My Chevrolet Astro van consistently pulls to the left when braking. Consistently means: always, the amount is proportional to the pressure applied to the pedal. It doesn't pull to the side if braking with the emergency breaks, which only applies the rear drum brakes.
Background: I am an overlander currently travelling through Chile, South America. This includes a lot of bumpy, unpaved roads and highways.
The last brake service for the front brakes was performed 23000km/5 months ago. Only 3000km/4 weeks ago, the rear drum were replaced, and I verified that there is no pull at all.
About 1000km/2 weeks ago I noticed the pull and saw a brake specialist. He adjusted a loose castle nut on the front right wheel, but the pull persisted. He told me to drive on anyway (he thought it wasn't that bad) and told me that the pull would most likely go away over the next 1000km. Otherwise, he could only pull apart everything, but he advised against this, so I went on. It didn't change (not worse, not better).
Today I saw another mechanic. He tells me that basically all he can do is perform a brake service on the front and/or rear wheels (both ~30$), put the car back on the road and see if there's still a pull. He too told me that he'd probably just drive on, without me having demonstrated the problem.
To me as an engineer without much knowledge of cars, this service seems like random fiddling rather than a diagnosis approach. So, finally here's my question:
What are the chances that an ordinary brake service fixes a pull as described?
Should I do a brake service or continue driving? I don't mind spending money on maintenance, but not without reason...
PS: During normal city traffic, I have to steer right about 15° to correct for the pull. On a highway going 100km/h, braking without correction would bring me to a halt about 2 or 3 lanes to the left (I didn't try!)
Update: The mechanic pulled all 4 wheels. Calipers and pistons all work properly, nothing unusual and no leaks of breaking fluid nowhere. With these out of the question, where should I look for?