Abstract: A pneumatic tire pressure monitor for a tandem set of tires has a housing for being affixed to the outboard tire wheel. The housing includes a first and second chamber communicating with the interior pressure of each pneumatic tire of the tandem set of tires. The housing further includes a third chamber containing hydraulic fluid for maintaining a reference level of pressure. A diaphragm is responsive to pressure differentials between the first and second chambers and the third chamber to activate a latch. The latch releases a visual signal member, which moves from a retracted position within the housing to an extended position in order to signal a drop in pressure in either or both of the tires. In an alternate embodiment, the monitor may be used to monitor the pressure of a single pneumatic tire.