G90-1260-001 | Genuine Paccar® Brake - Compact Trailer Hand Self Return

It sends compressed air pressure to the trailer’s brake lines (service line) when the driver operates the valve (lever, knob, or switch).

This allows testing or partial braking of the trailer without engaging the tractor’s brakes.

Because of safety, these valves often have a self-return or spring-return mechanism so when released, the valve returns to a neutral (off) position—stopping the air supply to the trailer brakes.

It is not intended to be used for parking or emergency full stops while driving (as using it improperly may cause the trailer to skid or jackknife).

The foot brake (service brake pedal) remains the main control for combined tractor + trailer braking in regular driving.

Air Leaks / Hissing Noise

You hear a continuous hissing or whistle from the valve or nearby when it’s supposed to be closed.

Air may leak when you apply or release the hand valve.

This suggests internal seals or diaphragms have failed or ports aren’t sealing.

Slow or Delayed Brake Response

When you operate the hand valve, the trailer brakes respond sluggishly or with a delay.

Brakes may not apply as quickly or as fully as expected.

This can indicate restrictions, internal binding, or partial failure inside the valve. 

Brakes Fail to Release (Dragging / Brake Drag)

After releasing the valve, the trailer brakes stay partially engaged or “drag.”

The valve is not fully venting air or not allowing return flow, so residual pressure stays in the brake circuit.

This leads to overheating, increased wear, and fuel penalty. 

Uneven or Inconsistent Braking

Sometimes the trailer brakes feel strong; other times weak.

One axle or side brakes more than the others when using the hand valve.

Inconsistent output may point to internal leakage or partial blockage inside the valve.

Loss of Air Pressure / Sudden Drops

When using the hand valve or in connection with it, you might see air pressure drop abnormally in the system.

The valve may be leaking internally into the atmosphere or into other unintended ports. 

Brake Fade / Diminished Effect Over Time

Initially braking works but then becomes less effective during repeated use.

The valve might be contributing to reduced air supply or internal leakage over repeated cycles. 

Physical / External Evidence

Corrosion, damage, or wear on the external body of the valve.

Seepage or oil/contamination around seals or connections.

Loose or damaged fittings and ports. 

Foot Brake / System Overrides Don’t Behave Normally

Using the foot brake overrides the hand valve as intended, but when the hand valve is faulty, you might see weird interactions (e.g. both trying to send conflicting signals).

The hand valve may give unexpected results in conjunction with the main brake controls.