We study the hydrodynamic behaviour of the asymmetric simple exclusion process on the lattice of size n. The dynamics is attached to reservoirs at boundaries. The reservoirs are weakened by a factor n θ with θ < 0. We prove that the spatial density of particles, under the hyperbolic time scale, evolves with the entropy solution to a scalar conservation law with boundary conditions. The boundary conditions are formally given by u(t, 0) = 0, u(t, 1) = 1 and are rigorously characterised by boundary entropy flux pairs [17] and the boundary traces [20].