We report here compression force measurements of diblock copolymers containing a short neutral hydrophobic sequence (polystyrene) and a long water-soluble weak polyelectrolyte sequence (poly(2-vinylpyridine)). This study focuses on the polyelectrolyte sequence and has been restricted to the case where the subphase is at pH 2. The lateral compressions were done using a Langmuir trough whereas the normal compressions were carried out with an atomic force microscope with a spherical particle attached to the tip. The thickness of the brush is considered as a function of the grafting density and is determined from the AFM force curves without reference to any theoretical model. At high grafting densities an "anomalous" behavior is observed, where the thickness of the brush decreases with increasing the grafting density. This is attributed to the annealed character of the polyelectrolyte.