I do I hour sittings of  breath concentration. About  15 minutes In my breath gets slow and almost un noticeable.  I also  get pleasant feeling at that  time .the feeling is just pleasant in My body.  QUESTION.  should I make that pleasant feeling my primary object of concentration  for the rest of the time in sitting or revert  to breath.  My mind wonders more when I revert to breath.

It might be a good idea to try to fill your breath with that pleasant feeling and make the breath really exquisite. 

And what happens when you use this pleasant feeling as object? Where is the object? have you blended it with breath? please experiment and tell us what happens.
~D

In Mindfulness, Bliss and Beyond Ajahn Brahm gives a good <checked myself> advice on this. Switch concentration focus from breath to luminosity/nimitta or pleasure/piti themselves only after they stop to wobble/pulse and become stable.

If I remember well he also advocates spending plenty of time with breath and letting go, because first level of pleasure/piti is piti born of letting go, so it will disappear in early jhana states if focused too early on. Unless it's allowed to act as a spontaneous factor abiding in the background/peripheral awareness. Second level of piti is piti born of concentration and this is stable even as an object and not background quality.

This works for me regardless of hard/soft jhana paradigm. 

I never get  a  visual  nimita. I've seen a  couple  of annoying lights  but I just ignore them and return  to. Breath  or pleasant feeling.