For the oil path from which oil is supplied to the vacuum pump, which supplies negative pressure to the vacuum brake booster, the union bolt is not suitable for the purpose, and therefore, a sufficient amount of oil may not be supplied to the vacuum pump. If this problem is left unattended, the vane inside the vacuum pump may wear, and the pump may break, in which case no negative pressure would be generated, and in the worst case, this could lead to a longer braking distance.