Steer brake pulling in rain.

2017 Cascadia. Truck brakes normally in dry weather. In rain it pulls to the left along with the steering wheel, no abs issues. Recently done a brake job - new shoes all around, adjusted, greased, psi adjusted as well. Mechanic says it could be slack adjuster.
My concern is that if it was a slack adjuster than It would pull in dry conditions as well, but it doesn't. Could be the drum, but it looks ok to my judgement.

Does anyone have an idea what it could be?

(I also have watered the drums intentionally at truck wash and it was pulling to the left for a while until it dried out)

 

The shoes are new all over, barely drove them (less than 5000 miles), didn't change the drums though.

 

I see what you are saying, braking performance overall is good, and I am getting issues only in wet conditions (heavy rain, wet drums\shoes) . Why then it's braking straight when it's dry.... Feels like it has oil in the drum in wet.

 

It is not the hub oil 100% , checked it all over, took it to mechanic to look at it as well. The drum is dry and truck brakes normally when it is dry outside. As soon as water gets on the drum - truck starts pulling when brakes applied.

 

In other words - it could be the drum imperfection preventing the water escape and causing lack of braking? It was just fine with other shoes I had. I could take a picture of the surface of the drum if that helps and post it here.