Coulomb Friction is a constant friction. As it pertains specifically to the aerospace field, the amount of coulomb friction will not change as the velocity changes. According to Design Aerospace, "coulomb friction represents friction associated with mechanical surfaces rubbing together and includes bearing friction, friction in flight control surface hinges." Like the human body, there is always some friction being experienced in any mechanical hydraulic system. Coulomb friction is the hardest type of friction to avoid, but since its effect doesn't change with the amount of velocity, it also might show the least effect over time.
Viscous friction deals specifically with the hydraulic fluid flowing through the system. Viscous friction accumulates as the fluids of the hydraulic system are pushed through the various passages and hinges in the hydraulics. The amount of viscous friction is directly related to the amount of velocity being put upon the hydraulics. The more velocity; the more friction and vice versa. Thus, viscous friction is not a factor when there is zero velocity.
According to Design Aerospace, "seal friction is the friction of the seals against the housing or pistons." Seal friction is also known as Stribeck friction. Unlike viscous friction, seal friction is high at a low velocity and low at a high velocity. Again, like the human body, the seals are like our joints that grind together as a person slowly moves through life.
