Abstract: A hydraulic brake system for automotive vehicles provided with a hydraulic power booster (1) and with a master cylinder (10) operable by the power booster (1). The booster piston (2) is designed as stepped piston with a pedal-close portion (5) of smaller diameter and with a pedal-remote portion (6) of larger diameter. Together with a housing (11), the annular surface (12) of the booster piston (2) at the piston step encloses an annular space (13) of variable volume. The annular space (13) communicates with an unpressurized supply reservoir (22), which communication may be locked, and wherein in a pedal-remote pocket bore (7) of the booster piston (2) a master cylinder piston (8) confining a working chamber (17) is guided in a sealed manner. An annular chamber (14) confined by the pedal-remote annular surface of the booster piston (2) is pressurizable by the pressure of an auxiliary pressure source.

Abstract: Hydraulic fluids the primary properties of which comply with the official specifications and, moreover, which have a good lubricating effect, a high oxidation stability and a high acid stability consist of about 10 to 60% by weight of a nitrogen-containing boric acid ester, about 5 to 30% by weight of an alkyl-polyethylene glycol tert. butyl ether and about 35 to 75% by weight of a glycol monoalkyl ether. The nitrogen-containing boric acid ester is a reaction product of an alkoxylated monoalkyl amine, orthoboric acid and optionally a glycol.