Why is this pair of electric brakes not working in series (as intended), but works in parallel?
The schematic shows B+ 48V power applied to pin 7 and both brakes in series with return to pin 8 which is connected to B- (0V) by either fowd or rev contactor.

This means 24V across each brake coil.

If you are seeing 40V, there may be a problem with the wiring or battery condition.  Only 1 diode drop should be expected ~ 1V rise above ground on Brake return when active.

Check contactor resistance or trace the voltage drop from Vbat to Brake voltage.

If all else fails, contact motor service company for details on how they validated the brake performance, as there is a problem.

Each brake can be tested for release threshold with a variable 24V supply and ought to be released &lt; 80% of Vbat min. (SWAG)