I had originally thought I might use just one for rebalancing the electrolyte level in my battery. (I somehow did not get the cells even when adding the acid to the battery initially.) But since then, I've used these for a lot of other things as well.
When reassembling my forks, I used the syringe plus a length of brake line for getting the fork oil level correct:
When bleeding my brakes, I used the syringe plus a 2-foot length of clear hose to vacuum-bleed the lines. I was astonished at how well this worked. Same principle as the MightyVac. Just pull back the stopper a bit to create suction, watch the air bubbles and brake fluid start to fill the hose. Keep the syringe held up vertically so that the air bubbles travel upward toward the air space in the line. And of course make sure you don't run the reservoir dry.
Of course the downside is that I don't know how long the plastic and rubber will hold up to volatile chemicals. For some tasks, I've been able to keep the chemicals inside tubing, using the syringe only as a vacuum source. But really, these are cheap enough that they're pretty much worth it even if they were one-time use for every job.
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