pettibone stuck in reverse

Hi, we have an old pettibone 20 multikrane that is stuck in reverse. hydraulic fluid level seems ok. i did a search and saw this question posted last year, but not many replies.us thought maybe i'd ask again. it seems that the krane has a gearbox for the 3 speeds, and that the direction [forward and reverse] are determined by a hydraulic valve. is it possible this valve suddenly stuck? also, as the power appears to be delivered to the transmission via hydraulics, is it better to check the fluid level when the machine is running-thanks