Air Brake Chamber Sizes Explained: Type 30, 24, 20 and 16

The type number is the diaphragm area

Air brake chambers are sized by the effective area of the diaphragm in square inches — that's the number you read on the can. A Type 30 has a 30 in² diaphragm, a Type 24 has 24, a Type 20 has 20, a Type 16 has 16. Bigger area means more force pushing the pushrod at the same air pressure, so a bigger type clamps the brake harder. It's the size of the muscle, not a brand or a model.

Which type goes where

• Type 30: the workhorse on the drive axles of most road tractors, and on many trailer axles — usually as a 30/30 spring brake.
• Type 24: common on steer axles and lighter applications.
• Type 20 and Type 16: smaller service chambers on steer axles and lighter-duty setups.
• The truck was engineered around specific sizes front and rear — the split between axles is deliberate brake balance, not a suggestion.

Why you match, not upsize

It's tempting to think a bigger chamber is 'more braking,' but that's the wrong instinct. The engineers matched chamber size to the slack adjuster length, the foundation brake, and the balance between axles. Put a bigger type on one axle and you've thrown that balance off — that axle grabs, the others do less, and now the truck stops unevenly. Match what came off. Same for the reverse: a smaller type shorts you on clamping force.

Size is only half of it — don't forget stroke and type

The size number gets you partway. You still have to match the stroke (long vs short) and whether it's a single service chamber or a 30/30 spring brake. A Type 30 long-stroke spring brake and a Type 30 short-stroke service chamber are both 'Type 30' and are not the same part. Read the full stamp on the old can, or send us the VIN.

Land on the right one

Tell us the type, the stroke, and whether it's a spring brake — or just the number off the old can — and we'll match it. PartStop stocks the common truck and trailer sizes as premium-quality aftermarket direct replacements, cross-referenced to Bendix, Haldex and Meritor, 6+ month warranty, same-day from Tacoma, singles and 2/4/10-packs.

Frequently asked questions