I have a 05 RX8 Shinka and I have been having this issue for 3 weeks ever since my clutch pedal got stuck to the floor while driving it.
I have scanned the forums, watched videos and have changed both the master cylinder and the slave cylinder. I have changed the master cylinder twice and have been pumping and bleeding. I have bled through the slave cylinder. It's nothing but straight fluid.

I have pressure bled. I have suction bled. I have bled in all possible ways along with fresh fluid and when I pump, the pedal is not bouncing back.
There is definitely some pressure there, but not enough to push the pedal back.

I have checked the pedal itself. I have taken it out, inspected it for any damage, I have checked the position of the rod on the back of the pedal that depresses the plunger in the Master Cylinder. With no resolve. Now just today, when I pump the pedal really fast with short strokes on the back end of the clutch, pressure builds and I will push it all the way and the pedal will bounce back, but when I depress it again, it doesn't bounce back.
You can feel there is fluid but the pressure is not building whatsoever, almost like it gains some pressure up to a point but never comes back. I made sure everything was installed properly, I have inspected all brake lines for tightness and making sure there is no fluid coming out anywhere.

I am at a loss at this point and I don't know what else to do at this point. Does anyone have any advice? I have been trying to figure this out all on my own but now I truly need some help. Please.

 

Maybe a broken clutch pedal bracket?
Do a search.
It was a recall...

 

as stated the clutch pedal is known to crack/break. there are brackets to "fix" it but honestly its cheaper to remove the pedal reweld it and bolt it back in. see if it squeeks or has lateral movement

 

So in regards to the pedal, we have actually already done this as we noticed it did have that lateral movement. We tac welded it and it is solid, no issues from the pedal at this point. What I'm thinking is that I may have to start showing pictures of everything that I have done But yes that one has already been resolved as we thought even that would cause some issues.