Nope. The rear seal is leaking. Since this is a newly-rebuilt master cylinder, the seal got torn by improper or careless installation procedures, there is a scratch on the metal push rod that got overlooked, or a little impacted debris got missed and cut the seal. Any reputable auto parts store will exchange it for another one. This does not happen very often, so I would not be worried about getting a replacement from the same supplier.
Be aware too that rubber seals are lubricated with Brake Assembly Lube to make installation easier. That is essentially real thick brake fluid. If you are seeing residue of that, you do not have to worry about it. The clue that the rear seal is leaking is the fluid level will keep on going lower in the reservoir.
Thursday, December 6th, 2018 AT 4:12 PM