Komatsu PC88MR-10 Shop Manual page 205

Operation when traveling downhill
If the machine runs away on a downhill, the motor runs idle and the pressure in the motor inlet port (MA)
decreases, and the pressure in chamber (D) also decreases through orifice (c).
When the oil pressure in chamber (D) decreases below the reaction force of spring (3), counterbalance
valve (2) starts to return to the right.
The oil in chamber (G) flows into port (PB) through orifice (f). At this time, back pressure is generated by
the throttle effect of orifice (f), and it controls the speed of counterbalance valve (2) returning to the right.
Counterbalance valve (2) moves to a position where the force caused by the machine weight and the
hydraulic force of motor inlet port (MA) are balanced with the hydraulic force of motor outlet port (MB).
When motor outlet port (MB) is throttled, the oil pressure on the outlet side increases and rotating
resistance is generated in the motor.
The speed is controlled in accordance with the pump discharged volume to prevent the machine from
running away.
PC88MR-10
10 Structure and function
Travel motor
10-127