Master Cylinder. If there are no external leaks I can't see how anything else could be at fault. I believe the condition you are describing is typical when the internal seals are begining to fail. When cold the seals shrink a little allowing the fluid to pass around them, after you pump it a couple of times and some heat is generated the seals swell back to close. It's dangerous to drive it in my opinion and should be replaced, TODAY.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz