No explanation from me! After struggling with this for an evening and then putting the post in, I removed the Master cylender for investigation. I did make sure that the reservoir was completly filled by tipping the whole assembly to get the fluid to transfer between chambers prior to installing the unit. After that I bled it with the engine off no problems, but I was extra cautious to monitor the level as I bled it.

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Deezel
87 300TDT
150,000 miles