Brake vacuum modulator traction control with pressure source variable as function of engine load during incipient wheel spin conditions

A vehicle braking system equipped with an adaptive braking system with traction control to control drive wheel slip during vehicle acceleration includes a conventional vacuum brake booster having a vacuum chamber normally connected to engine manifold vacuum and an atmospheric chamber connected to the vacuum chamber when the brakes are released but which is connected to atmosphere when a brake application is effected. The traction control system includes a control unit which actuates a solenoid valve under wheel slip conditions during vehicle acceleration to cut off communication between the vacuum chamber and engine manifold vacuum. The traction control system also cuts off communication between the vacuum chamber and the atmospheric chamber and initiates communication of the atmospheric chamber directly to engine manifold vacuum, thereby effecting a brake application proportional to engine manifold vacuum, which varies as a function of engine power.