With the older models , the check is to verify that the green hyd( normal braking) has taken over from the yellow hyd ( parking/altn ) . If the px is not at zero , then the transfer has not taken place . There is a mod in the newer MSNs where this check for transfer of hyd is not necessary .
Small point B , unless your operator does different , the FCTMs I have used indicate that the PF is to monitor brake effectiveness with the later MSNs , not the PM .
FD - It does appear that this mod corresponds with the NWS mod also.