So I been having trouble with rear brakes (DRUM) real sensitive you just touch and you can feel them grab, rain, gravel road you got to baby them. Its like the pedal is right there at the top supplying rear brakes before the front disc. Well I installed new injectors and noticed oil leak under boost. Can this be the cause of rear brakes being so touchy?

No it won't.  Sounds like you have a bad combination valve. 

Where is this combination valve?

 

It's on a bracket between the PDC and the ABS controller.  If you have a factory repair manual the description is at sections 5 pages 9-10.

Just a FYI I had two different trucks that had that problem and on both I found that the axle seals had failed and contaminated the shoes. One had no outside indication of a leak. Both I replaced the shoes ,the seal and cleaned everything and all was good.

If that's the case with leaky seals then before changing them make sure vent isn't plugged. I check all my vents early just to be sure. 

Did axle seals about 2 years ago, good thought.