Chevrolet 70 1969 Series, Руководство по ремонту

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POWER BRAKE 5-46
Figure 2 — In ta k e o f Air
and all the procedures are applicable to all models. The
difference in m aterial com position between models has no
effect on the m ethods of service or repair.
See “ Specifications” at end of this m anual for differen­
ces in tolerances and dimensions.
Air com pressor crankshaft turns continuously while en­
gine is running. A ctual com pression of air, however, is
controlled by the governor. Acting with com pressor un­
loader m echanism , the governor loads or unloads com ­
pressor when pressure in air system reaches a preset m a x ­
im um or m inim um .
OPERATION WITH UNLOADER VALVE
CLOSED (COMPRESSING) (Figs. 2 and 3)
During the downstroke, a partial vacuum is created
above each piston. Intake air forces open the inlet valve
and air fills cylinder.
As piston starts upstroke, the air pressure ’ on top of
inlet valve, plus inlet valve return spring force, closes the
inlet valve. As air above piston is further compressed,
pressure lifts discharge valve and com pressed air is forced
through discharge line into reservoir. A t start of dow n­
stroke, discharge valve returns to seat, blocking return
flow of com pressed air to cylinder as cycle is repeated.
OPERATION WITH UNLOADER VALVES
OPEN (NOT COMPRESSING) (Fig. 4)
W hen air in system reaches m axim um pressure for
which governor is set, air passes through governor into
unloader cavity below unloader pistons in com pressor cyl­
inder block. U pw ard m ovem ent of unloader pistons
caused by air pressure lifts both air inlet valves off inlet
valve seats. W ith both inlet valves unseated, the air intake
cavity in the cylinder block forms a passage between the
cylinders above the pistons. U pstroke of one piston ex­
Figure 3 — C o m p re ssio n o f A ir
hausts air into cylinder of other piston on downstroke,
without compression.
W hen pressure in air system is reduced to governor
cut-in setting, the governor releases pressure from beneath
unloader pistons. Pressure of unloader spring on unloader
spring saddle, acting against reduced governor pressure,
forces pistons away from inlet valves. As inlet valve
springs, in turn, overcom e reduced plunger pressure, the
inlet valves reseat and com pression is resum ed.
DISASSEMBLY OF CO M PRESSOR
The crankcase, crankcase bottom cover, cylinder block,
Figure 4 — U n lo a d in g C om p re ssor
CHEVROLET SERIES 70-80 HEAVY DUTY TRUCK SH O P M A N U A L