Brutus' reaction to Cassius' death is a bit of a puzzle. He starts off first of all citing the abstract significance of his friend's death in very rhetorical, formal terms:
O Julius Caesar, thou art mighty yet!
Thy spirit walks abroad, and turns our swords
In our own proper entrails.
Not the tears of horror you might expect. Brutus does then go on to say something very tender about Cassius (nad Titinius who has also killed himself, next to Cassius):
Are yet two Romans living such as these?
The last of all the Romans, fare thee well!
It is impossible that ever Rome
Should breed thy fellow. Friends, I owe more tears
To this dead man than you shall see me pay.
I shall find time, Cassius, I shall find time.
Brutus never does find time - he dies himself only ten minutes later in the play. But this onset of tenderness and compassion is offset immediately by some extremely pragmatic, uncaring, and clearly unmoved lines which immediately follow:
Come therefore, and to Thasos send his body;
His funerals shall not be in our camp,
Lest it discomfort us. Lucilius, come,
And come, young Cato; let us to the field.
Labeo and Flavius, set our battles on...
Ten seconds mourning, and then back to business: even Cassius' body is to be dispersed off somewhere else. It's a problematic ending - and it doesn't reflect particularly well on Brutus at all. Make of it what you will.
Hope it helps!