Cessna SKYMASTER 336 1964 Service Manual page 210

At serial 336-0055 & on, an O-ring has been
added under the mounting nut above the horiz-
ontal firewall, where the rear relief valve ex-
tends through the horizontal firewall. The
O-ring acts as a seal for the opening around
the mounting nut. This O-ring should be in-
stalled on all earlier serials.
Figure 14-4. Vacuum System
14-9