A vacuum breaker valve-discharge elbow assembly for a rotary drum filter of the pipe machine type is adjustably mounted to the drive gear housing for the filter via a separate mounting plate. A valve flange is secured to the mounting plate. The mounting plate is in turn transversely and axially adjusted on the gear housing via adjusting bolts to accurately seat the valve within its valve housing in the drum. The discharge elbow is then secured to the mounting plate independently of the valve. The valve can be rotationally adjusted on the mounting plate without disturbing the prior axial and transverse adjustment of the plate and valve by providing slotted fastener access holes on the valve flange and close-tolerance valve flange guide shoulders on the mounting plate. Cooperative calibration plates on adjacent valve flange and mounting plate surfaces facilitate precise angular adjustment of the valve.