Final answer:
When all wheels on a truck with air brakes lock up due to minimal pressure on the treadle valve, potential causes could be a bad application valve, a leaking limiting valve, an overadjusted compressor regulator, or brakes that are adjusted too high.
Step-by-step explanation:
If all the wheels on a truck with air brakes lock up when a small amount of pressure is applied to the treadle (foot) valve, several potential issues should be considered. The situation described could indicate a problem with the brake system. One possible cause is that the application valve, which is responsible for transmitting the pressure from the treadle valve to the brake chambers, may be defective or improperly adjusted (The application valve is bad). Alternatively, if the limiting valve is leaking, it might not manage the pressure properly, leading to the brakes applying too vigorously.
In a truck's pneumatic braking system, the force exerted by the driver is amplified and transmitted to each wheel in a controlled manner, typically following Pascal's principle, as represented by hydraulic systems in the reference information provided. An overactive regulator could result in too much pressure being delivered to the brake chambers (The compressor regulator is set too high), leading to the brakes engaging too strongly. Lastly, if the brakes are improperly adjusted and the clearance is too tight (The brakes are adjusted too high), they could lock up with minimal pedal force.
Therefore, to troubleshoot the cause, one should inspect the application valve, check the limiting and relay valves for leaks, ensure the compressor regulator is set at the manufacturer-specified pressure, and verify proper brake adjustment. In diagnosing and fixing such an issue, it's essential to have a comprehensive understanding of the truck's air brake system and to follow safety procedures.