Hydraulic Brake Boost System

WARNING: This page is about a different car, the 2008 Saturn Sky and 2008 Pontiac Solstice. However, it is still accessible from the selected car via links, so may be relevant.

If your vehicle is equipped with Electronic Stability Control (ESC) and the optional 2.0L turbocharged engine, it also has a hydraulic brake boost feature which supplements the power brake system to maintain consistent brake performance under conditions of low brake booster vacuum. Low brake booster vacuum conditions can include initial start up after the vehicle has been parked for several hours, very frequent brake stops, or high altitude driving. When hydraulic brake boost is active, you might feel minor brake pulsation or movement in the pedal but this is normal.

The Hydraulic Brake Boost system activates (only) during a brake apply when the brake module detects low vacuum from the vacuum sensor mounted in the booster. The boost function in this case will be provided by the ESC system to provide metered brake pressure to the wheels. When the system activates to build pressure, the ECU will run the pump motor, opens the Electronic Shuttle Valve (ESV), and closes the Isolation Valves (NO TCS Valves) at the same time. The ECU runs the pump in pulsed cycles and builds pressure in the wheels that exceeds the TMC (with low vacuum in the booster). As the brake fluid is drained from the TMC, the brakes pedal will move accordingly (pulsing feel in pedal). Pump speed and duration are controlled by computed volume models in the ECU (information includes low vacuum sensed, TMC pressure, and pedal travel through the BAS sensor). If the pressure is reduced or vacuum restored while a Low Hydraulic Boost is required the wheel pressure can thus be continuously adjusted to the TMC pressure by selectively lowering the current on the Isolation Valve (NO TCS Valve).