One Gauge Falling on a Downgrade
You are 40 miles into a run with a loaded three-axle tractor pulling a tandem-axle van, gross weight 76,000 lb. On level ground both gauges read 118 psi. Partway up a long grade the primary (rear) gauge begins to fall - 105, then 95 psi - while the secondary (front) gauge holds at 118. At the crest you hear a faint hiss near the drive axles that stops when you release the brake pedal. Each service application now drops the primary gauge about 12 psi, and it recovers slowly. You cannot see the leak from the cab. Two miles into the downgrade the low-air warning light comes on with the primary at 58 psi and the secondary still at 110. The grade is 5% and runs four more miles to a wide, level pullout. Traffic behind you is light.
1. Explain what the two gauges are telling you and why one circuit can drop while the other holds. What does the hiss that stops when you release the pedal tell you?
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The two gauges are separate circuits. The primary reservoir feeds the rear service brakes; the secondary feeds the front. They are charged from a common wet tank through one-way check valves, so a failure or leak in one circuit drains that circuit but is not supposed to drain the other. That is exactly what you are seeing: the primary is bleeding down while the secondary sits at 118, which is the design working as intended, not a gauge fault.
The hiss that stops when you release the pedal is the diagnostic clue. A leak that only sounds under application is on the service side - typically a brake chamber diaphragm, a diaphragm that has started to rupture, or a line or fitting that only sees pressure when you push the treadle. A constant leak (a drain valve, a supply line) would hiss all the time. So this is a service-side leak in the rear circuit that gets worse with each application, which is why each brake now costs you about 12 psi from the primary.
2. Describe what the truck will do in the cab as the primary keeps falling past 60, 45 and 20 psi, and why the secondary holding at 110 does not save you.
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The sequence is predictable and each step removes options:
- Near 60 psi: the low-air warning light and buzzer come on. This is your last clear notice that the system is not keeping up.
- Around 45 psi: the tractor protection valve closes, and the trailer supply valve pops out, which applies the trailer's spring/emergency brakes. The red knob will not stay in if you push it.
- Between roughly 20 and 45 psi: the tractor's own spring brakes apply and the parking-brake valve pops out. You are now stopping whether you want to or not, and it will not be a gentle stop.
The secondary holding at 110 is not a rescue. Front brakes alone cannot stop 76,000 lb on a 5% grade - the front axle carries only a fraction of the weight and the front brakes are sized to assist, not to carry the load. And the moment the protection valve closes you have lost control of the trailer brakes. Two gauges holding is good news; one gauge holding is a countdown.
3. You are two miles into a four-mile 5% grade with the pullout ahead. State your actions in order and justify each one.
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First, stop using the brakes as you have been. Every application is now costing about 12 psi from a circuit that is already at 58. Fanning or riding them will take you to 45 psi - the protection valve - in a handful of applications.
Second, get the speed down and keep it down without the service brakes: downshift early (one gear at a time, matching rpm) and use the engine brake or exhaust brake if fitted. Compression braking on a 5% grade will hold 25-30 mph in a low gear without spending air.
Third, commit to the pullout. Do not try to finish the remaining two miles on a leaking circuit. Plan to arrive slowly, brake firmly but as little as possible, and stop.
If the valve pops out anyway, do not hold the red knob in and do not fight the spring brakes. Steer, keep it in gear, and let them bring you to a controlled stop on the shoulder. Once stopped, do not move again until the leak is fixed - you had half a system and now you have less.