Technical 2008 Fiat Ducato clutch issue- new genuine slave cylinder

[Marmitethecat]

Ive got a 2008 Fiat ducato 3.0 and he is my list of work done.
- bought a 2nd hand gearbox,had it inspected and fitted with the existing slave cylinder that had only done 2000 miles, but it started leaking.
- bought a brand new genuine clutch slave cylinder and a brand new bleed pipe

when i pick up the van the clutch failed on the way home, I rebled the system so i could drive it back to the garage( they showed me to old slave cylinder which had a slight weap on the seal. They checked the system was bled properly and all appeared fine.
BUT what i have noticed is sometimes the pedal is light and full travel, sometimes its 3/4 and stiffer. If i flutter the pedal play that exists at the top 1cm the pedal starts to drop bout another 1cm and and randomly will drop to the floor.
When driving the van the clutch is sometimes either 'snatchy' most times fine,but more so the clutch pedal will randomly drop to the floor in slow traffic .. this is happening once every few days. wheni flick the pedal back up the clutch works again.
the fluid appears to have dropped in the brae res. ive topped it up to see if it drops again. There is no fluid dripping out the bell housing i an see. in the cab there was a wet patch where my heel sits, which smelt like brake fluid, but around the master cylinder it is dry.
Ive bought a new master cylinder, to see if that helps, as its the cheapest option.
But if its not the problem, why would my van be EATING slave cylinders every few thousand miles, since the gearbox was replaced????

 

[bugsymike]

Some of the symptoms suggest the master cylinder, the same as on brake master cylinders you can have a pressure /resistance as you hold the pedal down a bit, then relax it and push again and it drops to the floor, the same as if you held the clutch pedal down at a junction in gear and you start to feel it creep forward and it is hard to pull gear lever back into neutral.
Often this is with no loss of fluid or external leaks but if you carefully observe the reservoir as you press the pedal down the fluid drops slightly as normal, but without moving the pedal it becomes soft and watching the reservoir you can see the fluid return.
Generally the failing slave cylinder leaks lots of fluid.
Occasionally a master cylinder will leak down the pedal onto your shoes etc. from the back seal, but less common, it is the internal pressure seals that cause what I described.