Given these propensities of our hero, we can see that as he is speaking to his fellow conspirators in Act II, Scene 1, one could argue that he isn’t so much manipulating others as he is manipulating himself, and persuading his allies to conform to his idea of an honorable murder for the betterment of Roman society.

When Cassius suggests that they take an oath of secrecy, Brutus dissents, and calls upon this honor as rendering any oath superfluous.  He protests,

…do not stain
The even virtue of our enterprise,
Nor th’insuppressive mettle of our spirits,
To think that or our cause or our performance
Did need an oath;

Thus he persuades the conspirators to forego any secret oaths to each other or to their cause, since the cause and the men themselves are noble, and such a virtuous act, among virtuous men, does itself solidify each man’s dedication to the plan.  And so Brutus compliments the others, and challenges their own moral fiber not only as honorable citizens, but also as Romans; the necessity of an oath is only for lesser men than they, for citizens of lesser civilizations.  This challenge forces the others to accept his suggestion, for fear of compromising their own reputations.

Brutus uses a similar tactic in lines 162 to 183 to dissuade the conspirators from assassinating Mark Antony.  “Let’s be sacrificers, but not butchers,” he implores to Cassius, “for Antony is but a limb of Caesar.”  He believes that Antony is harmless without Caesar’s leadership and support, and would have Caesar’s death be clean and honorable.  He will not have them contribute to a limb-hacking bloodbath, and calls upon the men’s reverence of the divine, asking them to “carve him as a dish fit for the gods,” and behave in a manner that will “make our purpose necessary, and not envious; …we shall be called purgers, not muderers.”  In this manner he seeks to persuade the conspirators to temperance in their deed, and therefore he, Brutus, will maintain his reputation for honesty and he can continue to live in his bubble of idealism and honor.  Which, of course, turns out after the fact to be impossible.

I would argue that Cassius is not able to manipulate Brutus.  He attempts to manipulate him through flattery and even some bullying, but it seems to me that Brutus is the one that gets his way.  Two important places where Brutus overrides Cassius are in Act II, scene i, when the conspirators meet and Act III, scene i when he insists that Antony be allowed to speak at Caesar's funeral.

In Act II, scene i, first Brutus repudiates Cassius' suggestion that they "swear [their] resolution" to murder Caesar.  Brutus has quite a long response to this in which he asserts that they are Romans and, as such, have no need to swear.  It is their honor that will create their bond.  And then, more crucially, when Decius Brutus suggests that they murder Antony too, Brutus puts the nix on that as well:

Cassius

Let Antony and Caesar fall together.

Brutus

Our course will seem too bloody, Caius Cassius,

To cut the head off and then hack the limbs...

For Antony is but a limb of Caesar.

This, along with Brutus overriding (once again) of Cassius after the murder of Caesar and permitting Antony to speak at the funeral, could be argued as the two fatal mistakes to the success of the conspiracy.  In Act III, scene i:

Cassius

You know not what you do.  Do not consent

That Antony speak in his funeral...

Brutus

It shall advantage more than do us wrong.

Cassius

I know not what may fall.  I like it not.

Well, like it or not, it is not Cassius that has the upper hand, much to his own frustration.  So, I would argue that Cassius, try though he might, is not able to manipulate Brutus at all.

In Act 1, Cassius uses flattery to gauge Brutus's willingness to join the conspiracy.  Cassius expresses disgust that Caesar is so revered by the Romans, and he makes the point to Brutus that the names "Brutus" and "Caesar" are equal (and, therefore, that Caesar is no better than Brutus and doesn't deserve the power he's enjoying).  Further, Cassius reminds Brutus that his own ancestors were responsible for founding the Roman republic:

O, you and I have heard our fathers say

There was a Brutus once that would have brooked

Th' eternal devil to keep his state in Rome

As easily as a king.

Later in Act 1, Cassius further manipulates Brutus by writing letters (he uses different handwriting to make it appears as if the letters have come from various citizens) that express concern about Caesar's power.  Cassius leaves the letters for Brutus to find, because he knows that Brutus's love for Rome and its citizens will make him act in opposition to Caesar.