We measure the real and imaginary parts of the ac magnetic permeability of YBa2Cu3 O7-δAx (A = Cl, F, H) ceramics and powders as functions of temperature and ac magnetic field amplitude H0, applying uncommonly low and widely ranged excitation fields (1 mOe ≤ Ho ≤ 200 Oe). We determine the temperature dependence of two loss peaks, Hp(T) approx. (Tc - T)2.7 at low fields in ceramics and Hm(T) approx. Ts0 - T at higher fields in both powders and ceramics. The extrapolated field Hp(0) and characteristic temperature Tc depend on x and δ, whereas Hm(0) and Ts0 depend mainly on δ. The real part of the permeability indicates a two-step flux-penetration process with two threshold fields for flux penetration. The latter govern the temperature dependence of maxima in the imaginary part as well as the peaks' widths. We propose a field-dependent percolation description of this process, derived from a previous temperature-dependent percolation model. It involves two penetration depths, the classic Ginzburg-Landau one and a low-field abnormally large penetration depth due to chemical disorder. Correspondingly, two types of vortices describe high- and low-field dissipation peaks.