Brake Pressure Hold Valve: Description and Operation

Fig. 5 Hill holder (PHV) valve:

Fig. 6 Hill holder installation:

The Hill holder is essentially a Pressure Hold Valve (PHV), Fig. 5, built into one brake circuit, that maintains hydraulic pressure in the brake circuit when the vehicle is facing uphill and the clutch pedal depressed, Fig. 6.
The PHV lever, Fig. 5, is connected through a linkage to the clutch pedal and control a camshaft which provides the motion to the PHV driveshaft. The PHV driveshaft controls the clearance between the PHV inertia controlled ball and seal.

Fig. 18 Hill Holder Activated:

Fig. 19 Hill Holder Deactivated:

When the clutch pedal is depressed, the PHV driveshaft is pulled into the seal, allowing the ball free movement. If the vehicle is facing uphill, inertia will cause the ball to roll onto the seal, Fig. 7, thereby holding hydraulic pressure. When the clutch is released, Fig. 8, the driveshaft is forced into the ball chamber, unsealing the ball and releasing hydraulic pressure.