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:
Let Antony and Caesar fall together.
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:
You know not what you do. Do not consent
That Antony speak in his funeral...
It shall advantage more than do us wrong.
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.