If the problem doesn't go away after your brake fluid change, including proper bleeding, it's probably time for a master cylinder. The internal seals go, so there's no leakage, but it won't develop pressure the way it should. I did mine in about 45 minutes today, then ran out of daylight so I'll be bleeding it tomorrow. I got the part from this site. They had three or four manufacturers available and I decided to try the $38 Meteli cheapie.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.