The present invention relates in particular, but not limited to, a wind turbine hydraulic brake calliper. The brake calliper comprising a cylinder bore, a piston having a piston head and a piston skirt. The piston being received in the cylinder bore in an orientation where the piston head faces towards the brake pad. The calliper comprising a fluid connection connected to or connectable to a source of hydraulic fluid for leading hydraulic fluid into the void of the cylinder bore below the piston head (1b), so as to move the piston. The piston is formed at least partly and preferably solely from a polymeric material or from polymeric materials.