B737 Captain reported an indication of zero pneumatic duct pressure during climb out; resulting in a precautionary operation at a reduced altitude to destination.

Narrative

On climb out from ZZZZ; we noticed the left duct pressure indicator was reading zero. The #1 bleed switch was on; the left pack switch was in auto; and the isolation valve was in auto. There were no recall lights or pneumatic panel lights illuminated. In an abundance of caution; we leveled off at 180 to troubleshoot the issue. There were no non-normal Flight Manual (FM) procedures pertaining to the issue. FM directs to contact maintenance in the event of a pressure differential of more than 8 psi. We spent 20 minutes attempting to contact Dispatch via ACARS (through Dispatch) and VHF/HF; to no avail. During this time; we elected to slow to long-range cruise due to our higher fuel burn. This decision kept us well within the range of reserve + buffer fuel. We also elected to climb to FL240; further increasing our endurance. At no point did we feel the flight was unsafe. We were finally able to reach maintenance via radio approx. 200 miles south of ZZZ. They informed us the same issue on the same airplane had presented itself to multiple previous crews and that they would investigate it further upon arrival. Our Dispatcher joined the conversation and concurred with our decision to continue to ZZZ as planned.