I think, and this is an internet diagnosis so take it for what it's worth, you have air in the system, or the mc is bad. The sinking pedal isn't good. Could be the air compressing or the mc leaking internally.
Here is what I would do.
Get a helper to man and operate the pedal. Tell them you will be giving instructions., Down means push pedal down and hold until you tell them release. Then when the pedal returns to the up position they tell you up. This is so you don't have a bleeder open and suck air back into the system. You also need a 2x4 piece of wood. This is to place under the pedal so it doesn't go all the way to the floor. This enables your helper to push the pedal and bottom out on the wood. This was mentioned above in a prior post. Over extending the pedal can introduce air into the system.
Alrighty then, now that we are ready to bleed I hope you have your helper, some new, clean brake fluid, a brake fluid receptacle(I like a clear jar so I can see the bubbles), tubing ( i prefer clear here too)and wrenches/sockets, I would get the vehicle up on jack stands and preferably level so we can commence the bleeding!
We start at the mc. The pedal operator needs to know and understand they aren't "slamming" on the brakes. The pedal needs to be operated in a slow and continuous manner. When you say down, they need to apply slow even pressure to the pedal to force the fluid out of the mc ports until the pedal reaches the wood(you put the wood under the pedal didn't you?) then hold the pedal there until you tell them release. Jerky, erratic, fast operation introduces bubbles and this is what we ware here to try to get out, not put in. Not holding the pedal down is also counterproductive as it draws air/ old fluid back into the system. You don't want that.
With your PPE(safety glasses at the least) , you will have you mc on the car(I like to bleed on the car), the ports blocked/ not attached to the brake lines and you new fluid ready, fill the reservoir and have you helper operate the pedal. This isn't a 1 or to pedal cycle deal, you will operate it as many times as necessary until you don't see any, and i don't mean oh they were small, NO BUBBLES! Before you move on to the wheels. WHen there are NO BUBBLES, then and only then are you ready to move on to the wheels. Make sure you keep the mc reservoirs full as if at any time one goes dry you start over bleeding the mc, because you have introduced air into the system again. KEEP it topped off.
So once you have the mc bled, it is time to move onto the wheels. Starting with the furthest from the mc usually the pass rear. You will be there under the car with you helper there operating the pedal, making sure the operator can follow you down(Push and hold) and not release until you tell them. They need to hold the pedal down until you close the bleeder to prevent sucking air/dirty fluid back into the system. Once you have closed the bleeder have them release the pedal and let it return to the up position and them tell you it has returned. Open the bleeder again and have them push and hold. you should be getting fluid out of the bleeder into your hose and down to your clear receptacle, watching it for bubbles. I like to bleed until we push the old fluid out, but you need to monitor the reservoir to keep it topped off, as if it runs dry you MUST start again at the mc. Once you have the pass rear bled, then you can move to the dr rear and repeat the bleeding. Keep the mc topped off. Then move to the pass front and finally the dr front. Keep the mc topped off(Notice I keep saying this, IT IS IMPORTANT). If at any time you run it dry, you start over. If you dont you end up where you are with a spongy pedal because you have air in the system.
Let us know how you do.