When an excavator's swing brake fails to release, the operator will notice the machine is sluggish to start swinging, and then jerks into motion. If left unfixed, the friction plates in the swing brake will burn up, filling the swing drive housing with black, burnt oil and destroying the brake pack. The problem is rarely the brake plates themselves; it is almost always the swing brake sequence valve.
The sequence valve ensures that pilot pressure must first stroke the main swing motor spool before the brake releases. If the valve is out of adjustment or stuck, two terrible things can happen: either the brake releases too early, allowing the house to freewheel before the motor engages, or the brake releases too late, causing the motor to try to turn against a locked brake.
To test it, you must install a 600 PSI gauge into the test port on the swing brake housing. Have the operator start the machine and slowly move the swing joystick. Watch the gauge closely. The gauge should show zero pressure at rest. As the joystick is moved, the swing motor should begin to develop torque, and the brake gauge should hit ~300 PSI almost simultaneously, releasing the brake. If the gauge hits 300 PSI before the house moves, the brake is releasing too early. If the house tries to twist and shudders before the gauge climbs, the brake is dragging. Adjust the sequence valve's set screw until the brake release and motor torque are perfectly synchronized.
