What would cause my front hub to leak?

You can replace the glass separately with a window kit, if you find The window is dryrotted. Some of Those front caps are $30 jobber price, a $8 window kit sounds better to me.

 

Or hit it directly with water from a fire hose.....

 

Looks like everyone has chimed in with my responses. I would bet it was the plug, if thar
t doesn't work check the bearings and make sure not to overfill.

As far as the window kit I have had better luck replacing the cap they very in price depending on where and what type you get but I buy stem Co brand and spend about $12 each from fleet pride online truck tractor parts ebay store. I us ally have a couple kicking around since I ended up spending like $45 one time I was in a bind on one of my trucks

 

If you've added oil it could be too full, remember it expands when warm so it could be leaking from expansion.

 

If the plug doesn't work I would just replace the entire cap. The are more likely to leak around the base of the cap than around the window, unless the window itself is broken. When you buy the cap it has a new window, (already installed) gasket and plug for under $20. Make sure you are not overfilling and torque it in right sequence and correct inch pounds. If not you can crack new cap. Also make sure base is clean when putting new gasket on. Very cheap and easy fix.

 

I have had the caps leak from new. had to tighten the screws around the cap. I think it is a #10 hex bit. never had this prob in the past q.c. is not what it used to be.