Brake Pedal goes to floor

Think about the 30 pumps and those geysers. They are telling you a story,along with the low-pedal hitting the floor.
During all that pumping, you are moving fluid down the lines. It is obviously going somewhere cuz the geysers are proof that it s returning.
If the m/c was internally bypassing, there would be little to no twin geysers.
Also, as Tb says; the geysers should be very small, typically just roiling the surface.
-A geyser that breaks free from the fluid surface on a DB car, ,from the chamber that operates the front brakes, is telling you there is compressed air in the system somewhere.
-A geyser from the chamber that operates the rear brakes is telling you that one of two things is going on back there, Either; 1) there is air in that system, or 2) the rear slack-adjusters need adjusting.
-The many,many pumps is also telling a story. Usually this is also pointing to the rear slack adjusters needing adjustment. All that pumping is moving the rear shoes slowly out to the drums. If the pedal returns the m/c piston faster than the fluid is able to return,then eventually the shoes get to the drums. If the front system was OK, then at this point you would have a hard pedal. But the second you release the pedal, whoosh, the fluid is returned by the shoe return springs, and then you get to start over.
- The low pedal,going to the floor, is an indication that the front system has completely failed.

So, in my estimation, you have two problems; 1) the rear slack adjusters need tightening up to eliminate the many,many pumps, and then 2) get the air out of the front system.
There is a wild card. If the flex hoses have gone soft, they will introduce there own bs into the diagnostics, by acting like expansion balloons. I have only rarely seen this, so I seldom check for it until I have exhausted all other possibilities.
But I would adjust the rear slack-adjusters first. The operation of the m/c kindof dictates this. The front-brake system is directly attached to the foot-pedal,whereas the rears are not.At least not usually. The rear brake-system is hydraulically coupled, by a small pocket of fluid in the m/c. When you apply pedal, the pedal drives the rearmost piston forward, which immediately shuts off the compensating port. Then the fluid is pushed down the line to the front brake calipers. As this is going on the pocket of fluid trapped between the front and rear systems, hydraulically pushes the frontmost piston forward, which then shuts off the front compensating port. The fluid in that chamber then moves out to the rear wheel cylinders. If the rear slack adjusters are just a little loose, then no brake action occurs back there until you push seriously hard and push out all the fluid from the frontmost chamber, and then maybe you get a lil action back there. But if the slack adjusters are really loose, then there may just not be enough fluid moving out, and then you get zero brakes back there. In your case,it seems like with enough pumping, the rears do finally get enough fluid. But the reason you can actually do this is cuz the front system is not energizing, and you may actually be activating the rear brake system by it's safety feature.
The m/c is designed such that in the event of a hydraulic failure in the front system, then the rearmost piston will travel forward, and a small extension on the front side of it will drive the frontmost piston forward to engage the rear brakes. The proof of this going on is the very low pedal height, and it's hitting the floor is proof that the rear slack adjusters need attention.
--So that's what twin geysers, a low pedal hitting the floor and multiple pumping, tells me.
-To recap; loose rear slack adjusters and air in the front system. With a minor possibility of air in the rear system.Adjust the rears first. Crank them hard, and back off 3 strokes(not notches), fix the fronts, and then reset the rears last.That's how I would attack this.

- except for one thing; I would ensure that the m/c itself is purged of air.And that is why we bench-bleed m/cs. Once the master is mounted it invariably does not sit level, an so air gets trapped inside its chambers this air has got to come out through the compensating ports or be driven all the way down the lines to the end points. But as you can clearly see. the m/c sits well above the level of those end points, and driving air downwards is hard enough but trying to keep it moving downwards is almost impossible. So by far, starting with a fully operational m/c is very important,as is keeping it fully functional during the bleed-procedure,which means; do not let air get into the chambers from the top!
OK so go get 'em and good luck!