The leather needs to be renourished. After you dye, apply a couple of thin coats of neatsfoot oil to the dyed side (not the rough side). Allow it to soak in for at least a day before applying your finish. Don't overdo the oil...just wet a small cloth and wipe it on until the surface is wet, then let that soak in...do that twice and then stop, even though it looks like it would love to drink up some more.

Of course, it may be better in the long run to just replace them, unless we're talking about an historically significant garment. Serious cracks that compromise the strength of the strap will not be repaired by oil or wax.