We theoretically study a 1D elastic photonic crystal containing air voids as an opto-pneumatic medium. This medium is sensitive to weak deviations of the external pressure and, owing to its elasticity, can vary its geometry depending on the external conditions. We show that the reflectivity can be drastically changed at a chosen working frequency near the photonic band-gap edge or the reflection window. The resonance properties of such pneumatic photonic crystals made of glass, silicon, and mica with directly excited eigenmodes in the infrared region of frequencies are analyzed. The ways to determine small changes in the pressure on the micro- and nanobar scale are discussed.