Recently a new mechanism for passive Q-switching in fiber lasers based on cooperative dynamics of linear Rayleigh backscattering (RS) and Stimulated Brillouin Scattering (SBS) has been reported in Yb- and Er-doped fiber lasers with high pump powers (greater than 2 W). At such high pump levels, the intensity of the light generated inside the fiber laser cavity exceeds considerably the SBS threshold, so that the conditions for passive Q-switching due to nonlinear backscattering are easily achieved and, for this reason, no special optimization of the laser configuration is required. Here, we report on experiments in Er-doped fiber lasers with low pump power levels. The Q-switching operational mode of Er-doped fiber lasers was observed at pump power levels in the range of 30 - 100 mW. Throughout the experiments, we traced in details different stages of Q-switching pulse formation process: growth of the spontaneous radiation, lasing due to Rayleigh backscattering, appearance and growth of the first order SBS Stokes radiation and the second order Stokes radiation, lasing suppression due to saturation of the population inversion in Er-doped fiber by the SBS Stokes radiation. In general, the process was slower in comparison with previous experiments at high pump power level. The output pulse duration was in the range of 10 - 20 ns and the peak power of the pulses was less than approximately 100 - 200 W. For this reason, all nonlinear processes except SBS did not influence the pulse formation process. The experimental results are well explained by a theoretical model based on RS-SBS dynamics.