As I suspected, one of the accumulators, a supply side at the chamber, was found to have zero pressure.  Additionally, it would take no gas.  I replaced the Schrader Valve Stem and then it would take a charge and appeared to hold charge.  Nothing obvious on the stem but for 20¢, I'm not investigating further.

This is a pretty good case for the frequent HEPI fluid level check being an indicator of the accumulators' state of charge.  As I reported in 36356, the level was down by 7/16" while running.  While stopped, the level increased by only 4 or 5/16" when recharged.  Additionally, this accumulator was on the supply side which run at a higher charge, 69psi.  Had it been a return side accumulator which run at 35psi, this loss might not have been apparent.  However, I'd argue that the return side accumulators are marginal in their benefit.  Further, the much greater fluid volume in the corner station system might absorb the charge loss in a single accumulator without revealing that loss at the reservoir.

Bottom line, verify as best you can that the Schrader valve is sealing after the charge check and closely monitor the fluid levels for otherwise unexplained reasons of fluid level change.

Closing WP 6655 and FRS 8192.