Also check to see that the small port inside and on the bottom of the master cylinder is clear of obsrtuction. If it has crud covering/blocking it, it will not allow the fluid to return to the resevoir.
All the things the rest of the guys said are good considerations of what the problem is.
Let us know what you find smile

There is enough good in the worst of us and enough bad in the best of us that it does not behoove any of us to criticize the rest of us.
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Be yourself. If you are ever lost, It will be much easier to find yourself if you know who you are!