Truck Brake Rules: Pushrod Stroke, Lining Thickness and ABS
A commercial truck needs brakes on all wheels that can stop and hold it (49 CFR 393.40 and 393.42). Air brake pushrod travel may not pass the limits in 393.47(e), such as 2 inches for a standard type 30 clamp chamber. Linings on non-steering air drum brakes may not be under 1/4 inch, and the low air warning must come on at 55 psi or below.
Facts checked October 21, 2026
Key Points
- Every commercial motor vehicle must have brakes adequate to stop and hold the vehicle or combination (49 CFR 393.40(a)) and brakes acting on all wheels (393.42(a))
- Air-braked trucks, truck tractors and buses built on or after March 1, 1975, and air-braked trailers built on or after January 1, 1975, must meet FMVSS 121 as it stood when they were built (49 CFR 393.40(b)(2))
- The pushrod stroke of an air brake may not be greater than the readjustment limit in the table in 49 CFR 393.47(e). For a type not in the table it may not be greater than 80 percent of the rated stroke marked on the actuator, or its marked readjustment limit
- Brake lining on a non-steering air drum brake may not be thinner than 1/4 inch at the shoe center (or the wear indicator). Air disc pads may not be thinner than 1/8 inch (49 CFR 393.47(d)(2))
- The brake chambers on each end of an axle must be the same size, and the slack adjusters on each end must be the same effective length (49 CFR 393.47(b) and (c))
- Every air-braked commercial motor vehicle needs an air pressure gauge and a warning signal. The warning must be continuous whenever service reservoir pressure is at 55 psi or below, or at one-half of the governor cutout pressure, whichever is less (49 CFR 393.51(c))
- Truck tractors built on or after March 1, 1997 must have ABS. Other air-braked commercial vehicles built on or after March 1, 1998 must have it too (49 CFR 393.55(c))
- An air-braked commercial vehicle built on or after October 20, 1994 must have automatic brake adjusters (49 CFR 393.53(b)). Replacing one with a manual adjuster fails the annual inspection (49 CFR part 396, Appendix A)
Brake limits for air-braked commercial motor vehicles
| Item | What the rule says | Where | |
|---|---|---|---|
| Pushrod stroke, clamp-type 12 chamber | 1 3/8 in (34.9 mm) standard stroke; 1 3/4 in (44.5 mm) long stroke | 393.47(e) | |
| Pushrod stroke, clamp-type 16 chamber | 1 3/4 in (44.5 mm) standard stroke; 2 in (50.8 mm) long stroke | 393.47(e) | |
| Pushrod stroke, clamp-type 20 and 24 chambers | 1 3/4 in (44.5 mm) standard stroke; 2 in (50.8 mm) long stroke. A type 20 or 24 with a 3 inch (76 mm) rated stroke: 2 1/2 in (63.5 mm) | 393.47(e) | |
| Pushrod stroke, clamp-type 30 chamber | 2 in (50.8 mm) standard stroke; 2 1/2 in (63.5 mm) long stroke | 393.47(e) | |
| Steering axle lining, air drum brake | Not under 3/16 in (4.8 mm) for a shoe with a continuous strip of lining, or 1/4 in (6.4 mm) for a shoe with two pads, or worn to the wear indicator | 393.47(d)(1) | |
| Non-steering axle lining, air drum brake | Not under 1/4 in (6.4 mm) at the shoe center, or worn to the wear indicator | 393.47(d)(2) | |
| Air disc brake pads | Not under 1/8 in (3.2 mm) | 393.47(d) | |
| Hydraulic and electric brake lining | Not under 1/16 in (1.6 mm) on non-steering axles. Steering axles: 1/16 in (1.6 mm) or less is too thin | 393.47(d) | |
| Wedge brake adjustment | The scribe mark on the lining may not move more than 1/16 in (1.6 mm) | 393.47(f) | |
| Low air pressure warning | Continuous warning at 55 psi (379 kPa) and below, or at one-half the governor cutout pressure, whichever is less | 393.51(c)(2) | |
| Tractor protection valve | Must operate automatically when towing-vehicle air pressure is between 20 and 45 psi | 393.43(a) | |
| Stop from 20 mph, tractor-trailer combination | Service brakes: within 40 feet of the start of pedal movement. Emergency brakes: within 90 feet | 393.52(d) |
What brakes must a commercial truck have?
Every commercial motor vehicle must have brakes that can stop and hold the vehicle or combination (49 CFR 393.40(a)). That means a service brake, a parking brake and an emergency brake system, each meeting its own rule. Brakes must act on all wheels (393.42(a)). A few exceptions exist. A truck or tractor with three or more axles built before July 25, 1980 does not need front-wheel brakes. A semitrailer or full trailer with a gross weight of 3,000 pounds or less needs none if its axle weight is no more than 40 percent of the towing vehicle's (393.42(b)). All brakes the vehicle has must work at all times (393.48(a)). A lift axle is the exception while it is raised, but its brakes must work whenever it is lowered and the tires touch the road (393.48(c)(6)).
How far can an air brake pushrod travel?
