This is the most common failure of the glycol pump. After years of service running 24/7 @ 1725 rpm, most pumps will experience a rear seal failure. When this seal fails, the pump will leak glycol very slowly into the front bearing area of the motor. Eventually, this will cause the motor to fail, among other issues.
Even if you catch the pump failure prior to the motor failing, it is recommended that you replace the motor as well. It is a certainty the motor will fail shortly after the pump, these motors have a very low tolerance for moisture exposure to the bearings. Experience has shown us that pumps & motors should be done in pairs.
Take the example of a motor failure but the pump still moves fluid and is not leaking; because the pump has already “worn in”, introducing a new motor with optimum torque and bearing position will cause the pumps vane wheel to deflect to the point it “chatters” on the inner casing. Because the “wheel” is made of graphite, as it wears on the casing it will flood the system with the abrasive graphite particles. If this should happen, it will require a complete glycol replacement, cleaning of the unit bath and flushing of the circ lines. If you have graphite infiltration to the fluid and this procedure is not performed you will continually have premature failure of the pump due to the abrasive particles running through the new pump. The graphite particles will also tend to be deposited in quick bends in the glycol circuit like, u-bends, elbows and tower loops. This restriction adds to the overall operating back-pressure the new pump has to overcome. Pumps running with excessive back-pressure or on high restriction circuits will have a lower life expectancy.
Knowing how things work will save you time and money, besides its just fun to know stuff!