Fully agree with planenut
See my earlier reply about the Stahlbus valves.
It is a good habit to take the brake-drum away from the rear wheel.
That should be simple else there could be something wrong.
And inspect the brake, so you have a good idea about the condition of your rear brake.
Brake paths still good, no oil around the brake cilinder, the condition or readjusting the automatic handbrake gaff, the springs etc.
