the high pressure portion of the compressed air generating system is supplied via the inputs of relay valves 7 and 9 thus the high pressure supply connection 7b of the first relay valve 7 is connected via a line 11 to the high pressure storage tank iii and the counterpart connection 9b of the second relay valve 9 is connected to the high pressure storage tank iv via a conduit or line 12 the valves 7 and 9 include the respective output brake connections 7c and 9c which form the respective fluid connections for the brake cylinder 13 of the rear axle and the brake cylinder 14 of the front axle respectively the control portion of the auxiliary locking brake system extends from the three stage protective valve 3 which is associated with the low pressure circuit of the compressed air generating system that is a fluid path extends from the output of valve 3 through a line 15 a manual brake valve 16 and a line 17 to the low pressure control connection 18a of a third relay valve 18 a high pressure supply connection 18b of the third relay valve 18 is connected to the three stage protective valve 4 which is associated with the high pressure portion of the compressed air generating system via a line 19 the high pressure output brake connection 18c.