I parked my bike up in November (ish) for the winter. While I had her off the road I replaced the mechanical water pump seal as it had started dripping from the telltale hole in the crankcase. I ordered all the bits from Oz at Zed Parts. As well as the actual seal I bought the shaft o rings and the pump housing o ring and a new circlip. I reassembled everything and sealed it all up with RTV. I refilled the system with 50/50 pure antifreeze and distilled water and then covered her up. I decided to get her ready for the road today and the bloody thing is still dripping when the motor is running? Now I’m certain I’ve done everything correctly so why do you think it’s leaking? As soon as the salt disappears off the roads I’ll take her for a short run as someone has suggested that they do need to “run in” Despite this being my first bike mechanical seal I’ve done dozens of mechanical seals on big pumps when I was working and they have always sealed immediately. 

Mechanical seals don't need to be "Lapped in" so no running up of the engine would fix a newly installed seal. The whole theory of the mechanical seal is that the seal faces are lapped to less than 2 light bands and there's nothing smaller than 2 light bands that will cross the faces of the seal.
Without knowing how you handled the seal components or the method of installation i.e. lubrication used and where, it's hard to help here. There are several areas of the mechanical seal that need to work properly to achieve a seal.
The stellite face has to seal to the housing so the leak could be across that sealing face. If the housing was not cleaned back to as new, that might be the problem.
The Stellite face when removed from the packaging as a rule of thumb, needs to be cleaned with either methyl hydrate or lacquer thinner and when ready for installation the face of the seal should be a mirror finish and I truely mean a mirror finish. You should not see any stains, on the face what so ever. When handling the stellite face, you should be wearing nitrile gloves. the acid in your skin can damage the face of the seal and what may have happened here, is that you installed the seal and didn't run up the engine and there was no flushing of the seal with the antifreeze and maybe, just maybe the acid from your fingers had a chance to etch the stellite over the past few months.
The carbon seal face needs to be inspected before installing for cracks  or chips on the perimeter of the seal. There's also a seal surface between the carbon face and the seal drive shaft that the o ring rides on. Lots of reports of a damaged drive shaft at that o ring seal area. O-rings needs around 15% compression to be able to do their job so knowing that, and knowing that the seal drive shafts have needed replacement in the past, I would measure the shaft and ensure that 15% compression is there.
I always ensure that when installing o-rings that I use a silicone based o-ring lube or if that's not handy at the time, use the fluid of the system being sealed as a lubricant. In this case, I would use the antifreeze. Both the stellite seal face and the carbon face seals have to be rolled into position so as the seal is installed, the o-ring has to squirm into position instead of being in position and then compressed. The o ring has to distort as it's being compressed/positioned and if the o  ring is dry when installed, it will not relax and orient itself so that the entire seal is sealing properly.
At this point, I hate to say it, but you really should disassemble the seal and inspect it and particularly the shaft in the area of the carbon face o-ring. 
Let know how you make out. Here to help.
KB
 

I've replaced the water-pump seal "numerous "times over the 43 years.  Some seal right from the get-go; one  never did and ironicly,  I reused the current one after the top-end rebuild and it never showed a "drop" !.  I've also had a weep start after the fact - left it alone and the weeping quit within a tank-full or two (?)  No apparent rhyme or reason.  
Kawboys' correct !  "Cleanliness" is not an option - it's mandatory, for your best chance of a leak free install.  For myself that also includes changing the coolant every 2 years.
Between those changes:   Whenever I need to drain the system for maintenance I always use a (automotive-paint) filter when refilling the rad.  It's  sometimes surprising to see what a filter will remove. 

Thanks for the replies, when I replaced the seal I was very careful not to touch the mating surfaces and I give them a quick wipe over with a clean lint free cloth sprayed with brake cleaner and I then dipped them in neat antifreeze before fitting them. I used rubber grease on the shaft “O” rings and I sealed the rear (fixed) seal with RTV. I refilled the system with fresh neat antifreeze and fresh distilled water mixed to 50/50. I didn’t filter the solution though! I’ll give the bike a short run as soon as I get a dry salt free day and report back. 

Filtering the coolant is just something I do.   The few times I've had to dump the engine oil for work requiring an empty  oil-pan, I filter the oil too - if I'm reusing it. 
 There's usually little specks of something, in the filters. 
Why put "anything" back into the system?  

Watching this thread.... on my ToDo list.

Has anyone used the mechanical seal out of Germany?