It was a problem with a quick disconnect fitting on the gauge. Pressure build up on gauge side of the fitting was able to close the valve so it couldn't read more than about 28psi. I'd heard that the fittings with Schrader valves caused problems so had got rid of them, but didn't spot the quick disconnect internals. Once I removed the qd fitting it all worked.

I'll try the battery charger suggestion next time I test. I guess it could affect heater voltages too.