The pushrod stroke may not be greater than the readjustment limit for that chamber (49 CFR 393.47(e)). The limit depends on the type of chamber. A standard stroke clamp-type 30 chamber has a limit of 2 inches. A type 20 or 24 has a limit of 1 3/4 inches. Long stroke chambers get more. For a type that is not in the table, the stroke may not be greater than 80 percent of the rated stroke marked on the actuator, or greater than the readjustment limit the maker marked on it. The annual inspection standards say to measure with the engine off and reservoir pressure at 80 to 90 psi, with the brakes fully applied (49 CFR part 396, Appendix A). Any brake over the limit is rejected.
How thin can brake linings and pads be?
On an air drum brake, the lining on a non-steering axle may not be thinner than 1/4 inch at the shoe center, or worn to the wear indicator if the lining has one (49 CFR 393.47(d)(2)). On the steering axle the rule is 3/16 inch for a shoe with one continuous strip of lining, and 1/4 inch for a shoe with two pads (393.47(d)(1)). Air disc brakes need at least 1/8 inch. A hydraulic or electric brake may not be under 1/16 inch on a non-steering axle, and on the steering axle it must be thicker than 1/16 inch. The annual inspection also rejects a lining that is not firmly attached to the shoe, or that is soaked with oil, grease or brake fluid (Appendix A, 1.a(6)). Drums and rotors may not be thinner than the maker's limit (393.47(g)).
When must a truck have ABS?
A truck tractor built on or after March 1, 1997 must have an antilock brake system that meets FMVSS 121 (49 CFR 393.55(c)(1)). Every other air-braked commercial vehicle built on or after March 1, 1998 must have one too, including trailers, converter dollies and single-unit trucks (393.55(c)(2)). A hydraulic-braked truck or bus built on or after March 1, 1999 must have ABS (393.55(a)). Newer rules add warning lamps and wiring. A tractor built on or after March 1, 2001 must carry the trailer's ABS malfunction signal to a lamp in the cab. A trailer built on or after March 1, 1998 must have an ABS lamp on the outside of the trailer (393.55(d) and (e)). A missing or dead ABS lamp, or one that stays on, fails the annual inspection.
What air pressure warning must a truck have?
Every air-braked commercial vehicle, and every vehicle that tows one, needs an air pressure gauge the driver can see from the normal seat, and a warning signal (49 CFR 393.51(c)). The signal may be audible or visible. It must warn continuously whenever pressure in the service reservoir system is at 55 psi or below, or at one-half the compressor governor cutout pressure, whichever is less. A truck that tows an air-braked trailer must also have a tractor protection valve that works automatically when pressure falls between 20 and 45 psi (393.43(a)). A towing vehicle needs two ways to apply the trailer's emergency brakes. One is automatic at that pressure range, and the other is a manual control the driver can reach from the seat (393.43(b)). A trailer's brakes must apply automatically and at once if it breaks away. They must stay applied for at least 15 minutes (393.43(d)).
How well must the brakes stop the truck?
The rule sets a stopping test at 20 miles per hour on a dry, level, hard surface, with the vehicle in the middle of a 12-foot lane (49 CFR 393.52). A tractor-trailer combination must stop within 40 feet from the point where the driver starts to move the brake pedal. A single-unit truck over 10,000 pounds GVWR, other than a tractor, must stop within 35 feet. On the emergency brake alone the limits are 90 feet and 85 feet. The service brakes must also develop a braking force of at least 43.5 percent of gross weight, and decelerate at 14 feet per second per second or more. A performance-based brake tester may be used to measure braking force.
What brake defects fail an annual inspection?
Appendix A to 49 CFR part 396 lists them. The main ones are no braking action on an axle that needs brakes, a missing brake, missing or broken shoes, springs or pushrods, loose air chambers, and an audible air leak at a brake chamber. A brake over its stroke limit fails. So does a mismatch of chamber size or slack adjuster length across a steering axle. A drum or rotor with an external crack that opens when the brake is applied, or a missing piece, fails. Damaged, bulging, crimped or leaking air hose and tubing fail. So do a missing or dead low pressure warning, and a missing or inoperable tractor protection valve. A brake found out of adjustment must be checked to find out why the automatic adjuster is not working, and the cause must be fixed. It is not acceptable to adjust an automatic adjuster by hand without first correcting the underlying problem.
Related Free Tools
Annual DOT Inspection Checklist
Every item from the federal minimum periodic inspection standards as a checklist, with your next due date and what the report must show.
Preventive Maintenance Scheduler
Track all service intervals by odometer. Never miss a maintenance item again.
Pre-Trip / Post-Trip DVIR Checklist
A 49 CFR 396.11 compliant Driver Vehicle Inspection Report with defect tracking.
Used Truck Inspection Checklist
What to check before buying a used semi truck: paperwork, engine, frame, tires, and a real test drive.
Frequently Asked Questions
What is the pushrod stroke limit for air brakes?+
It depends on the chamber type. For a standard stroke clamp-type 30 chamber it is 2 inches. For a type 20 or 24 it is 1 3/4 inches. Long stroke chambers are allowed more. The full table is in 49 CFR 393.47(e). For a type not in the table the limit is 80 percent of the rated stroke marked on the actuator, or the readjustment limit marked on it.
