We report on the dynamics of surface-attached polyelectrolyte (PEL) brushes under compression and varying conditions of added salt and pH. Fluctuations of the brush thickness are probed by bringing the sample in contact with a small sphere attached to the cantilever of a scanning force microscope (SFM). The noise power spectral density (PSD) of the cantilever motion reflects the interaction between the surface-attached polyelectrolyte layer and the sphere. The brush-sphere system mimics the interaction between a brush-coated solid wall and a colloidal particle. A shrinkage of the brush height was induced by addition of salt and variation of the pH. Close to the collapse the brush is highly compressible with an increased dissipation between the brush and the cantilever. This suggests that polyelectrolyte brushes under near-collapse conditions could be efficient rheology modifiers for colloidal dispersions.