Riddle me this....

If the pushrod is too long it puts the piston in a position where it does not allow fluid to return the reservoirs.

There are procedures in the FSM but basically the the pushrod should be a few thou shorter than the distance inside the master cyl. The keeper inside the mc might be there too and it might also need to be removed.

Also there might be a residual pressure valve inside the forward most port on the master cyl. It would have been there from when the master was used on a disk drum setup.