The RockShox Reverb seatpost can compress up to 5mm from full extension. If there is movement beyond this amount, it could point towards air bypass around the internal floating piston.
For C1 and AXS Reverb, the Vent Valve can be used to purge air bypass and return the post to a firm position at full extension. See our SRAM Tech videos for the C1 Vent Valve or AXS Vent Valve procedures, or consult the C1 Reverb Service Manual/AXS Reverb Service Manual for step-by-step instructions.
NOTE: The Vent Valve is not to be used regularly and is only to be used if the seatpost compresses abnormally while seated (more than 5mm). For A1, A2, and B1 Reverb and Reverb Stealth, this will need a rebuild to correct the symptom. You can find service documents at the SRAM Service Site or consult your local dealer for assistance repairing or setting up an in-house service.