I usually replace the draw hose those crack and lose draw, the metal housing for the spring and ball bearing can come loose, that will cause you to loose draw.
Those two things are mostly my problems.
I have a hydro force from Cobbs that has lasted well over 3 years.
I've had to replace the hose to the gun once.
And my gun leaks doah!
So I use the ball valve alot.
I hate the square handled guns on some sprayers.
To much hand fatigue.
I think those are the ones
Saigers uses.
They don't last any longer either.