a deceleration sensing valve is provided which includes a valve body adapted to move under its own inertia as the deceleration of a road vehicle exceeds a given value , and a valve seat on which the valve body seats . the valve seat is located rearwardly of the valve body as viewed in the direction of movement thereof under its inertia , whereby there can occur no change in a control pressure derived by the deceleration sensing valve , in particular when the vehicle is empty , as it does occur in the prior art in response to any slight shift in the timing when the valve body becomes seated . consequently , when used in combination with a proportion valve , the deceleration sensing valve of the invention assures a stable braking characteristic when the vehicle is empty as well as when the vehicle is occupied .