I think that your tree may have Bacterial Wetwood or Slime Flux which is caused by bacteria that entered the tree either through the roots and the water the roots take up or through an old wound or injury.  As the bacteria builds up in the tree, it oozes out and leaves a black wet mark on the wood and a vertical line that can look like a crack.  Here is an excellent fact sheet on Bacterial Wet Wood that explains what the problem is, and the fact that the tree will survive despite it unless it moves into another part of the tree which will cause the leaves to die off.  http://extension.psu.edu/pests/plant-diseases/all-fact-sheets/wetwood-or-slime-flux   If the tree does start to lose a lot of leaves and branches, then it is time to re-evaluate and call in an arborist.  My ‘rule of thumb’ is when at least 1/3 of the tree is dead, call a licensed and insured arborist to safely remove the tree.    You always want to hire a licensed and insured arborist so they will have the proper training and safety practice.