PS Magazine - TB 43-PS-572
PS, The Preventative Maintenance Monthly
ISSUE 572
JULY 2000
PS 572
7
JUL 00
PS 572
6
JUL 00
M172A1 Semitrailer . . .
Lube Brake Camshafts
O
perators, grease fittings on the M172A1 lowbed
semitrailer’s brake camshaft rods get skipped at lube
time because they’re hard to find.
But without lube, the camshaft rods seize up. The
rods have to slide and rotate inside their nylon bush-
ings and bearings. If they can’t, the brakes lock up.
That can cause an accident.
Keep the brake camshaft rods lubed. The fittings are
behind the inner tires of each axle.
M939-Series Trucks . . .
Stencil numbers near valves
You can stop that show-stopper by
draining the air tanks at the end of
each day. Here’s the right way to drain
M939 air lines:
Always drain all four tanks.
There’s more water in the wet tank for
sure, but all tanks hold moisture.
Drain tanks in the order called for
on Page 2-67 of TM 9-2320-272-10
(Aug 96).
To make the draining order easy to
remember, get your mechanic to sten-
cil sequence numbers on the truck near
the valves. Use CARC paint, NSN
and wait for air pressure to build. The
primary gauge maxes out, but the sec-
ondary gauge won’t budge or only
gives you 10 psi or so.
What’s up? Chances are that mois-
ture has corroded the spring inside the
secondary relay valve so it won’t let
pressure build to rear brakes. You’re
stuck until the relay valve is replaced.
8010-01-229-7540, and the 1-in sten-
cils from the No. 1 Common shop set.
Open each petcock, one at a time,
until the air pressure is gone.
Close all the petcocks after you’ve
drained the tanks. Leaving them open
doesn’t get rid of water—it creates
more. That’s because cool nights and
warm days build condensation inside
the tanks. When you close the tanks
just before you operate, the water is
trapped inside, ready to clog air lines
and jam relay valves.
Check secondary relay valve for corrosion
ou go to your M939-series truck to
prepare for a mission. You crank it up
hmmm, i've
got
primary
pressure,
but the
secondary
gauge
hasn't moved
.
what's up?
corrosion
has
jammed up
the
spring
on my secondary relay
valve.
you
never drained
my air tanks!
Give fitting 4 to 5
shots of grease
...secondary
not OK?
Primary
gauge
OK but.
..
PS 572
6
JUL 00
3
4
1
2
DRAIN
AIR
TANKS
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