Full power hydraulic assistance device

An hydraulic assistance device comprising a housing provided with a stepped bore inside which piston means is mounted slideably, the piston means displaceable by means of a brake pedal between a first position communicating a first chamber, connected with a brake circuit, with a low-pressure reservoir and isolating the first chamber from a high-pressure fluid source, and a second position isolating the first chamber from the low-pressure reservoir and communicating the first chamber with the high-pressure fluid source, the piston means displaceable by the brake pedal in the event of failure of the high-pressure fluid source to produce a pressure increase in a second chamber connected with the brake circuit, characterized in that the piston means comprises a bush in which is mounted slideably an actuating piston, and high-pressure valve means, the actuating piston extending from the bush and into a reduced diameter section of the stepped bore, the actuating piston displaceable by means of the brake pedal so as to open the high-pressure valve means, the actuating piston and bush disposed within the second chamber which comprises an enlarged diameter section of the stepped bore so that in the event of the failure of the high-pressure fluid source a seal disposed about the actuating piston is displaced therewith to engage sealingly a substantially radial surface of the bush which is displaced by the actuating piston to displace a greater volume of fluid from the second chamber to the brake circuit than would be displaced through utilization of the reduced diameter section of the stepped bore, the booster further comprising a push rod connected with the brake pedal, a first spring disposed between the push rod and actuating piston and a second spring disposed between the actuating piston and bush, the seal disposed radially inwardly of the second spring.