When using wood glue, it's important to get glue inside a crack or split so it will hold with maximum strength. This is challenging to do without a wood glue injection pump. These pumps allow you to use air pressure to push the glue deep into the split wood. We use these in our furniture repair business, and nothing works better.
These are sold in a 2-pack and can be washed with water for reuse.
When gluing up split wood with wood glue, there are 3 things needed for it to work properly:
1) the two surfaces must mate well and tight when clamped
2) there must be enough adhesive to cover the full split
3) after the glue is applied, clamping pressure is needed for the glue to bond
Injecting glue into splits can be done with blunt tip syringes, but with limited reach for small splits. Dental floss or thread can also help spread glue in some situations. Neither of these methods work as well as using these wood glue injection pumps.
See the wood glue injection pump in use on the Fixing Furniture YouTube channel.