I have a dump truck hinge piece that snapped in half when it got bent backwards too hard. Is there a type of glue that would be appropriate to repair the piece? Would superglue be a good choice? Obviously, it's never going to look new again, I'm more just concerned about the piece being functional if we wish to build more dump-trucks. Or is it a lost cause and after I go to all the trouble to glue it, it's just not going to hold?
Here's a picture of the broken part. Ldraw seems to list this hinge as parts 3324e + 314e
LEGO is very good about replacing broken parts - for free even. Visit the web-site and look on the bottom of the page under Customer Service for a "broken or missing parts" link (I can't check at current locale).
This is worth trying even if the set is old or you didn't acquire it brand new. LEGO doesn't usually care where you got the set; but if it's not recent there's a risk that the part is not produced anymore, and then they won't be able to replace it. But try anyway, they're usually very friendly.
You can also try calling Customer Service directly.
LEGO bricks are made from ABS. Therefore the most effective adhesive would be ABS cement from the hardware store that sells it for plumbing repairs. ABS cement in theory would chemicallyc weld the bricks back together.
It's sometimes possible to build a jig out of other Lego blocks to hold the broken pieces firmly together to allow the glue to set and the result to be very good. I use my worst-condition bricks for jig building because they almost always get glue on them or some kind of damage.
The location of the break on your hinge makes it a difficult fix, but you could try to make a basic jig by:
Putting a couple layers of plastic wrap around the 2x8 portion (to keep glue off)
Using a thin 2x4 plate at the end of the 2x8 bit of the hinge
Using a 2x6 plate atop the previous plate to secure the 2x4 broken bit of the hinge in place
Then carefully align the broken hinge bits and apply a glue which will weld the break together.
I've had success repairing cleanly-broken parts (snapped without bending) using plastic cement -- the sort that contains methylethylketone.
Make sure the broken surface is free of dirt and other contaminants.
Brush a small amount of plastic cement onto both fracture surfaces. Make sure not to accidentally get any anywhere else -- this stuff works by briefly dissolving a thin layer of plastic at the join, and can cause damage if not used carefully.
Hold the two parts together for about 30 seconds to let the cement set.
Leave the repaired brick alone for a while (I usually let it sit overnight) to let the join fully harden.
That said, your hinge is a tricky repair. That's the weakest part of the brick, and it's quite possible the join won't be strong enough. It's also close enough to the pivot point that you risk gluing the hinge permanently in place.