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In alkali rf-discharge lamps used for optical pumping in atomic clocks and magnetometers, a buffer-gas (Kr or Xe) allows electrons to extract energy from an rf-field, and these energized electrons eventually produce alkali resonant light. Contrary to naïve intuition, rf-discharge lamps can lose their noble-gas buffer over time. Recently, we began a long-term experimental program to better understand the mechanism of noble-gas loss in rf-discharge lamps, and needed a nondestructive means of measuring buffer-gas pressure changes in sealed glass cells. For this purpose, we settled on the Kazantsev, Smirnova, and Khutorshchikov (KSK) technique, which is based on inferring buffer-gas pressure from the collision shift of an alkali ground-state hyperfine transition frequency νhfs. Here, we discuss the basic KSK technique and two modifications that we have implemented for its improvement: use of a diode laser for optical pumping, and extrapolation of νhfs to zero magnetic field. Testing our system's long-term performance with a very low pressure reference cell (i.e., 3.3 torr Xe), we find a reproducibility of 0.2% and an absolute accuracy of 5%. Further, our systematic drift is less than 1 mtorr/month.