We report here Langmuir isotherms of diblock copolymers containing a short neutral hydrophobic sequence (polystyrene) and a long water-soluble polyelectrolyte sequence (polyvinyl-2-pyridine). This study focuses on the polyelectrolyte sequence, which is a weak polyelectrolyte when unmodified and strong polyelectrolyte when quaternized. In pure water, the surface pressure of the quenched brush is proportional to the osmotic pressure of the counterions. In water at pH 2, the annealed brush isotherms are those of a polyelectrolyte brush in the strong screening limit. In water at pH 3, a plateau appears in the annealed layer isotherms, revealing a transition from a state where the polyelectrolyte sequence is adsorbed at the interface to a strong polyelectrolyte brush state. It is not clear whether this transition is a first-order transition or a continuous compression-induced solubility of the PVP chains.