3030 Brake Chamber Sizes: Type 20, 24, 30 and Long Stroke

A 3030 brake chamber, also written 30/30, is a spring brake chamber whose service and spring-brake diaphragms each have 30 square inches of effective area. The label gives size, not stroke: federal rules cap a standard Type 30 at 2 in (50.8 mm) of pushrod stroke and a long-stroke Type 30 at 2.5 in (63.5 mm).

Last updated: October 29, 2026

Key takeaways

• 30/30, or 3030 without the slash, means 30 square inches on both the service and the spring diaphragm.
• Federal stroke limits: 1.75 in for a standard Type 20 or 24, 2 in for a standard Type 30, 2.5 in for a long-stroke 30.
• Square ports, a trapezoid tag or stamped LS data mark a long-stroke chamber; an unmarked one counts as standard.
• Measure applied stroke at 90-100 psi with the engine off, and cage a spring brake only with its own release tool.

What do the numbers on a 3030 brake chamber mean?

The type number is the diaphragm's effective area in square inches. Bendix's piggyback spring brake service data, SD-02-4500, uses the 30/30 as its example: 30 square inches on each diaphragm. A spring brake chamber is a pair of chambers in one assembly; Ontario's air brake handbook places the service section nearest the pushrod, with the spring section piggybacked behind it. Our guide to choosing between T, DD and spring chambers covers the other designs.

Bendix multiplies delivered pressure by effective area, so 60 psi in a Type 30 service chamber gives roughly 1,800 lb at the pushrod. The same rule gives about 1,440 lb for a Type 24 and 1,200 lb for a Type 20, so a size change alters the braking force at that wheel.

On the spring side, air holds the power spring compressed and the spring applies the brake when that air leaves; Ontario's spring brake chapter puts the usual release pressure at about 60 psi (414 kPa). Ontario calls 30 the most common size.

Which markings separate a long stroke brake chamber from a standard one?

Most type numbers come in standard and long-stroke versions with different limits. Most North American long-stroke chambers follow SAE J1817, which defines three identifiers and requires at least two on each chamber, according to CVSA Inspection Bulletin 2014-02:

• Square air ports or a square embossment around them. Standard ports are round; Lode-King's 2018 trailer bulletin uses this check on its "size 3030" chambers.
• A trapezoid tag near the air fitting or a clamp bolt, showing the rated stroke, in any color.
• Stamped or labeled data, often L or LS after the size, as in 24L or 30LS.

Ontario calls square ports the most visible and permanent sign. CVSA does not accept chamber thickness, a square pushrod hole or tag color as proof, and an unmarked chamber is deemed standard. Type 20 and 24 come in two long-stroke versions, rated at 2.5 in or 3 in; a square embossment 0.5 in high marks the 3 in one.

"Short stroke" is a search term, not a federal category, so match a short stroke brake chamber to the standard limits only once its tag or data sheet confirms the rated stroke.

What stroke limits apply to Type 20, 24 and 30 chambers?

The limits come from 49 CFR 393.47(e), the rated strokes from CVSA's pushrod stroke brochure. Rated stroke is a design figure, generally 0.5 in (13 mm) above the limit, so the tag number is never the pass mark. When a label is unreadable, the clamp ring's outside diameter identifies the type.

Clamp-type chamber sizes, rated strokes and federal readjustment limits

Type	Outside diameter	Standard stroke: rated / limit	Long stroke: rated / limit
20	172 mm (6 25/32 in)	2.25 in / 1.75 in (44.5 mm)	2.5 in / 2 in (50.8 mm), or 3 in / 2.5 in (63.5 mm)
24	184 mm (7 7/32 in)	2.25 in / 1.75 in (44.5 mm)	2.5 in / 2 in (50.8 mm), or 3 in / 2.5 in (63.5 mm)
30	206 mm (8 3/32 in)	2.5 in / 2 in (50.8 mm)	3 in / 2.5 in (63.5 mm)
36	229 mm (9 in)	3 in / 2.5 in (63.5 mm)	Not listed
30, Bendix DD-3 (buses, coaches)	206 mm (8 1/8 in)	2.75 in / 2.25 in (57.2 mm)	Not listed

An actuator missing from the federal tables may not exceed 80% of its marked rated stroke or the readjustment limit its maker marks. SAE J2899 letters, found on some chambers since 2018, do the conversion: E is 2.50 in rated with a 2.00 in limit, F is 3.00 in with 2.50 in.

How do you measure applied pushrod stroke?

CVSA and the Ontario adjustment chapter describe the same routine, and Ontario asks drivers to check adjustment at least daily.

1. Chock the wheels and release the spring brakes.
2. Bring both gauges to 90-100 psi (620-690 kPa) and stop the engine; above 110 psi (758 kPa), the readings are wrong.
3. Identify each chamber's type and stroke version.
4. With the brakes released, chalk each pushrod where it leaves the chamber.
5. Get a helper to apply and hold full pedal while you measure how far each mark moved.
6. Compare each reading with the limit; if the tanks drop below 90 psi, rebuild pressure and resume.

