Join Date: Nov 2003
Location: Port Elizabeth, South Africa
Your symptoms sound like the slave or the master cylinder, given that you replace the slave, it's probably in the clutch MC. The bore of the cylinder is most likely rusted and the edge of the rubber washer has been hurt & the fluid is cycling past the washer. Pumping will temporarily build up pressure as you pump more fluid than can leak past, but as you hold the pedal the fluid continues to leak past til you have no more clutch pressure.
Time to take the cylinders off, both clutch master & slave & examine both - I refurbish all my cylinders in stainless steel & use silicone fluid - some curse it, I swear by it, but it does depend on your personal preference.
If there is rust in the MC, you need to overhaul both the MC & the slave, if only to flush any contaminated fluid out the slave. It's an easy job, just mucky - remember that ordinary brake fluid strips paint very effectively, so take extreme care here !!
Secondly any dirt / contamination will damage the edges of the washers, so cleaning everything is critical, especially the cylinder that comes back from being sleeved in stainless - don't imagine that the engineering or hydraulic shop is going to clean it out 100%
Your ONLY washing & flushing fluids that will not damage hydraulic washers are Methylated Spirit or Meths, that purple stuff you had in the Bunson burners at science class or White Spirit. I use 3 plastic ice cream tubs next to each other, 1st; 2nd & 3rd wash with Meths... Wash in tub 1, rinse in tub 2 & rinse again in tub 3. You would be amazed at what lurks in corners in the cylinders. An old toothbrush will loosen the crud nicely.
Re-assemble the cylinders & bleed carefully - you should be good to go.
Warning: Years ago Mercedes Benz out here had a contamination issue in their assembly line and ended up throwing away 400+ ABS units as it was cheaper than cleaning them out -their Lab determined that 0.02% of mineral oil contamination in brake fluid was enough to damage the rubbers in the system, so clean hands & parts using only Meths. Be careful of touching any greasy suspension stuff while you are under the car, if in doubt, wash hands again.