Pressure Test

Thoroughly clean area around hydraulic pressure switch on side of power flow regulator and remove pressure switch. See Fig 1 .
Install a 0-1000 psi pressure gauge in place of pressure switch.
Start engine and check for leaks. With engine running, pressure should be at upper switching specification. See HYDRAULIC BRAKE BOOSTER TEST SPECIFICATIONS table.
Apply brake pedal until pressure drops to lower switching pressure specification. Check that pressure quickly returns to upper switching pressure specification. See HYDRAULIC BRAKE BOOSTER TEST SPECIFICATIONS table. If any faults are found up to this point, power flow regulator is faulty and should be replaced. See POWER FLOW REGULATOR under REMOVAL & INSTALLATION.
Turn ignition switch to OFF position. After 5 minutes, pressure drop should be no greater than specification. See HYDRAULIC BRAKE BOOSTER TEST SPECIFICATIONS table.
If pressure drop exceeds specification, go to FLUID LEAK TEST . If pressure drop is within specification, go to HYDRAULIC ACCUMULATOR TEST .

Fig 1: Locating Hydraulic Booster System Components

Courtesy of BMW OF NORTH AMERICA, INC.

HYDRAULIC BRAKE BOOSTER TEST SPECIFICATIONS

Application	psi (kg/cm² )
Lower Switching Pressure	522-595 (36.7-41.8)
Maximum Allowable Pressure Drop After 5 Minutes	73 (5.1)
Upper Switching Pressure	754-827 (53.0-58.1)