I did a quick example which worked and I find it weird yours that it doesn’t fill. By default the quadric should be GLU_FILL so there should be no need to call it.
did you make a glPolygonMode(GL_FRONT_AND_BACK, GL_LINE) call if not you could try and use glPolygonMode(GL_FRONT_AND_BACK, GL_FILL)
try commenting out gluQuadricDrawStyle(quad, GLU_FILL) and see if it works.
I tried commenting it out, but that didn’t make a difference. I also tried adding in the glPolygonMode call, but that didn’t work either. I really have no idea what’s going on…
I put gluDisks on the top of the cylinders, which at least makes it look right, but it’s still very strange that the fill isn’t working.