Pontiac FIREBIRD 1972 Service Manual page 87

1A-28 1972 PONTIAC SERVICE MANUAL
1 ST STAGE EXISTING CONDITIONS
System 1s off, pressure equal on both inlet and
outlet & presaure
is
approx. 70 psi (normal
day of 70
-
800 F)
I
2
Vacuum bellows
-
contracted due pressure
'
I
@
Needle Valve
-
open
O
ktng
over 28.5 psi
2ND STAGE EXISTING CONDITIONS
-
PISTON OPENS
-
System is on, compressor is pulling down pressure,
therefore outlet side (compressor side) has lower
pressure than inlet side.
Vacuum Bellows is contracted because outlet pressure
is still over 28. 5 psi.
3
Piston
-
IS
open because pressure on bottom of
1
piston (iiilet pressure) is n o : greater than
pressure on top of piston (outlet pressure).
3 RD STAGE EXISTING CONDITIONS
-
BELLOWS CLOSES
-
Compressor has pulled outlet pressure down
to 28. 5 PSI therefore:
@
Vacuum Bellows
-
expands pushing needle
valve closed, causing
@ P r e s s u r e on top of Piston
to begin
Fig. 1A-22 P.O.A. Valve
-
Basic Operation