How do you measure air brake pushrod stroke?+
The annual inspection standards say to measure it with the engine off and the reservoir pressure at 80 to 90 psi, with the brakes fully applied (49 CFR part 396, Appendix A). Compare the result with the readjustment limit for that chamber type.
How thin can semi truck brake lining be?+
On a non-steering air drum brake, not thinner than 1/4 inch at the shoe center, or worn to the wear indicator. On the steering axle, 3/16 inch for a shoe with a continuous strip of lining, or 1/4 inch for a shoe with two pads. Air disc pads must be at least 1/8 inch (49 CFR 393.47(d)).
When was ABS required on semi trailers?+
Truck tractors built on or after March 1, 1997, and other air-braked vehicles, including trailers, built on or after March 1, 1998, must have an antilock brake system (49 CFR 393.55(c)). A trailer built on or after March 1, 1998 must also have an exterior ABS malfunction lamp (393.55(e)).
At what psi does the low air warning come on?+
It must warn continuously whenever pressure in the service reservoir system is at 55 psi or below, or at one-half the compressor governor cutout pressure, whichever is less (49 CFR 393.51(c)(2)).
Do all semi trucks have air brakes?+
No. The rule covers hydraulic, air, vacuum, electric and surge brake systems (49 CFR 393.40(b)). Air brakes are used on heavy tractors and trailers, and air-braked vehicles must meet the air brake rules in 393.47, 393.50, 393.51 and 393.53.
How many brakes does a semi need?+
The rule says brakes must act on all wheels (49 CFR 393.42(a)). There are exceptions, such as small light trailers and a truck or tractor with three or more axles built before July 25, 1980 that does not need front brakes (393.42(b)).
Can you replace an automatic slack adjuster with a manual one?+
Not on a vehicle built on or after October 20, 1994 that must have automatic adjusters (49 CFR 393.53(b)). The annual inspection rejects an automatic adjuster that has been replaced with a manual one (49 CFR part 396, Appendix A).
What do inspectors check on the brakes at a DOT inspection?+
The annual inspection standards list the defects. They include missing or broken parts, loose air chambers, air leaks, a brake over its stroke limit, thin or oil-soaked lining, cracked drums, damaged hose and tubing, a dead low air warning, a bad tractor protection valve and ABS lamp faults (49 CFR part 396, Appendix A).
More federal trucking rules
- Hours of Service Rules for Truck Drivers
- ELD Mandate: Who Needs an ELD
- DOT Roadside Inspection Levels Explained
- DOT Drug and Alcohol Testing Rules
- Driver Qualification File Requirements
- DVIR and Vehicle Maintenance Rules
- New Entrant Safety Audit Requirements
- Hours of Service Exceptions and Exemptions
- Motor Carrier Insurance Requirements
- CDL Classes, Endorsements and ELDT Training
- DOT Physical and Medical Card Requirements
- Drug and Alcohol Clearinghouse Requirements
- Cargo Securement Rules for Trucks
- Hazmat Basics for Truck Drivers and Carriers
- USDOT Number and Biennial Update Requirements
- Safety Ratings, Compliance Reviews and DataQs
- Federal Truck Size and Weight Limits
- California Clean Truck Check (HD I/M)
- Truth-in-Leasing Rules for Owner-Operators
- Form 2290 Heavy Highway Vehicle Use Tax
- Truck Driver Phone, Texting and Seat Belt Rules
- Railroad Crossing Rules for Truck Drivers
- Truck Emergency Equipment and Warning Triangle Rules
- Truck Windshield, Dash Cam and Mirror Rules
- CDL Disqualification: Major and Serious Offenses
- Truck Tire Rules: Tread Depth, Retreads and Pressure
- Trailer Reflective Tape (Conspicuity) Rules
- Driver Conduct Rules: Alcohol, Drugs, Fatigue, Passengers and Pre-Trip Checks
- English Language Proficiency for Truck Drivers: The 391.11(b)(2) Rule
- Weigh Station Rules: Who Must Stop and What Happens If You Skip
- Truck Lighting Rules: Required Lamps and Reflectors (49 CFR 393)
- Driver Road Test Rules: The Company Road Test and Certificate (49 CFR 391.31)
Related terms and pages
Sources
- 49 CFR 393.40, Required brake systems (eCFR)
- 49 CFR 393.42, Brakes required on all wheels (eCFR)
- 49 CFR 393.43, Breakaway and emergency braking (eCFR)
- 49 CFR 393.47, Brake actuators, slack adjusters, linings/pads and drums/rotors (eCFR)
- 49 CFR 393.51, Warning signals, air pressure and vacuum gauges (eCFR)
- 49 CFR 393.52, Brake performance (eCFR)
- 49 CFR 393.53, Automatic brake adjusters (eCFR)
- 49 CFR 393.55, Antilock brake systems (eCFR)
- 49 CFR part 396, Appendix A, Minimum periodic inspection standards (eCFR)
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This is a plain-language summary for reference, not legal advice. Check the official rule text before you rely on any detail. Ask a compliance professional about your own operation.