Vacuum Check Valve - GMC 4000 Series Service Manual

On all vehicles with vacuum power assisted
hydraulic brakes a check valve is used somewhere
in the vacuum line between the engine intake mani­
fold (gasoline) or the vacuum pump (Diesel) and
the vacuum power cylinder (see fig. 23).
Purpose of check valve is to seal vacuum in
power cylinder (and in vacuum reserve tank, when
used), assuring sufficient vacuum for at least one
power brake application in case the engine stalls.
Check valve can be tested for leakage by d is­
connecting power cylinder vacuum line from valve
fitting and connecting a vacuum gauge, using a
length of hose between gauge and check valve. Start
engine, run at idle for a few seconds, and note
reading on gauge. Stop engine and observe rate of
vacuum drop. If drop exceeds 1 inch in 15 seconds,
leakage must be considered excessive, and check
valve must be repaired or replaced.
There are three basically different type vac­
uum check valves used. One is repairable and two
are not. The repairable type has a cast metal body
and is used on Tilt Cab and P4500 Model trucks.
The non-repairable types have a sealed plastic
body and are used on all other models.
Check valves (plastic, non-repairable type)
used with dash mounted boosters are integral with
vacuum chamber assem bly. Check valves used on
"S" Models and some conventional Models are
mounted separate from booster cylinders.
TO EN G IN E OR
V A C U U M PUMP
CHECK
VALVE
TO POW ER
CYLINDER
C O N V E N T IO N A L M O D E L S
G M C S E R V I C E M A N U A L ______________________
V ACU U M CHECK VALVE
"S" Models, conventional, and Tilt Cab Models is
the same. Disconnect all lines and hoses and re­
move bolts or screws which fasten valve to vehicle.
Discard malfunctioning valves removed from con­
ventional and "S" Models and replace with new
valve. These are non-repairable. Valves removed
from Tilt Cab and P4500 Models can be repaired.
power cylinders disconnect vacuum supply hose
from check valve manifold assembly and pull man­
ifold assembly out of grommet in vacuum booster
chamber. Discard malfunctioning valves since this
type cannot be repaired. When installing this type
valve always use a new rubber grommet. Coat
grommet lightly with grease prior to installation.
panel below master cylinder assembly and on op­
posite side of panel.
the engine and is mounted on a bracket attached to
the frame crossm em ber on diesel models. On gas­
oline models, the valve is fastened t6 a bracket
which is bolted on cab rear support, inside the
channel on left-hand side (see fig. 23).
cated in the engine compartment and is fastened
to a bracket attached to the fire wall (see fig. 23).
using dash mounted booster is part of vacuum
chamber assembly.
REAR CAB
SUPPORT
CHECK VALVE
Figure 2 3 — V a cuum Check V a lve s Installed
HYDRAULIC BRAKES
Removal and installation of valves on P4500,
To remove check valve from dash mounted
Valve on the P4500 Model is mounted on dash
Valve on the tilt cab model is at the front of
Valve on conventional and "S" Models is lo­
As previously stated, check valve on models
M O U N T IN G
BRACKET
ifS m r i
I j j r o P O W E R
' M
CYLINDER
TILT C A B M O D E L S
gec 5A .
Page'i83
TO EN G IN E O R * —
V A C U U M PUMP