Its hard to know what to tell you because your not clear in your
descriptions of your problems.

IE you knew the master was bad because you had no power.  Does that mean the
truck wouldn't stop worth a darn with a low pedal possibly that fades,  or
the pedal was hard as a rock?
Hard as a rock is not a master problem but a power booster issue.  And it
doesn't necessarily mean the booster is bad, it could be a bad hose, low
vacuum, or the check valve at the booster is bad.

The "smell" coming from one wheel tends to say ether the caliper hung up,
the brake hose went to pot, or a wheel bearing is goosed.  A bad wheel
bearing will let the wheel and rotor lean over and the brakes will be
supporting the vehicle, they get hot and stink, hot enough the fluid boils,
and regardless the caliper seals get cooked. You will also have a low pedal
on intial application as the rotor leaning ovwer pushes the piston all the
way back in the caliper bore.   Then again the caliper slide could have hung
up, or the caliper piston could be stuck which will also over heat the
brakes to the point it could boil the brake fluid which will result in the
no pedal scenario you described.  A bad brake hose can do the same thing.
They go bad and peal on the inside, this blocks the hose like a heart valve,
fluid can flow in but it cant get out.  Brakes over heat, fluid boils.
Caliper gets cooked as well, and always replace hoses in pairs from the same
manufacturer. There is a plus and minus spec on inner hose diameter, one
brand may be to the plus, the other to the minus, and this can cause a pull
on intial brake application. Not a real issue on dry roads, but a panick
stop on slippery surface could cause a skid.   The over heating can also
result in piston seal failure which will also result in no brake pedal, but
the  front brake resovoir of the master will be empty, thats the larger one.
The smaller one is for the rear brakes,  .
I'll get greif here, but replace calipers in pairs, and replace the caliper
hard ware. Myself I go with  loaded calipers, they come with everything.
Raybesto's has some really good ones, NAPA's arent bad ether.  Autozones
Durastop suck.
Another thing to keep in mind, Fords have two calipers that are the same
except for piston material.  The standard has the "plastic" pistons, the
"police" have steel pistons.  The police style cause less issues than the
standard and cost about the same.  There is a difference in the pads, the
clip on the inner pad is different between the two styles of calipers, other
wise they are the same.  And under no circumstances mix the two styles on
the same axle unless you like going for wild rides, and love brakes that
pull weird when they get hot.

If you think the master is leaking unbolt it from the booster and pull it
foreword, do you see brake fluid, or is there a trail down the booster from
the bottom of the master?

On replacing the booster, not a hard job its a nut and bolt thing, one
vacuum line, remove the master, under the dash four bolts hold the booster
to the fire wall.

Whitelightning

Please post a pic of your truck, so I can get off the road if I see
you comming.
You say the axles were just replaced and the rear brakes done? What
did the mechanic have to say about the rest of the brake system?
You really should have the brakes done by someone with experience.
Brake pedal hard? Not an automatic booster problem, could be oil from
leaking axle seal had saturated the rear shoes.
To test booster; with engine off, apply brake pedal 4 or 5 times then
release pedal. Now apply pedal, if needed (pedal goes to floor), pump
pedal till some resistance (braking force) is felt and continue to
hold pedal down. With pedal still held down, start the engine. If
booster is working, the pedal will drop some.
Loose front wheel bearings will cause low pedal, to test, raise wheel
off ground so it is free to spin, grab wheel at top and bottom and see
if you can rock it sideways. A c-hair of play is fine, more than that
may require an adjustment.
Another source of low pedal is rear shoes not adjusted correctly.
It may be that the passenger side brake got hot if it was the only
brake working.