Air Over Hydraulic Brakes: How They Work and Differ From Air Brakes
How air over hydraulic brakes work, how they differ from full air brakes and from car-style hydraulic brakes, and how the CDL rules treat them.
- Published
Short answer
Air over hydraulic brakes use compressed air to push the hydraulic fluid that applies the brakes at the wheels. The driver's foot controls an air valve, the air does the pushing, and the wheel ends are hydraulic: calipers or wheel cylinders on fluid lines, as on a car. It is a separate system from the full air brakes on a tractor-trailer, where air itself works a brake chamber at each wheel.
How do air over hydraulic brakes work?
The system has an air side and a hydraulic side, joined by a booster.
- Air side. An engine-driven compressor fills storage tanks, much as on a full air system. When the driver presses the pedal, a valve sends air from a tank to the booster.
- Booster. The booster turns air pressure into mechanical force on a hydraulic master cylinder. Different makers build it differently, and it goes by several names, such as air-hydraulic booster or air-hydraulic power cylinder.
- Hydraulic side. The master cylinder pushes brake fluid through steel and flexible lines to the wheel ends, where calipers or wheel cylinders apply the pads or shoes. Releasing the pedal releases the air, and the fluid returns.
The stored air supplies the force that the driver's leg would otherwise have to supply.
How are air over hydraulic brakes different from full air brakes?
The difference is at the wheel end. A full air brake system has a brake chamber and, on drum brakes, a slack adjuster at each wheel, and a trailer connects through air lines. On air over hydraulic the service brakes at the wheels are hydraulic, so wear is taken up by the caliper or the wheel cylinder and its adjuster, not by a slack adjuster setting.
That changes what a technician checks. Air-side faults, such as a leaking line, a weak compressor or water in the tanks, look like air brake faults. Hydraulic-side faults, such as low fluid, a leaking line or wheel cylinder, or air trapped in the fluid, look like hydraulic brake faults. A soft pedal and a slow air build-up are different problems with different fixes, so find out which side the symptom is on first.
Parking brakes also vary by truck. Some builds carry air-applied spring brakes, so do not assume there is no chamber at a wheel, and some use another arrangement. Check the manufacturer's documentation for the unit in front of you.
Which trucks have air over hydraulic brakes?
They are found on some medium-duty trucks. Whether a given truck has them depends on the maker and the build, not on its weight class alone. Read the truck's documentation or look for the booster and the master cylinder, not the weight rating on the door.
Do air over hydraulic brakes count as air brakes for a CDL?
For the air brake restriction, yes. For the full air brake restriction, they are a separate category. Under 49 CFR 383.95(a), if an applicant fails the air brake knowledge test or takes the skills test in a vehicle without air brakes, the State must indicate on the CLP or CDL, if issued, a restriction from any type of air brakes, and air brakes include any braking system operating fully or partially on the air brake principle. Under 49 CFR 383.95(b), if an applicant takes the skills test in a vehicle with air over hydraulic brakes, the State must indicate a restriction from a CMV with a braking system operating fully on the air brake principle. The section defines air over hydraulic brakes as a system operating partially on the air brake and partially on the hydraulic brake principle. This page does not cover the rest of the section or tell a driver what a CDL allows. Read the section on eCFR.
What should a technician check on an air over hydraulic system?
Check both sides, and keep them apart in the record.
- Air side: air build-up time, leaks, tank drains, the dryer, and the warning devices the truck has.
- Hydraulic side: fluid level and condition, hoses and lines, caliper or wheel cylinder leaks, pedal feel, and lining or pad wear.
- The joint between them: the booster and master cylinder, where air-side force becomes fluid pressure.
Bleeding, fluid type, adjustment and any specification come from the manufacturer's service documentation for the specific build, and this page does not give them.
Before you work on these brakes
Stored air and hydraulic pressure can both be present on one of these trucks, so a line that looks idle may not be. Before you loosen a line, a fitting or a booster, follow the manufacturer's procedure for that component.
Log the work in Blue Collar Parts
When a technician logs brake work on one of these trucks in Blue Collar Parts, the record can show the air-side and hydraulic-side findings separately, so the customer who confirms it can see which side the work was on. Learn what Blue Collar Parts is.
This page describes how air over hydraulic brakes work in general terms. It is not a service manual and does not establish regulatory compliance. Use the equipment manufacturer's current service documentation for equipment-specific procedures, and the current eCFR text for requirements.