It’s fitted in the brake line
Is it some sort of valve ???

 

It’s a brake pressure reducing valve for the rear brakes, if it’s not leaking then don’t mess with it, they are not available anymore.

 

Ah ok
So if this is removed the rear brakes will be over working
I was going to upgrade aswell

 

Not sure on the technicalities on that especially if your up grading the brakes, if your upgrading to the 4x4 disc & callipers the position of the valve was on the base of the ABS pump for some reason, again the reasoning for this I don’t know.