That is sorta normal; during the first inch or so of pedal travel,the piston in the MC is moving past the C-port to close it off. If you whack it in that inch, yeah I can imagine a lil geyser but just a lil one is all I can imagine.
Brake fluid does not begin moving until the C-ports are closed.
Below is a picture to help you understand, Those blue springs are sitting in brake fluid and so is all the area in between them.
As you push on the pedal, brake fluid first gushes into the rear chamber until the C-port is closed. Then the fluid trapped ahead of the rear piston hydraulically couples that rear piston to the front one, and begins pushing it past the front C-port. Now we are ready to start braking. The fluid in the front chamber now moves out, while the fluid in the chamber between the springs may or may not be moving out, but when the front piston encounters resistance, you keep on pushing down on the pedal. NOW for sure the chambered fluid starts moving out.
That is how it is supposed to work.
But if there is air trapped in the inter-piston chamber, then it just gets compressed. You still get some brake action, but nothing really happens until the front piston encounters resistance. By this time you may be pressing hard enough to drive the the front piston mechanically. Look inside the rear spring and see the pushrod in there. When this happens, the pedal is very low, and the air in there can be at a very high pressure; so when you release, it explodes back into the reservoir.
So, the best way to set this up is, firstly without trapped air,lol, is so that the rear shoes start back-feeding into the MC as early as possible, so the front piston has only a limited range of motion. In that way, assuming no air in the MC, the rear piston can also get to work early, and so you will get a high hard pedal.
But as you can see, air in the inter-piston chamber just kills it.
Hope this helps.
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