Atlas Copco XAS 185 KDU T4F HOP Instruction Manual page 19

When the air consumption is resumed, the unloader valve
gradually opens the air intake and the speed regulator
increases the engine speed.
The construction of the regulating valve is such that any
increase (decrease) of the air receiver pressure above the
pre-set valve opening pressure results in a proportional
increase (decrease) of the control pressure to the
unloading valve.
Part of the control air is vented to atmosphere, and any
condensate discharged, through the vent holes (VH).
- 19 -