QUESTION:
On a hydraulic system, if there was any problem with one fitting or line
would that
mean the breaks on all four wheels (dual-axel trailer) would stop working?


If the problem with the fitting allows all the fluid to be bled out of the
master cylinder reservoir, then yes. All the wheel cylinders get their
fluid supply from the master cylinder reservoir. The braking systems on
each wheel are not isolated from each other. I've had exactly that happen.
Had a small leak in a brake line connection somewhere on the trailer and
didn't know about it. Eventually, the master cylinder reservoir was bled
essentially dry. At that point, no trailer brakes. Makes for an
interesting discovery on a 14K+ pound boat and trailer rig. Found the
leaking connection, tightened it, refilled the reservoir, and was good to go
again. System self bled the air in the lines during the next several
braking attempts.