Brake Pressure Testers for Commercial Vehicles, Agricultural Vehicles, and Off-Highway Vehicles

Frequently Asked Questions About Brake Pressure Testing on Commercial Vehicles and Machinery

Pneumatic brake systems operate using multiple pressure circuits and components, the function of which depends on proper pressure build-up and a sufficiently leak-tight compressed air system. Mechanical brake pressure measurement helps assess actual pressure conditions and supplement electronic diagnostic data. Here, we answer frequently asked questions about brake pressure testing on trucks, buses, trailers, as well as agricultural and off-highway vehicles.

• What is a pneumatic brake system?

  In a pneumatic braking system, compressed air is used to actuate the wheel brakes or brake actuators.
  
  Such systems are particularly common in heavy-duty commercial vehicles, buses, and trailers.
  
  Modern vehicles often combine pneumatic actuation with electronic control and regulation systems.

• What is the difference between a pneumatic brake and a hydraulic brake?

  In a hydraulic brake system, braking force is transmitted via brake fluid.

  In a pneumatic brake system, compressed air serves as the working medium.

  This results in differences in:

  • Operating pressures
  • Components
  • Connections
  • Test procedures
  • Suitable measuring instruments

  For passenger cars with hydraulic brake systems, separate Brake pressure testers for passenger cars are therefore available.

• Which vehicles are equipped with air brakes?

  Pneumatic or electropneumatic braking systems are found in particular in:

  • Trucks
  • Buses
  • Tractor-trailers
  • Trailers
  • Semi-trailers

  In addition, pneumatic braking systems are also used in various agricultural, construction, and specialty machines.

  Whether a specific machine operates using a pneumatic, hydraulic, or combined system must be determined based on its technical design.

• Why is the brake pressure of a truck's air brake system measured?

  A mechanical pressure measurement shows the actual pressure at the respective measurement point.

  It can help, for example, with:

  • Functional testing of the compressed-air brake system
  • Comparing different pressure circuits
  • Testing pressure build-up and pressure release
  • Troubleshooting in cases of unusual brake behavior
  • Inspection after repair work
  • Comparison with electronic diagnostic values

  The evaluation is based on the specifications of the respective vehicle or brake system manufacturer.

• What is the normal brake pressure for a truck?

  There is no single brake pressure value that applies to all trucks and all measurement points.

  The expected pressure depends, among other things, on:

  • Vehicle
  • Brake system
  • Measurement point
  • Operating condition
  • Actuation
  • Reserve pressure
  • Electronic or pneumatic control

  The measured values must therefore always be compared with the technical reference values for the respective vehicle.

• What is the reservoir pressure in a compressed-air brake system?

  The compressed air system stores compressed air in dedicated air reservoirs.
  
  This air supply powers the vehicle's pneumatic systems.
  
  The system pressure must therefore be distinguished from the brake or control pressure measured at a single brake or control line.

• What is the difference between reservoir pressure and brake pressure?

  Storage pressure refers to the pressure available in the compressed air system or the storage tanks.
  
  Brake or control pressure is generated or varies depending on the operation and the specific brake system.
  
  Both values can be important for diagnostics, but they provide answers to different questions.

• Why can multiple pressures be measured simultaneously in an air brake?

  Pneumatic brake systems have multiple circuits, lines, and connection points.
  
  Multiple pressure gauges allow different pressures to be monitored simultaneously and compared with one another under the same operating conditions.
  
  This makes it easier to diagnose discrepancies between different parts of the brake system.

• How do you connect a brake pressure tester to a truck's air brake system?

  The Leitenberger BDP and BDPD test kits are connected to the designated connection points of the brake system using appropriate compressed air hoses and couplers.
  
  Which measurement points are used and how the test is performed depends on the specific vehicle or brake system.
  
  Before taking measurements, the correct connections and permissible pressure ranges must be verified.

• Why do the test kits include coupling heads?

  Coupling heads allow connection to the appropriate pneumatic lines or connections of the compressed-air braking system.
  
  Depending on the test situation, this makes it possible, for example, to include different circuits or tractor and trailer connections in the measurement.

• Can a brake pressure tester also be used to check for leaks?

  Measuring pressure can assist in a leak test.
  
  For example, this allows you to observe whether a pressure that was previously built up drops to an impermissible level within the specified test period.
  
  How the leak test is conducted and what pressure loss is permissible depends on the specific brake system and test procedure.
  
  A detected pressure loss initially indicates a deviation. The actual leak must then be located.

• Why is it important for a compressed-air brake system to be airtight?

  Leaks can cause compressed air to be lost, preventing the system from reliably building up or maintaining the required pressure.

