The glm submodule sets some default priors which might very well not be appropriate for every case of which yours is one. You can change them by using the family argument, e.g.:
pm.glm.glm('y ~ x', data,
family=pm.glm.families.Normal(priors={'sd': ('sigma', pm.Uniform.dist(0, 12000))}))
Unfortunately this isn't very well documented yet and requires some good examples.