Rear braking pressure is governed by the loading on the rear suspension through the rear height corrector but shifting the manual height selector to max height should bypass that.
If the flow is still poor then there is likely some blockage in the pipes. If both are the same I'd suggest that it is in the line from the rear height corrector to the doseur valve or the one from the doseur to the rear brakes. Opening some of the unions should confirm this.
Do take the usual care while working under the car and on the hydraulic system
Help. Rear Suspension wont go down and MOT tomorrow
Moderator: RichardW