Polyelectrolyte brushes were formed at the interface between hydrophobized mica and water by adsorbing amphiphilic diblock copolymers consisting of a hydrophobic and a charged block. We measured force-distance profiles with the surface force apparatus (SFA) for several polyelectrolyte brushes of different lengths as a function of aqueous 1:1 electrolyte concentration. Long-ranged electrostatic double-layer forces were not observed in the force curves, and our results suggest that more counterions than predicted by the Manning theory are condensed onto the polymer backbones inside the brush. The brush height observed with the SFA follows the predictions of the mean field theory: at low ionic strength the height is independent of the external salt concentration c., whereas at high external ionic strength the height shrinks with a weak power law L-0 proportional to cs(-1/3). All force vs separation data for three different brushes, with various salt concentrations, collapse with reasonable accuracy into two reduced plots of data in the osmotic and salted brush regimes, respectively, the variables of which are developed in the scope of the mean-field brush theory.