Non mopar brake question

Sorry this isn't about a mopar, just need some advice before I start throwing money at it
2007 Honda CRV, it isn't leaking or losing brake fluid, no leaks around the master cylinder, no warning lights are on ,ABS is working fine power brakes work good
The problem is when you hold the brakes at a stoplight, and put more than normal pressure on the pedal, it slowly goes almost all the way to the floor, its still stopped, no warning lights come on, but I can't put my foot between the pedal and the floor
If I'm right the master cylinder can cause this, even though it isn't leaking, whatever separates the front brakes from the rear brakes is allowing brake fluid to seep between the 2 on the piston(sorry I don't know the correct terms)
question is, can a bad master cylinder do this? Local discount parts dealer says it can't, I'm pretty sure it can, I think I've even had it happen before, at almost 60 years old, and too many cars can't remember when and where