The Master Cylinder is fairly conventional in its design.

Factory Diagram of Master Cylinder:

Text from the shop manual:
It is horizontally fitted by means of a reinforcing plate, added to the scuttle panel.

It is actuated by new clutch pedal gear through a pushrod with constant length.

The main component is a housing in which a piston slides, fitted with 2 cups and a check valve. The piston defines an oil pressure chamber, whose volume varies according to the piston position.

The chamber is supplied as required through a hole located in the rear upper part. After a 1 mm stroke of the pushrod, the pressure chamber is isolated from the supply circuit. So as to enable a rise in the fluid pressure in the chamber.

Since the pushrod is no longer in contact with the master cylinder when the petal returns to its normal position. The fluid flows from the supply circuit to the operating circuits and inversely.

 

Here are some pictures of the Master Cylinder
 

 

 

The pedal mechanism is different also.

The hydraulic clutch requires a pedal mechanism that differs from the standard CX cable-operated clutch.   A cable operated clutch necessitates that the pedal pull the cable to declutch.  The master cylinder on the hydraulic system needs a pushing action.

The cable mechanism had a spring whose operation was typically Citroen;  unusual, but elegantly effective.  The point of maximum extension of the spring was midway through the clutch's travel.  When declutching, the spring assisted the driver by  pulling  the pedal down to the stop.  When reclutching, the spring pulled the pedal back to its engaged position.  There is a separate return spring on the clutch bellhousing also.

Several photos of the cable and hydraulic pedal mechanisms.  The cable mechanism is always on the left. 

 

 
Cable Clutch pedal mechanism
Hydraulic Clutch pedal mechanism

Cable Clutch Mechanism

Hydraulic Clutch Mechanism
Notice how much more robust the hinge / pivot mechanism is on this one.  It had to handle a lot more force.  The hydraulic mechanism, with the lower forces involved, could manage with a smaller pivot.
 
 
 
View from the engine side of the firewall.  I was test-fitting the clutch at the time.  Don't worry, I cleaned the rust off the threads before the final installation.
Test Fitting.
At some point, a NOS master cylinder appeared on ebay.
 
Also thanks to ebay, I found the shop manual which referenced a reinforcement plate between the master cylinder and the firewall (scuttle panel).   So I took the cable mount, and some bracketing, and made a reinforcement plate.  The steel of the bracketing is thicker than the base of the cable mount, so it should be OK.
Couldn't resist taking a picture when I realised that the bracket was 100mm.  Somebody was a real stickler for the tiniest of details.
As installed.  The Krylon 'hammered-metallic' green is a pretty good match for Citroen LHM green, considering it comes in a spray can.