Safety device for an oil hydraulic braking system

Claim 1

A safety device for an oil hydraulic brake system comprising a housing containing a first bore and a second bore, first, second and third ports opening into said first bore, said first, second and third ports being connected to a front master clinder, and said second and third ports communicating with said second bore through passages formed in said housing, a rear wheel cylinder and a rear master cylinder, respectively, a piston slidably disposed in said first bore, said piston having a first effective pressure-receiving surface of a first area communicating with said first port, said first surface being adapted to receive a first force exerted by the oil hydraulic pressure of the front master cylinder in a first direction, a second effective pressure-receiving surface having a second area larger than said first area and communicating with said second port, said second surface being adapted to receive a second force exerted by the oil hydraulic pressure of the rear wheel cylinder in a second direction opposite to said first direction, a land portion adapted to provide communication between said second port and said third port when said piston is displaced in said second direction, a cam portion adapted to actuate an electrical contact when said piston has been displaced in either direction, a spring adapted to exert a force to said piston in said first direction within a limited stroke, said force being not less than the balance between said second and first forces, and a proportioning means disposed in said second bore and communicating with the rear master cylinder and the rear wheel cylinder through the passages formed in said housing and said second and third ports, said proportioning means adapted to transmit hydraulic pressure of the rear master cylinder to the rear wheel cylinder with a reduced increasing rate when the hydraulic pressure of the rear master cylinder is above a predetermined level.