NA 1.6, Bought the car in October 2017 and no issues with the clutch. It was engaging i’d say halfway up the travel but these past 2 weeks it’s been intermittently engaging straight off the floor, maybe half an inch, and the last few days has been solidly like that. Checked the fluid and it’s at the max line. Looked at the slave and it is covered in oil or something (could be due to my slow CAS oil leak yet to fix). I can pull back the rubber boot and it’s quite greasy but again can’t tell if its clutch fluid or oil.
Other than that I have no trouble selecting gear apart from when the car isn’t up to temps, and no slipping on the clutch. Is my clutch engaging there a problem? I’d guess something’s going on as it wasn’t engaging there when i bought the car.
My first guess ( as I suspect yours was by looking under the slave cylinder rubber ) is that there is a leak on the salve. I doubt the oil is from the CAS as it is on the other side of the engine.
After market replacement slave is only £15. That would be my starting point.
My master cylinder went bad in my 99 mx-5 first my slave cylinder went so i replaced that because it would stream out as soon as you would put any pressure on the clutch after that was replaced and I replaced the lines as well, my clutch still would act like it had no pressure at all. (Even with the line being properly bled.) So i replaced the master and it fixed my problem.
If the reservoir is not losing fluid, then the problem is in the master. With pedal on the floor symptoms, if it was the slave it would be dropping fluid on the road each time the pedal was pressed.
Reconditioning ALL the seals in it with a repair kit is possible, but it is usually better to replace the master with a new one or a guaranteed refurb.
As the others have said it is a good idea to replace the pipe and the slave at the same time - clean new system = no problems.
A word of warning, when I replaced a master and slave, soon after, the rubber flexible hose failed. These have to be ordered from Mazda. Aftermarket Goodrich hoses are hopeless. These cars are getting pretty old now, and that rubber hose is exposed to heat, oil etc.
Old hoses can cause a problem all by themselves. I’ve seen a brake hose that had lost its structural strength and it would inflate like a balloon, causing a very spongy pedal.