Technical Article - 504 and 504 Hydraulic Clutches

The hydraulic clutch used on the Peugeot 504 and 505 is a very clever design but there is a problem that the rules usually applied to hydraulic brake systems do not apply and this causes unwanted challenges for many DIY enthusiasts.  Brake systems are designed to collect any air in the system at the slave cylinders where it has to be 'bled' off by opening a bleed nipple that is located at the highest point of each cylinder.  The fundamental feature of the clutch hydraulic system is that there is an unobstructed 'up hill' path for air from the slave cylinder through to the reservoir.  A bubble sneaking past a worn slave cylinder seal will rise to the top of the slave cylinder, into the pipe and up to the bottom of the master cylinder, into the master cylinder and upwards to the pressure equalisation hole just ahead of the front seal, and finally up to the reservoir.  Once the system is partially 'bled' and you have some pedal resistance, the system is essentially self bleeding.  By the way, it is common for old slave cylinder seals to admit a little air into the clutch hydraulics and the self bleeding action usually gets rid of it. Some of us have developed a soft clutch after extended periods of highway driving, which seems to cure itself.  This is an example of the air leaking in faster that the self-bleeding can get rid of it. 

Given the above description you can see why reverse bleeding is recommended in the manuals.  However there are better and easier ways to get the job done: With the bolt on type of slave cylinder you simply unbolt it, let it hang down and open the bleed nipple. If the system is OK then the cylinder will fill with fluid and eventually pour out of the bleed nipple. At this point close the nipple and refit the cylinder. Doing this will necessitate pushing the slave cylinder piston in, displacing fluid up the pipe and into the master cylinder.  Now the system is partially 'bled' and you will have some pedal.  Start the engine to provide some vibration to facilitate the movement of bubbles, press the clutch pedal to the floor, wait 5 seconds, release the clutch pedal and repeat until a full pedal is obtained. With the clip on type of slave cylinder you achieve the initial bleeding by releasing the clips that locate the cylinder, turning it up side down (bleed nipple pointing upwards) and allowing it to move forward. Open the bleed nipple and when fluid flows out of the nipple, close it and move the cylinder back to its normal position (with bleed nipple down). Complete the bleeding as outlined above.

What can go wrong with this procedure? Essentially anything that prevents fluid flowing from the reservoir down to the slave cylinder; a blocked pressure equalisation hole in the master cylinder or a flexible hose which is internally obstructed. The best test for this is to open the bleed nipple at the beginning and make sure that fluid flows out freely. The other potential problem is a worn slave cylinder seal that allows too much air into the system as you are trying to bleed it out.  Another thing to watch out for when there does not seem to be enough clutch travel to properly disengage, is a worn pedal to master cylinder push rod.  These rods wear where they go through the pedal and sometimes wear through and leave you stranded with no clutch.

Bill McNamee

30/10/2003