I am old school and back in the day, if the brake pedal sinks to the floor, the master cylinder is bad, or the brake line is leaking, or a caliper is leaking.
Since there is no leak anywhere, I was sure it was the master cylinder.

I changed the master cylinder and it still does the exact same thing. Pedal to the floor. What baffles me is that even when the engine is not running, the brake pedal still goes to the floor.
I heard if you bleed the brakes, some cars need to be in ABS brake bleed mode. Can someone explain how in the world the valves in the ABS module can allow the brake pedal sink to the floor?

 

Your description with no leak could be from a bad master cylinder or a weak line where the line bulges when pushing on the brake pedal.

If your MC was bad, you could just have introduced air into the brake system. You would need to bench bleed the new master cylinder and bleed all the lines to the caliper.

If the flexible brake line was weak, you would need an assistant to press the brake pedal and observe the rubber line on each wheel for an obvious bulge.

 

Thanks Honda. I'll try to get back to it in the next few days. Just surprised a NEW master cylinder could be bad.
I only had 2 in the last 40 years or so.

The MAIN thing is that the pedal sinks even with the engine OFF. So I am thinking MC.
Every car that I know of with the engine off, you can pump it 1 time, then the pedal will not depress again.

 

I forgot to mention the ABS shouldn't cause the issues you are seeing.

You are correct that the new master cylinder could be bad out of the box. I typically try to use Honda parts when the price isn't astronomically high vs some of the aftermarket parts out there.

 

Sounds like a huge amount of air in the system someplace. what exact procedure did you use to bleed after you replaced the MC? Was a complete bear for me to bleed, even with 2 people. What I ended up doing was the heavy grease thing. Clean off around your bleeders. Takea Q tip and get the heaviest grease you can. Put some grease around the bleeder where it meets the caliper, Put a piece of hose into a large, clean brake fluid bottle about 1/3rd full of clean fluid. Back off the bleeder a bit, not too much. SLOWLY pump your pedal, but DO NOT pump it all the way to the floor. Keep filling the reservoir. I was able to get all the air out that way.

Right after the replacement, even after bleeding with two person method, still pedal was almost to the floor. Took over a quart to get most of the air out.

 

Well, we tested the master cylinder by disconnecting the brake lines and plugging the outlets with 2 bolts. Pedal is solid.
So we are looking at excessive air somewhere or there is something funky about the ABS module.