If you are a member of SCOF, there is a very extensive Technical section with in depth explanations on how to adjust the parking brake. When I owned SPF 1938 I followed that guidance and was able to adjust mine so it would hold tight under any condition.
If you aren't a member of SCOF you are missing a lot of good information and interaction with other SPF owners, so get out there and join.
With the proper adjustments, the SPF parking brake works just fine.
