The clutch master cylinder will typically fail from it's internal seals.  The leak may or not be obvious, so if you are experiencing clutch hydraulic problems and the clutch slave cylinder and clutch slave hose is not obviously leaking, it's most likely the clutch master cylinder.  Seal rebuild kits are an economical alternative, but corroded and pitted bores will make any rebuild fail.  It is safer to buy a new or rebuild clutch master cylinder and just replace your original unit.  We recommend Mazda remanufactured units over big auto parts chain stores for better quality.

With any of the clutch hydraulics, if your set (clutch master cylinder, clutch flex hose, and the clutch slave cylinder) is still original from the factory, it is highly recommended to replace them all at the same time if you can afford it.  It is common to end up replacing all of them once one fails, as you end up replacing the weakest link in the chain and they all leak once you start replacing each aging component.
Clutch slave cylinder Problems page
Clutch slave flex hose Problems page

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