Does the bridge need removed?
I have an Alvarez RD20 that my wife & daughter bought for me. This summer was hot and I noticed during a string change that the corner of the bridge has started to lift just a bit. Probably could slide the corner of a business card in but not a credit card. Can I just inject some glue with a syringe & clamp or does the bridge need to come off for complete re-glue?
on a flattop guitar a lifting bridge often is the sign of a different problem, especially on a pin-style bridge: it usually means the top is starting to belly -- meaning it's developing an outward curve (belly) in response to string tension. the bridge, being stiffer than the top, usually begins to pull off at the upper, lower and back edges as the glue is not strong enough to hold against increasing forces as the strain increases. a modern Alvarez guitar such as yours probably has a two-piece bridge assembly which may make lifting less likely; but clearly it's not invulnerable. which part is lifting?: the "tailpiece" with the pins or the forward part with the saddle?
If you really like this guitar, and want properly fixed for the long run, I recommend you take it to a guitar tech for a consult. do this before you try anything, as once you do something like gluing, the damage be worse or more difficult ($$$) to repair. if the top is bellied, it may need to be repaired (e.g., brace repair, addition of a bridge doctor â€¦). if only mildly bellied, correctly re-gluing the bridge, adjusting the saddle height and going to lower tension strings may take care of it.
to your original question: you could try to inject some glue, but chances of long term success are slim. (and there is always the "made it worse" risk.) first problem is that unless you remove the original (old) glue, your new glue will not likely adhere well. the exception would be if the original is a hide glue, in which case you could heat/steam the bridge/joint to re-flow the old glue and add some more hot hide glue. this is not amateur work. the second problem, is that even if the bellying is not serious, the bridge may need to be removed and reshaped to match the new curve of the top. that will relieve stresses, making chances of a long term fix better. and removing the bridge also will allow proper removal of the old glue from both the top and bridge. other points: removing the bridge on a bookmatched top is tricky. the grain run-out is in the opposite direction on each half, so the direction for inserting the lifting/prying tool is the opposite for each half of the bridge. done incorrectly, the top is easily damaged. you need a good clamping system to hold the bridge while the glue sets and cures. unless you build this, it will cost a few $$$ for a long-jawed clamp, and a bridge caul to distribute the forces â€¦ in addition to the tools to remove the bridge and old glue, and gluing materials. unless you are planning on doing this more than once, probably not worth it as compared to a pro repair (tho, if you own a 12-string, chances are â€¦)
-=tension & release=-