the invention relates to an assembly made of a compensating tank and a master cylinder for a hydraulic motor vehicle brake system , wherein the compensating tank has two fastening attachments , each with an opening , wherein openings align with an opening in the master cylinder , and the compensating tank is fastened by a detachable connection by a fastening pin that extends through the aligned openings . in order to improve the installation and possible dismantling of the fastening pin , and at the same time to maintain an optimized assembly weight , the invention proposes that the fastening pin be made of plastic and interact in a positive - or force - locking manner with at least one of the openings of the fastening attachments .