Rebuilding the pressure regulator
Release the hydraulic pressure pressure and undo all the conduits leading to the pressure regulator (have a rag handy, there is going to be a lot of LHM leaking out).
Remove the regulator coupled with the main accumulator. If the retainer ring does not come out easily from the regulator, try to use some special tools (looks like a miniature crowbar) and a jig to press the plug inside and you might be able to take the ring out without any further intervention. This ring holds a lot of pressure, hence it is very strong and will not be too willing to come out. Get something hard and pointed, and a small hammer. Press the plug in (a C-clamp works great for this), 1 mm will suffice. Be sure to keep the small missing section of the ring, where the ends meet, free of dirt and obstruction. Now wedge your pointed tool against one end of the spring and give it a tap with the hammer. If the ring moves, you can probably dislodge it this way.
If not, you will have to use more creative measures. Put some sort of plug into the conduit holes to avoid dirt getting inside. You need a good hacksaw or even a grinder to make a narrow incision radially, at an angle, where the blue part is shown. Before you do that, notice that the spring has a small section missing. Your hacksaw or grinder will have to grind right into that missing section, until you practically touch the 45 degree beveled outer edge of the plug. After you have made the incision, you need to use the hard pointed tool with the hammer to get even just a little bit of the ring to cover the incision. Then, you put something pointed, such as a large needle or even a nail, into the incision under the ring, and pop it out. After that you can get it completely out with a small flat-blade screwdriver or similar tool. Of course, you need to press the plug in to be able to get the ring out. However, do not pop the spring out just yet.
Clean the outer surfaces thoroughly, be careful not to get cleaning fluid inside yet. Use a brass wire brush or old toothbrush to get all the rust and debris out of the inner lip and around the spring that holds the plug in, otherwise it will be very difficult to get the plug out.
Separate the accumulator from the rest of the unit. Have a clean glass or jar filled with gasoline handy before proceeding with the next step. You'll also need a 6 mm socket wrench.
Remove the rear plug carefully, its spring is very strong. If it doesn't come out on its own, do not remove whatever tools you used to press it in with, just loosen it and tap on the regulator a few times, or even on the plug. It should eventually come out. Put the parts into the jar, everything except the O-ring, a new one has to be used. You should have the plug, the spring, a small part like a plate and possibly one or more thin rings.
Peek into the other end of the regulator, where the accumulator were. There is a small piece of metal inside secured by a 6 mm bolt, partially covering two holes. Take it out carefully, do not turn the regulator over, otherwise you will lose the ball. When you have removed the bolt, cover the opening with your hand and turn over the regulator. A bolt, a small piece of sheet metal and a small ball will land in your palm. Also, you might get the cut-in piston, too, but this is unlikely because the dirt will probably hold it in its place. Place all parts into the jar.
Push in the bottom end (where the plug was) of the piston, it should pop out the other end sufficiently for you to take it out. Again, put it into the jar.
Unscrew the two hex bolts, a little at a time to maintain balance of the plastic cover, opening up the cut-off spring, plate and piston assembly. Note that there is an O-ring under the plastic. Take all the parts out, including the thin rings at the bottom of the plastic cup, and put them into the jar. Leave the plastic outside.
Now you should be able to carefully pull out the piston with your fingertips. If you have to use pliers, do it gently (that part of the piston does not touch anything so scratches are not a problem). Remove all the conduit seals from inside the holes, you will need new ones anyway. Remove the pressure release bolt as well but be careful, there is another small ball inside and it is different from the one from the cavity where the accumulator goes, don't mix them.
Clean everything that remains thoroughly with gasoline or brake pad cleaner. When you are done, immediately douse everything with fresh LHM, if the housing is exposed to air, it will rust right away. Pay special attention to the hole where the little ball was, there should be no deposits left there. Clean the plastic parts with detergent and water, then dry them thoroughly. Clean all parts in the jar similarly with gasoline or brake pad cleaner. Be careful to get all deposits off the little ball. When you are done, get a small container and fill it with fresh LHM, and submerge the two pistons and the O-rings in it.
Now you can start re-assembling everything. Get a thin piece of brass first. When you put the ball back into its place, hold it there with the brass, and give that a good tap with a hammer. Get the ball out, put it back in, and do it again. Remove the ball once more. Pour some fresh LHM into the accumulator side of the regulator, replace the ball and while holding the other end shut with a finger, slide in the thick piston. It should go in without any resistance at all. Now put in the piece of sheet metal and secure it with its bolt.
Assemble the cut-out piston assembly. Douse the hole with LHM before putting the thin piston inside and remember to replace the O-rings. Don't worry if the thin piston slides way into the hole, it is more or less self-adjusting. The spherical end should stick out of the hole. Again, when tightening the hex bolts, do each a bit at a time, keeping the plastic cup parallel. Assemble the cut-out parts, starting with plate, spring and plug. It is very inmportant to replace the O-rings with new ones, otherwise the regulator will leak. Douse the inside rim with fresh LHM to help the O-ring slide in without getting pinched. At first, it is enough to just aim the plug in the right general direction. It is very difficult to press it in by hand!
Press in the plug slowly, and at the right moment, slip in the ring. Put the other small ball and pressure release bolt back into their place. Refit the accumulator and refit the whole unit to the car. Start engine and tighten the pressure release bolt.