  Possible causes include, for example:

  • leaky lines
  • damaged hoses
  • plug-in or screw connections
  • valves
  • brake actuators
  • other pneumatic components

  If a noticeable pressure loss occurs, the cause must be systematically identified.

• What happens if the pressure in a compressed-air brake system is too low?

  The specific response depends on the particular braking system.
  
  Pneumatic commercial vehicle braking systems have various safety features. For example, spring-loaded brake actuators may be designed to be released by compressed air and to return to mechanical operation when the pressure is released.
  
  Therefore, warning thresholds, minimum pressures, and test procedures must always be evaluated according to the specifications of the particular vehicle or braking system.

• What is a spring-loaded brake?

  In a spring-applied brake, a mechanical spring generates the braking force.
  
  Compressed air is used to overcome the spring force or to release the brake.
  
  This allows the corresponding brake actuators to perform parking and emergency braking functions.

• Can an EBS diagnostic tool replace mechanical brake pressure measurement?

  Not entirely.

  For example, an electronic diagnostic system can:

  • Read error codes
  • Display sensor values
  • Provide ECU information
  • Control components
  • Perform calibrations or service functions

  Mechanical pressure measurement, on the other hand, shows the actual pneumatic pressure present at the measurement point.

  Both methods therefore complement each other in professional troubleshooting.

  For trucks, buses, and trailers, the mechanical test can be combined, for example, with the Jaltest multi-brand diagnostic system for trucks, buses, and trailers.

• Can brake pressure also be checked on agricultural vehicles?

  Yes, provided that the vehicle or machine in question is equipped with an appropriate pneumatic braking system.

  The specific design varies depending on the manufacturer and the machine.

  For electronically monitored agricultural vehicles, the mechanical pressure test can also be combined with the Jaltest Diagnostics for Agricultural Machinery and Vehicles.

• Can the brake pressure be tested on construction equipment and off-highway vehicles?

  Yes, for machines equipped with the appropriate pneumatic braking system.

  However, off-highway vehicles feature a wide variety of braking systems. Therefore, before taking any measurements, it is necessary to determine which braking system is installed, which measurement points are designated, and which setpoints apply.

  If necessary, the electronic diagnostics can be supplemented by the Jaltest Diagnostics for Construction Machinery and Off-Highway Vehicles.

• What is the difference between analog and digital truck brake pressure testers?

  Analog testers display pressure using mechanical dial gauges.
  
  Digital testers use electronic digital pressure gauges.
  
  The current Leitenberger BDPD digital test kits have a measurement range up to 25 bar and feature an easy-to-read digital pressure display.
  
  A factory calibration certificate is available as an option for the digital pressure gauges.

• What is the difference between BDP 01 and BDP 02?

  The BDP 01 is the more comprehensive analog test kit.

  It contains:

  • two pressure gauges rated up to 16 bar
  • one pressure gauge rated up to 25 bar
  • three compressed air hoses, each 3.5 m long
  • four coupling heads

  The BDP 02 includes:

  • two pressure gauges rated up to 16 bar
  • two compressed air hoses, each 15 m long
  • two coupling heads

  Which kit is more suitable depends on the intended test procedure and the required measurement points.

• What is the difference between BDPD 31 and BDPD 32?

  Both versions use digital pressure gauges rated up to 25 bar.
  
  The BDPD 31 is designed for multiple measurement points connected simultaneously and includes three digital pressure gauges, three 3.5-meter compressed air hoses, and four coupling heads.
  
  The BDPD 32 includes two digital pressure gauges, two 15-meter compressed air hoses, and two coupling heads.
  
  The choice depends on how many measurement points are needed simultaneously and what hose length is appropriate for the intended test setup.

• Are the analog brake pressure testers calibrated?

  Leitenberger offers the current BDP 01 and BDP 02 models as uncalibrated versions.
  
  A factory calibration certificate is available as an option for the digital BDPD devices.
  
  The metrological and documentation requirements applicable to a specific testing purpose should be clarified before selecting a device.

• Can the devices also be used on trailers and semi-trailers?

  Yes, provided that the respective pneumatic braking system and the available connections are compatible with the intended test procedure.
  
  Especially in the case of a tractor and trailer, it may be useful to monitor several pressure values simultaneously.
  
  The specific test procedure depends on the design of the braking system.

• What other tests are recommended as part of a commercial vehicle brake diagnosis?

  Depending on the fault symptoms, the following may also be required:

  • Electronic ABS/EBS diagnostics
  • Inspection of lines and couplings
  • Leak detection on pneumatic components
  • Inspection of brake actuators
  • Mechanical inspection of wheel brakes
  • Inspection of brake rotors and brake pads

  For additional equipment and tools, see Test and Service Equipment for Brake Systems.