dear klm:
i agree 100% with walter. it's a matter of coordinate several different movements: fingers, wrist and forearm. let me suggest a way of practicing that:
- pick up measures one to five (until the b sharp).
- explore the bar slurs. there are four. try to play the fragments as legato as you can, and end each one with a slight movement of the wrist (upwards). notice that there is no arm movement: only the wrist. that will create a subtle break in your line, separating each fragment, but not creating a rupture.
- when you're Ok with that, add a forearm movement (or a larger wrist movement) to finish the part. that will create both a more definite separation, and give you momentum to attack the next part (a varied repetition, btw).
- final, add the espressivo touch that walter talks about. basically, you can use a bit of pressure in the first note, releasing in the second. very important is keep the wrist down, because if you move it upwards in this small slur, you will destroy the line - and thus the whole effect Brahms planned here.
exagerating those movements in practice usually helps me a lot, but as you get acquainted with the relations between notes, music, movements, it's cool to moderate them. this practice can easily lead to a histrionic performance if you keep using large gestures all the time.
best!