I think its a 231 its runs a pump. I think it has a brake but it isnt working the shaft spins some when it shouldnt.I saw a parts brake down but i didnt see a brake. It is air operated and the truck has 200.000 on it.
I've not seen one with a brake, but I've seen a couple where the teeth get chewed up from guys not paying attention engaging or disengaging them. If its air shift, and the air pressure falls too much, the pump will disengage. Can happen when hooking up air lines to a detach trailer with the pumps in (air all goes to the trailer), or we had one in a crane- if the pumps are in, guys would shut the crane off for lunch but didn't kick the pumps out. Then when the air builds up, the gears will grind until you clutch it to let them fall in. When the teeth get chewed up, they sometimes won't want to disengage. We swapped the air shift out for a manual lever in the crane, guys were grinding it all the time - and by guys, I actually mean- I had it happen to me, and then I asked the regular operator about it, and he said it happened all the time, the teeth were in bad shape when I opened it up.
Well for a 231 I find the parts breakdown here: http://partsmanuals.org/chelseapartsmanuals/Chelsea-231-Parts-Manual.pdf Quick looking at it appears to be a clutch engaged PTO. In other words it is always "in gear" and shifts in and out of service by engaging or disengaging the clutch. We had some of this style on some off-highway trucks and the clutch on those would at times drag just enough to turn the pump they drove when pump was not doing any work. But as soon as you engaged the hoist the pump would stop unless the clutch was supplied with pressure. Seems like I recall the clutch on ours were worked by oil pressure from the transmission on at least some of the trucks. And here is the site for the Chelsea PTO manuals I found, might be of help to someone: http://partsmanuals.org/chelsea-pto-parts-manuals/ Note: They also have Muncie and other manuals there if you look around some!