I'm not sure I understand what you are trying to achieve, but I understand what's happening in your code. You've identified _pooled as a class variable. The values in this variable will be treated as categorical rather than continuous.
If you want these values treated as numeric values instead of categories, you should remove the class statement. If you want to use the original values (5, 15, 50), then you should use the dosegrpc variable in your proc glm.