Are these sentences OK?
I'll call you before I leave. / I'll call you before I'm on my way.
I would have felt bad if I had kept quiet about it.
If I allow you to do it, then I'll have to allow everybody do it.

Question

Are these sentences OK?
I'll call you before I leave. / I'll call you before I'm on my way. 
 I would have felt bad if I had kept quiet about it.
 If I allow you to do it, then I'll have to allow everybody do it.

Answer

Yes, these sentences are correct. However, for your second sentence it would be more natural to say, "I'll call you before I head out." or "I will let you know when I am on my way." :)

Answer

also more natural to say "I'll call you before I leave." or "I will let you know when I leave." or  "i will let you know when I am leaving."