The tall geyser indicates one of two things
a) too much slack in the rear adjusters or
b) air in the rear system.
as to a) if the rear pistons go out a long ways, then when the return springs haul them back it pushes all the fluid back up the line ending in a geyser. For diagnostic purposes I often crank the rear adjusters up hard.
as to b) Air is compressible so if you pump the system up with air in it, it will behave like a big spring when you lift off the pedal, and blow the fluid back into the master.
So now you have a diagnostic tool. If the geyser persists with the rear adjusters cranked hard, I would look for air.
But if it's less or gone, then adjust the rear to normal.
If you clamp all the pistons into the bores front and rear, then the only flex in the system will be the hoses ; so,with braided lines, the pedal will be high and hard, and almost NO geyser is possible unless there is air in the system. So there is another diagnostic tool.
If the pedal becomes high and hard initially, but falls over time, with NO visible leaks, then the MC is bypassing fluid and is no good.
If you have a booster; Do not go two-footed crazy in pressing the pedal; it is possible to break the control valve inside the booster and render it useless.
