Topic approved.

Your post is confusing to say the least.
1. You say your rear brake loosing pressure while doing off road ride,..is it Tire or Brake loosing pressure ?. If it is tire then the Tire bead and rim seating is at fault.

2. If it is Rear Brake which is not acting well After an off road session, then check as follows.
a. Check for oil leakage in any of the joints, ie., banjo bolts, rear master cylinder, caliper etc.
b. if no leak found then check oil level before and after the off roading. If there is less level check for leaks, if not try a few normal braking on level roads and check for braking returning to normal.

The possible reason for for your problem other than mentioned could be that your ABS is re-calibrating itself as the terrain changes and the requirement changes with it.
Just my Thought..

Ita my Brake that is loosing pressure. all things are ok as checked by service center.

as mentioned by you, ABS might be calibrating, Is it possible in FZ25 as it does not have any riding modes or something.

What are the conditions during which you are facing loss of braking force?

I assume by loss of pressure you mean you are not able to press the pedal fully or even after pressing pedal brake force is not adequate/sufficient.

During off roading the the wheels may rise above ground which causes wheel speed to suddenly decrease and under these conditions the abs thinks there is wheel jams
and activates abs.during this you may feel brake pedal hard or loss of brake pressure even after fully pressing the pedal.

As far as I know it is common.

You may try disconnecting ABS/abs fuse during off roading and check if it is due to ABS intervention or any other issue.

Its like loosing pressure , so when i push pedal it just simply go down with no pressure then i build pressure by pressing it again and again.

But after i take a break it becomes normal.