I had a spare used regulator and wanted to make sure it was still serviceable and not all corroded so I decided to open it up. I used Hans Hehl website article as a guide. ( http://www.hehlhans.de/tipp42.htm )

What I found interesting is that between the lever that applies force to the regulator and the actual piston is a small spring ( travel 3.5 mm ) , item #7 in diagram ( http://www.hehlhans.de/bilder/autos/g240/alb/pic.jpg ) which gives one the impression that the piston in regulator is free to move and therefore serviceable. This is not necessarily the case.  

 With reference to Han's diagram , remove lever #6 by removing the outer 19mm nut holding the lever to the actuator. Be careful as the nut is quite thin and fragile. Remove the dust cover and the actuator #7.  Your will see a cir-clip and a 12 mm piston head . This piston head only travels  around 4.7mm  - that is when fully depressed the top surface is 4.7 mm below top edge of outer cylinder ( see photo ) of the regulator and is flush with cylinder when fully extended. You should be able to press the cylinder in by using your fingers but will need to external pressure to extended it back.  I found that by applying air pressure t  the port that goes to the brakes one can extend the piston back.  If piston is free to move one can arguably consider the unit as serviceable if there  are no specific reasons why not. Interestingly by apply air pressure to the supply port the piston travels all the way down.

So if piston does is not free one needs to dismantle further. Firstly remove cir-clip . My air pressure was not high enough to force the inner and outer pistons out so I used a grease gun which I attached to the port connected to the rear axle. One needs to also block off the other port ( i used bolt M10x1mm ).  Now one has to in a step wise fashion restrain the outer piston ( by holding it in position using something like a 13mm socket head mounted in a vice ) and applying pressure with grease gun until inner piston hits its limit. Then release the outer piston a bit and reapply pressure and eventually both pistons come out.

 One needs to be aware that grease is far from ideal in that it is not compatible with brake fluid and brake seals so clean up asap once all is dismantled  or preferably use brake fluid as fluid instead of grease. One needs to also carefully clean up the inside of the 12mm piston by applying brake cleaner through the 4 small bleed holes ensuring that  the small spring and valve works at the bottom of the 12 mm piston. Mine has a travel of around 0.7mm. 

Re-assembly is just the reverse. I have not purchased new seals yet but I 'm sure they are obtainable from a brake shop.

Seals are as follows ( using Hans as reference )

for outer annular piston - o-ring 18mm x1mm

inner piston  o ring 12mm x 2mm and brake lipped seal inner 8mm outer 13 mm

Peter

(View of outer and inner piston with cir-clip removed.JPG)

(Inner piston at max depth 4.7mm.JPG)

(Actuator.JPG)

(Outer annular piston.JPG)

(Inner piston.JPG)

(Cylinder .JPG)

View of outer and inner piston with cir-clip removed.JPG (55KB - 0 downloads)

Inner piston at max depth 4.7mm.JPG (41KB - 0 downloads)

Actuator.JPG (17KB - 0 downloads)

Outer annular piston.JPG (56KB - 0 downloads)

Inner piston.JPG (12KB - 0 downloads)

Cylinder .JPG (58KB - 0 downloads)