This means that structural_simplify is able to simply all the equations. You can check observed for the solved expressions. If you want to evaluate it, then you can use substitute. You can also obtain the observed variables by
julia> sys = structural_simplify(model);
julia> prob = ODEProblem(sys, Pair[], tspan);
julia> prob.f.observed(valve.a.p, prob.u0, prob.p, 0.0)
2000.0
julia> prob.f.observed(reservoir_right.a.p, prob.u0, prob.p, 0.0)
1000.0