crank system help

I just bought a 77 bon air pop up trailer. When I crank it up...it won't stay up. I have to attach pliers to the crank and jimmy the pliers behind the tire to get it to stay up. Does anyone know what the crank system looks like, or where to find info on how to fix it myself.

Thanks everyone.

It's basically a simple crack and a nothed gear with a stopper....maybe the stopper is broken??

Its a ratchet mechanism.
it should be right beside the crank. turn it to engage the ratchet. you have to disengage to lower the top obviously.

https://s337.photobucket.com/albums/n380/sithdad/Lift%20System%20Repair%20Guide/

I was looking for info on lift systems as this is something I would want in case I get stuck, low and behold I found the site above. Hope it helps in any way

Good Luck

I replaced cables on my trailer on the weekend. it was very easy actually.
the toughest part is threading the new cable.
here is the procedure I developed.

1: Start with the top lifted as High as it will go. I used a 2 ton trolley jack and some 2x4's to hold up the broken cable end.
2: I cut the old cable to a clean fresh end. butt the new cable end to end and tape them together. Keep the tape thickness down. it must go through a pulley and a hole in the floor. With a helper pulling and me pushing the new cable, we managed to thread the new cable up through the hole. but, in hindsight, with the top supported you can unscrew the metal channels and lift them off the floor to access the cable hole.
3: extend the two adjuster eyebolts to their longest setting. pull the cable through the side rail and pulleys and through the eyebolt and tension it as tight as you can manage by hand. Clamp with a bolt clamp. (It will most likely have to be moved)
4: lower the roof support. so cables are supporting the roof.
5: crank down the roof so its about 2-3 inches from bottom. measure all around and adjust tensioners to set height.
6: if you don't have enough adjustment. lower the roof onto a 2x4 under the lip. this will remove tension from the cable with a lot of slack so you can adjust as mush as needed. don't cut the cable until you have it set properly. I left 8 inches of cable for future tweeking.

Wow...Thank you Joust for sharing, that is real fantastic information !