Spongy Brakes
 

I have an odd issue that is baffling me.
I sometimes have a spongy brake pedal. a couple times it has even been soft enough to have the cluster "brake" light come on. this problem just started out of the blue and was not a direct result of a parts change.

I first checked all brake parts for obvious leaks, which there were none.
I believed it was the master cylinder so i replaced it with a new unit. I bench bled it properly then installed it. i then pressure bled all 4 brakes starting from furthest to closest to the master cyl. I then used my autel to bleed the ABS module and re-bled all 4 calipers.
After all of this there was no change to the problem.

I then replaced all 5 rubber brake hoses and repeated the brake bleed procedure. this also did not fix the problem.

This truck has zero rust and all hard brake lines are solid with no leaks.

The only other think i can think of is the power brake booster, but i have never seen one of these cause spongy brakes when failing.

Anyone else every have a problem like this?