If the clutch does not separate well or the pedal fails without resistance, then it is clear that air has entered the hydraulic drive. Simple removal of air in this case will not help - you should find and repair a leaky place. After that, you can do the removal of air. Other reasons why the clutch does not separate correctly are listed in the list of faults at the end of the chapter.
Anyone who wants to check the hydraulic clutch just in case should look for brake fluid leaks on the master cylinder (at the top of the clutch pedal) and in the working cylinder (next to the gearbox).
Clutch cylinders wet with oil are leaky and must be replaced. But without a thorough cleaning of the area around the working cylinder, the location of the oil stain on it can be determined with great difficulty.
Removing the clutch master cylinder
Remove as much fluid as possible from the brake fluid reservoir (like an old syringe).
Unscrew the shelf on the left under the dashboard (chapter «Salon»).
Remove the heating hose underneath (one screw, one bracket).
Remove the inlet hose and allow the remaining liquid to drain into a rag.
In the engine compartment, to the left of the brake booster, unscrew the pressure hose to the working cylinder.
Remove the two hexagon socket head screws securing the master cylinder
Pry off the pivot pin lock washer (pedal/master cylinder connection), squeeze out a finger.
Now you can remove the cylinder.
When installing a new cylinder, adjust the rod that presses on the piston of the master cylinder so that the clutch pedal is located about 10 mm above the brake pedal.
Be sure to pay attention: the return spring must correctly return the pedal. With the pedal in its original position (rest) must not rest against the bracket of the pedal unit.
To adjust, loosen the lock nut on the push rod and turn the push rod left or right around its own axis. Then tighten the lock nut again.
Bleed air from hydraulic clutch (pump).
Note: The clutch pedal in the Audi 100 has a spring above the dead center. It helps when you press the pedal, but after releasing the pedal, it depresses it again to its original position. If the pedal does not return, this means that air has entered the hydraulic drive or the pedal bearings or the spring are stuck.
Removing the clutch slave cylinder
Raise and secure the left front side of the vehicle.
Unscrew the pressure hose of the hydraulic actuator of the working cylinder - be careful, brake fluid flows out, which must be collected.
Loosen the fixing bolt on the cylinder, pull the slave cylinder out of the gearbox housing towards the rear.
If necessary, use a rust removal spray and pry bar.
Lightly lubricate the new cylinder on the unpainted body walls, apply MoS2 paste on the drive tappet.
Insert the slave cylinder into the gearbox housing and push in until the mounting bolt can be screwed into the threads on the gearbox housing (25 Nm). Use only a special screw with a rotating tip.
Bleed hydraulic clutch.
Bleeding the hydraulic clutch
If you do not have at your disposal a device usually used for this purpose in the workshop, you can bleed the clutch in the same way as the brake system (chapter «Brake system») or, almost losslessly, in the following way:
Open the bleeder valve of one of the front wheels and the clutch slave cylinder valve each by about one and a half turns.
Connect both valves with a hose.
Now depress the brake pedal slowly and carefully several times so that the brake fluid from the front brake is forced into the hydraulic clutch.
Do not press hard, otherwise the hose may come off!
Monitor the level of brake fluid in the reservoir.
If air bubbles from the clutch hydraulic no longer rise from the brake fluid reservoir, both valves can be closed and the hose removed
Check brake fluid level!
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