chamber, and never cut one open — replace the complete assembly. Caging the spring is strictly a controlled shop procedure.
Warning signs of a failing chamber
- Audible air leak at the chamber when brakes are applied (service side) or released (spring side) — a torn diaphragm.
- Truck pulls to one side under braking — one chamber doing less work.
- Slow brake release or dragging brakes — a weakening return spring or internal leak.
- Visible rust-through, dents in the clamp band, or a pushrod that no longer moves squarely.
Measure stroke at every PM
Pushrod stroke is the legal wear measurement for air brakes: mark the pushrod at rest, make a full-pressure application, and measure the travel. If it exceeds the adjustment limit for your chamber type, the chamber, the slack adjuster, or the foundation hardware needs attention. Out-of-adjustment brakes are among the most common out-of-service violations at roadside inspection — a chamber and slack adjuster are cheap insurance against a parked truck.
Replace in axle pairs when in doubt
Diaphragms on the same axle have lived the same life. If one has failed from age rather than damage, its partner is next — replacing both sides at once keeps braking balanced and saves a second labor bill. All PartStop chambers are premium-quality aftermarket direct replacements covered by our 6+ month warranty.
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