  We study four distinct second-order nonlinear equations of Rabelo which
describe pseudospherical surfaces. By transforming these equations to the
constant-characteristic form we relate them to some well-studied integrable
equations. Two of the Rabelo equations are found to be related to the
sine-Gordon equation. The other two are transformed into a linear equation and
the Liouville equation, and in this way their general solutions are obtained.
