I am not sure what type of brake cleaner you are activating the base with here. If you are referring to TCE, and are doing a tetra trap type of extraction, you should use water to mobilize the base. TCE won't do it. If the brake cleaner you are using is a mix of solvents including acetone and alcohol, and you are doing a waterless a/b type extraction, try using alcohol or acetone to mobilize the base. This might improve the yield substantially.
A second thing that should help your yield is to put the pill mass on low heat, such as a coffee warmer, stirring often, and let it sit and simmer for about thirty minutes. use low heat and give it time to work. Scottydog suggested this to me and it seems to work very well with the HCl pills. There is no reason it shouldn't also work with the sulfate pills.