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The tires shall be free of major defects when examined in accordance.
with MIL-STD-1224. The inner tubes shall be in accordance with
MS35392-26. Valve caps shall be of metal and shall be equipped with
elastomer inner seals. Balancing.  Brake drums, hubs, and tire and wheel assemblies
shall be inherently in balance to preclude excessive tire wear and elimi-
nate wheel tramp and drumming at the road speeds specified herein.
3.13.4 Chock blocks.  Two wood or commercial metal chock blocks with
captive chains and two wire cradles shall be furnished. The captive
chain shall be wound in a figure-eight manner around the cradle ears
and the hook snapped to the lower cradle loop.  The length of the captive
chain shall permit the chock blocks to be placed ahead of or behind the
semitrailer tires.  The chock blocks shall be of a minimum size as shown
in figure 1 and shall withstand the imposed forces of being run over by
the semitrailer.  The cradles shall conform to figure 2. Installation
of the cradles shall be with the two cradle ears uppermost and the short
loops fastened to an approximately vertical, flat surface.
3.13.5 Brake system.  The screening unit shall be equipped with a water-
proof, straight airbrake system in accordance with ICC Motor Carrier Safety
Regulations, sections 193.40 through 193.43 and 193.45 through 193.50, ex-
cept as otherwise specified herein.  The brake system shall include ser-
vice and emergency couplings (front and rear), relay emergency valve,
air reservoir, cylinders or chambers, foundation brake assemblies on each
wheel, and all other components and plumbing required for a complete,
matched airbrake eystem.  All brake system components and plumbing shall
be located, installed, and protected to prevent damage or malfunction
from water, ground obstructions and debris when transported, from falling
aggregate when operating, and to obtain maximum efficiency and ease of
maintenance.  Service and emergency couplings shall be furnished and shall
conform to MS35746, except that they shall be of the polarized or non-
interchangeable type, color coded blue for service and red for emergency.
A spring-type, metal dust cover shall be installed on each coupling to
prevent contaminants from entering the opening. The dust cover shall
not cause interference when another coupling with a dust cover is con-
nected.  In addition, metal identification tags conforming to NS53007
shall be installed near each front and rear fixed coupling. The relay
emergency valve shall conform to MS53004, except that the rapid bleed-
down vent shall not be furnished with these straight airbrakes. The
air reservoir shall conform to SAE J10 and shall have an air storage
capacity of not less than two times the recommended air volume for truck-
trailers in SAE J813.  The reservoir shall be equipped with a remote

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