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 I agree with you, kgould. Believed "knowledge" and "truths" become our reality. Michelle, you ask, "if everything is really just "a construction of the brain" then is it considered an illusion?" Illusions expose the possibility that our vision is not always veridical, or, does not accurately portray the world all the time. Something is considered an illusion when our perceptions of that thing do not meet our expected perceptions of reality. However, one must consider a standard for reality in order to give this definition meaning; the standard for reality and veridicality depends on one's expectations and background beliefs about what is normal. So, to answer your question, if everything is really a "construction of the brain", it is still our reality and not an illusion because we accept it as truth. 

Here is an example concerning the convergence of railroad tracks that I hope helps to illustrate my point:

Some might consider the convergence of railroad tracks to be an illusion because we "know" they run parallel in reality, but they appear to converge to us.

Objects that are farther away take up less space on the retina and thus appear smaller to us. The farther away railroad tracks get, the less space they take up on our retina and the smaller they appear; convergence is the constancy of this fact. Although we know the train tracks remain parallel in reality, we expect them to appear to converge because it seems normal to us and does not defy our expectations of perception. In fact, if we saw an image of train tracks that remained parallel, we might consider that image to be an illusion because it would defy our expected perceptions of reality, since we expect points farther away in our visual field to appear smaller.

The convergence of train tracks is an example of a phenomena that is our reality rather than an illusion because it goes along with our expectations of what is normal.