The method I use, often, for such cases, is to tape the surface, if possible, then wet any exposed surface, to keep the glue from adhering onto the surface. Squeeze/press the glue into the crack, as best as possible, and, if I think I need more glue deeper into the crack, I use the air hose to blow the glue down into the crack. This method works best for me in most cases..

I never thought to use suction, on the backside, but that should work, if you can access the backside. For me, the air hose seems quicker and more convenient.

Comments about syringes and needles:
Syringes and needles aren't (generally) perscription items. They are over-the-counter.

Your local pharmacy has TB syringes with 5/8", 21 gauge needles, sometimes lower guage needles (larger needles) are available, separately, as well. You likely have to buy in bulk, 100 per box. Any size needle fits onto all syringe tips, so syringe size doesn't matter. If the hole in the tip of the TB needle is too small, for easy flow of the glue, snip off the sharp tip of the needle, then pry open the shaft hole.... the shaft end having been smashed flat by the snipping.

A local farmer's co-op may have larger needles, & supplies, for service to livestock owners. Many farmers do their own routine injections and such.

You don't have to suck the glue from the needle end, to get it into the cylinder. Remove the piston from the upper end and pour/squirt the glue into the cylinder.... a cleaner procedure, than sucking from the needle end.

Maybe your physician might sell you a syringe and large needle, also.

Your local surgical supply house will likely not sell you, directly, those sorts of items, unless you know someone who works there. If available, purchase a damaged box at a discount.... now outlawed, here, that's what rooster-fighting folks did, buy damaged boxes for their injections.

Sonny