My 51 F8 dump box is moving slowly. I suspect it needs so fluid, but I am quite unsure where to fill since it seems to have no reservoir. The only visible opening seems to be on the cylinder itself and that would require the box to be raised to access. That seems like a not very good idea, so perhaps there is some other answer. Can anyone give me advice.

 

I'm afraid you answered your own question. There is no other way other
than to lift the body high enough to get oil in there on the "top" filler
plug. Either by using jacks, or a loader or a chain fall. Make sure its
prop'ed good. Keep adding fluid until the piston fully extends and working
it up and down until the system stabilizes. And also you may get soaked.
Most plugs have a pin hole to vent the air. The best solution is- to add
hydraulic oil when you first notice the body not going all the way up.

The CAUTION: many have been killed not propping bodys.!!!

Real life example; couple yrs ago I have 17 ton of asphalt (with a L9000)
I back up to the paver, just about 3/4 the way up the high pressure line
(hose) exploded ! The body still with most of 17 ton came down in a split
sec. bent the chassis the cab shield flatted the roof blew out all the glass
and door frames and I cant get out-I crawled out thru the windshield.
The crew said the front end came up about 4 feet in the air sounded like
a bomb. And being a L9000 with a 3306 Cat -thats a heavy front end, but
still no match for a 17 ton slam...

So be careful my friend.......
sam

edit the truck was totalled and towed away still with about $2000 asphalt in it.

 

Yes, as said above, the reservoir is part of the cylinder. If you don't see any major leaks any where, it may be a problem with the pump or valving. Usually, if low on oil they will raise normal speed, but only part way.
And again.....please be careful under there!