Primary Circuit Failure at Highway Speed
Driver Ruiz is southbound on a two-lane state route at 55 mph in a 2017 tri-axle dump truck (66,000 lb, air brakes on all three axles, no trailer) loaded with gravel. Coming up on a reduced-speed zone he applies the brakes and hears a sharp hiss. The red lamp and buzzer come on immediately. The primary (rear) gauge falls from 112 psi to zero in about eight seconds; the secondary (front) gauge holds at 105 psi and the compressor keeps it there. The brake pedal now travels almost to the floor with light effort, and the truck is not slowing. The parking brake valve is still pushed in, the engine is running, and there is a wide gravel shoulder 300 feet ahead, then a bridge with guardrails and no shoulder. Traffic is close behind him.
1. Explain exactly what braking Ruiz still has, what he has lost, and why the truck neither slowed sharply nor set its spring brakes by itself when the primary gauge hit zero.
Read the worked answer
This is a dual-circuit truck. The primary circuit feeds the rear service brakes; the secondary feeds the steering-axle service brakes, and on most trucks it also charges the parking/spring-brake circuit.
What he still has: the secondary circuit at 105 psi, so the front service brakes still apply and the compressor is still feeding that circuit. He also still has spring brakes, because their control circuit is still charged.
What he lost: all rear service braking. The pedal goes near the floor because the primary side of the foot valve now has nothing to compress; the travel is longer but the remaining effort is doing less work. At 66,000 lb, front-axle brakes alone provide a small share of the total braking force, which is why the truck is not slowing.
Why the spring brakes did not set: they apply when their own circuit loses pressure, and that circuit is still at 105 psi. The spring brakes are not triggered by the primary gauge. They would come on only if the secondary/parking circuit bled down to roughly 20 to 45 psi.
Why the warning came on: the low-pressure switch on the failed primary circuit sees less than 60 psi and triggers the lamp and buzzer, which is exactly what it is for.
2. Ruiz's first instinct is to pump the brake pedal, then to reach down and pull the yellow parking knob at 55 mph. Evaluate both actions.
Read the worked answer
Pumping the pedal: wrong. There is no air in the failed circuit, so pumping cannot rebuild pressure there; it only wastes the seconds he has and strokes the foot valve repeatedly, which can bleed down the secondary circuit that is currently his only service braking. The correct input is a firm, steady pedal, held.
Pulling the yellow knob at 55 mph: a last resort, not a first move. The parking valve dumps the spring-brake chambers, and at highway speed that can lock the drive wheels, producing a skid, a flat-spot or a jackknife on a two-lane road with traffic behind. Spring brakes are also not progressive in the way a foot valve is; they go from released to fully applied as the air dumps.
The judgment: use the front brakes first, downshift as speed falls, and get slowed before the shoulder. Use the parking valve to hold the truck after stopping, or progressively at low speed if the front brakes alone clearly will not stop him before the bridge. A controlled spring-brake application at 20 mph is far better than crossing a no-shoulder bridge with no brakes.
3. Sequence Ruiz's next 30 seconds of actions, and explain what he should not do once the truck is stopped.
Read the worked answer
In the next 30 seconds:
- Hazards on. Keep the engine running - the compressor is holding the secondary circuit, and an engine brake or retarder, if fitted, still helps.
- Off the throttle, firm and steady pressure on the foot valve, no pumping.
- Downshift progressively as speed drops, matching engine rpm and not over-revving.
- Aim for the gravel shoulder 300 feet ahead, before the bridge, not the bridge or the curve beyond it. A stop on the shoulder with a short walk is better than any stop past the guardrail.
- Once below highway speed, if the truck is still too fast, apply the parking valve progressively and steer to a stop.
After stopping: set the parking valve fully, chock the wheels, leave the hazards on, and set out reflective triangles (roughly 10 feet behind the vehicle, 100 feet and 200 feet back on a two-lane road, or as the terrain requires). Call for repair. Do not crawl under the truck, do not try to back onto the road, and do not release the parking valve to "roll to a better spot" - with the primary circuit open, the truck has no rear service brakes and the spring brakes are the only thing holding it.