Lesson 2/Learning Event 3
Brakes Apply Too Slowly
If you depress the brake pedal and the braking action does not occur
immediately, it indicates there is a bind or weakness some place.
First, ensure you have sufficient air pressure and there is no water
(condensation) in the compressed air system.
Check all air and hydraulic lines to ensure they are not kinked or
bent to a point where they will restrict the flow of air or hydraulic
fluid.
Check the brake shoes at their anchors. They should move freely and
not bind.
Brakes Dragging
This malfunction is noticeable when you are roadtesting a vehicle.
The vehicle holds back when you are trying to accelerate, and when
you disengage the clutch, the truck will not coast freely.
First, check the brake pedal free play adjustment to ensure you have
the proper clearance. If the pedal does not allow the master
cylinder piston and primary cup to clear the compensating port, the
brake fluid cannot come back to the master cylinder reservoir from
the wheel cylinders.
Next, check the brake adjustment of each wheel to ensure there is
enough clearance when the brakes are released.
Restricted air and hydraulic lines can also prevent the brakes from
releasing.
Ensure the brake shoes are not sticking on the anchor pins.
Check the master cylinder for operation. If some foreign material,
such as oil or dirt, is mixed with the brake fluid in the reservoir,
it can plug the ports or warp the rubber cups. This will prevent the
fluid from returning to the reservoir.
Sticking pistons in the wheel cylinders can also cause dragging
brakes. If the pistons or cylinders are corroded, the cylinder
assemblies should be replaced.
As a last resort, check the operation of the airhydraulic cylinder.
It is possible that the air valve is not allowing the compressed air
in the cylinder to escape. If tests indicate improper operation, the
airhydraulic cylinder should be replaced.
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