If your mx5 has abs, it may need to go on the computer to be bled via the abs pump. My golf was an absolute cow & had to go on the vag system. If it hasn't got abs, I would start checking for loose fittings. Maybe the system is sucking a little bit of air from somewhere, and air compresses easier than fluid. If this is still playing up after a quick check, I would strip the master cylinder and fit new springs & seals to it. Master cylinder rebuild kits are available on the net for about £25. Oh, have you tried bleeding the clutch ? Think it all works off the same system on the mx5. But you may need to confirm this.
