From experience : When these mechanical seals fail ..... you'll know it ! However: Having had my '80 since new and having replaced 3 seals to date I can state this: From time to time mine will weep very slightly but generally only for a very short period. Maybe for a minute when warming up the engine and not necessarily after a winter of not running. No rhyme or reason. It's rare but it happens and it always stops.
If the coolant is pooling on the floor - The seal is likely "toast". If it's merely a minor weep; "Just" wetting the crank-case ledge, leave it to see if it stops. There's a good chance it will. With this type of seal I believe clean coolant is important critical. Whenever I've had to do work that involves draining the radiator, I ALWAYS filter the coolant if I'm reusing it. There is always a couple of very small bits of "something" in the filter. I'll add that I consider my cooling system to be otherwise "squeaky clean" !
To avoid writing a manuscript, I'll leave it at this.
1980 KZ 1300 sr# KZT30A-009997
Always High - Know Fear !
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I have a slight occasional weep from mine. I'm toying with the idea of using waterless coolant as it runs at 75% or less usual system pressure. I had noticed the seal weeps just as the system starts to pressurise on mine. The fact that waterless coolant is a 'lifetime coolant' appeals too, as opposed to replacing the traditional mono-ethylene glycol / water mix coolant every 2 years as recommended.