Before TS gets in here. I found several sources of information. This one looks the most useful
COMMON WATER PUMP FAILURES & THEIR CAUSES | FlowKooler Water Pumps - Chevy, Dodge, Jeep, Ford, & more
Exerpt here for your consideration:
1. LEAKING FROM THE WEEP HOLE: To understand leaking from a weep hole let's begin with some common questions and answers.
What is the purpose of a weep hole?
The two holes you see in the water pump casting are called weep holes. The upper weep hole acts as an air vent. It allows air to evacuate the casting system and prevents the build up of humidity around the bearing. Also the vent allows atmospheric pressure into the pump and the seal remains seated.
The lower weep hole exists to allow fluid to collect or drain out of the water pump to protect the bearing integrity. In a horizontal centrifugal pump there is little space between the bearing and the seal so fluid build-up could potentially threaten the bearing. Weep holes permit this coolant to evacuate the system and protect the bearing. Weep holes also allow atmospheric pressure into the pumps and help the seals to remain seated.
Does a drip from the weep hole mean seal or pump failure?
Perhaps. Seals will drip as the seal lap in but to understand a more prolonged or intermittent drip you have to understand a seals design. Seals purposefully drip because capillary action draws fluid from the seal face. As the seal drips, it helps wash loose debris out and reduce the heat load. Most drips evaporate or fall on the road and are not noticed.
Naturally, a more pronounced drip indicates a compromised seal and impending bearing failure.
What causes a seal to prematurely fail?
Mechanical seal have spring-loaded assemblies to keep the carbon seals intact and rubber parts that may disintegrate if the engine runs hot or overheats. If a system overheats to the point of boiling out and the system is permitted to run dry, the polished sealing faces can wear and warp. A worn seal face allows fluids to escape and leak out the weep hole. Most pumps will leak catastrophically shortly after a boil-over.
Electrolysis may cause filming and/or crystallization on the seal face and cause the seals to permit fluid into the weep chamber. High mileage vehicle tend to have a greater incidence of seal failure due to pH imbalance that compromises the seal face causing fluids to leak out the weep hole.
Corrosive inhibitors are made up of silicates which plate metal surfaces. The degree of plating that actually occurs varies but as it does, the silicate levels deteriorate over time and the coolant becomes more corrosive. As the corrosion inhibitors deteriorate and the pH of the coolant drops to 7 or below, the result is electrolysis and plating. For this reason most mechanics and recommend a flush & fill at 24 months/30,000 miles. Unfortunately, few car owners flush and fill and radiators and water pump seals become compromised.
It seems every few months a new chemical additive for the cooling system comes on the market. Seal manufacturers find concentrations of gel from these additives and/or filming deposits build on the seal face or they find carbon rip out on the ring. These all contribute to premature seal failure.
If there are abrasive particles present in the cooling fluid can affect the wear resistance of the seal. Sand is the most common.
2. BROKEN WATER PUMP HOUSINGS AND SHAFTS COMMONLY CAUSED BY: Broken shafts are result of excessive vibration & unbalance and principally due to:
Bent, cracked, or broken fans
Fan not squarely mounted on the shafts
Cracked or bent pulleys due to improper handling or installation
Overtightened belts cause overload on the bearing and impose a powerful bending force on the shaft causing it to deflect substantially from true center rotation resulting in imbalance and early shaft fracture