Pulling on the slack is only a screen. The New York CDL manual treats more than about 1 inch of hand movement at the pushrod as a sign adjustment is needed, and CVSA says free stroke over 3/8 to 3/4 in (10-20 mm) can mean a deeper fault. Excess travel, Bendix adds, cuts braking efficiency and diaphragm life and wastes air.

Under CVSA's criteria, brake violations at 20% or more of a vehicle's wheel ends put it out of service. In the 2025 Brake Safety Week, inspectors removed 2,296 of 15,175 vehicles (15.1%) from service for brakes, and that 20% condition was the top reason, with 1,199 violations. Self-adjusting slacks are required on US vehicles built after October 20, 1994; an over-limit reading on one points to worn clevis pins, cam bushings or linings, or a cracked bracket, which manual readjustment only masks. Our chamber failure and replacement guide covers how such faults show up at inspection.

How do you cage a spring brake chamber safely?

Caging is the mechanical compression of the power spring so the brake stays released without air, needed before any chamber work and to move a vehicle whose spring brake has failed. Learning how to cage a brake chamber starts with respect for that spring: Ontario warns that tampering, damage or corrosion can release it violently. These steps follow Bendix's SD-02-4500 piggyback procedure.

1. Chock the wheels and stand beside the chamber, never in front of or behind it.
2. Remove the dust cap from the keyhole at the center of the spring housing.
3. Take the T-bolt release tool, washer and nut from the side pocket.
4. Insert the bolt through the keyhole into the pressure plate, turn it a quarter turn clockwise and pull to seat it.
5. Fit the washer and nut and tighten with a hand wrench as the service pushrod retracts; Bendix forbids impact wrenches.
6. On this Bendix unit, about 2.9 in of thread beyond the nut shows the spring is fully caged.

Never cage a structurally damaged chamber; Bendix says to replace the whole assembly. Do not remove the spring section's crimp band, and never strike or drop a spring brake. Tool length and nut torque differ by design, so take them from the sheet for your own chamber. A caged chamber gives no parking or emergency braking, so after the job Bendix has the spring uncaged, the tool stowed and the dust cap refitted.

Which parts around the chamber should you check at the same time?

Section 393.47 requires both ends of an axle to carry the same chamber size and slack adjusters of equal effective length. Bendix adds that a renewed diaphragm or spring means the same work on the opposite chamber, and that the slack arm should meet the pushrod above 90° when released, nearing 90° at the limit.

The New York manual has drivers check that chamber hoses are not cut or worn by rubbing, and Bendix wants them secure with slack for axle travel. VADEN supplies replacements in its brake cylinder hose range, such as the 413.03.0009 brake cylinder hose for Volvo trucks; confirm the OE reference first.

Air disc brakes adjust inside the caliper, so stroke is not read at the chamber, as Bendix's 2025 Brake Safety Week advice explains. Torn piston or guide-pin boots let water and grit in, so check them at each brake service; VADEN's caliper bellows and dust boots include the 4123012 brake bellows seal.

Frequently Asked Questions

Is a 3030 brake chamber the same part as a 30/30?

Yes, the slash is simply dropped; Lode-King's trailer bulletin writes "size 3030". The numbers give diaphragm area only, so the name does not reveal standard or long stroke. Check the ports, tag and stamped data, and match the chamber on the other end of the axle, which federal rules require to be the same size.

What is the stroke limit for a Type 24 brake chamber?

49 CFR 393.47(e) caps a standard Type 24 at 1.75 in (44.5 mm) of applied stroke. A long-stroke Type 24 is allowed 2 in (50.8 mm), or 2.5 in (63.5 mm) if its rated stroke is 3 in. Measure at 90-100 psi with the engine off and the pedal held fully down.

Does a long stroke chamber brake harder than a standard one?

No. Force is pressure times diaphragm area, so long-stroke and standard Type 30 chambers deliver the same nominal force at the same pressure. The gain is travel: 2.5 in (63.5 mm) of legal stroke instead of 2 in (50.8 mm). Lode-King recommends long-stroke 3030 chambers for its small drum brakes so new linings can bed in without being adjusted tight.

Why is the number on the stroke tag larger than the legal limit?

The tag shows rated stroke, a design value, while the regulation sets a lower readjustment limit, generally half an inch (13 mm) less, according to CVSA. A long-stroke Type 30 tagged 3 in is out of adjustment beyond 2.5 in. CVSA's stroke brochure says never to use the rated figure as the adjustment limit.

How long can a truck run with a caged spring brake?

Only as far as the nearest repair. Caging is an emergency measure, and a caged chamber gives no parking or emergency braking. Ontario's air brake handbook tells drivers who find one, recognizable by the protruding caging bolt, to have the vehicle inspected and repaired immediately.