Since I have been driving, 1969, I have bought a reman just one time. All the others, I re-manufactured myself. There is nothing to it, and none of the ones I ever did leaked, unless the lid was not clamped down adequately, which I adjusted with a pliers.
I have one still in use since 1994.
Another from 2004. But this one is running silicon which is really hard to keep from sweating. Since the silicon does not attack paint, I just wash it down periodically.
To be fair, I have always run a booster, so the sealed end sits inside the booster, and water cannot get to it, so it does not rust there.
That rear seal is not supposed to be under pressure and should never leak brakefluid. Air pressure outside is always higher than what the seal should be seeing on the other side.
Check out this image; if you are getting brake fluid out the back, it is most likely because your piston is over stroking and the Compensating port is dumping back there. If you seem to have a long pedal stroke then I would bet money on it. You might need to put the pedal back where it belongs.
Check out the supplied image.
In this drawing the C-ports are the same as the inlet port.
Imagine the rear piston stroking forward until the inlet port dumps behind it.This would be bad and AFAIK, can only happen if the rear wheel cylinders are over-traveling, which is not supposed to happen, as there are limiter stops in there to prevent that. So then, it would fall on the front system over-traveling. With disc front brakes this is not likely to happen except in the case of air in the system, or catastrophic failure of a brake hose. When this happens the rear piston moves waay forward until the pin on the front of it, (you can just make it out inside the spring, in the provided image) contacts the back of the front piston. You can clearly see that if this was to happen, the C-port would dump fluid behind the front seal of the rear piston. That fluid now collects in that cavity, and is pushed back and forth on every brake application. There is no seal on the back of that piston, so it is bound to find it's way, sooner or later, past the rear guide, and then the only thing keeping it there is the dust seal at the back. Since the M/C is set on an an angle and low at the back, eventually the fluid will find it's way out.
Now; I'm not saying this is your problem, cuz there are other more common reasons for leakage at the back. But I am saying that if your pedal is simultaneously low, that it could be, especially if you say that every M/C that you have installed, has done it.
Your pedal should be high and hard, no exceptions. And if you have a booster, then on the first start-up of the day, your pedal should drop about an inch or a lil more, under your foot, as the vacuum comes into it.
