ORIGINAL: badazz68stang

i've been drivin my coupe alot here lately for the first time. ITs a blast to drive around. But i've noticed when i barely put my foot on the brake peddle and apply gentlte steady pressure, the peddle will sink to the floor and not hold very good, and i will have to pump them up to hold. BUt if i just suddenly hit the peddle it stops really good. Any ideas what this could be? Maybe a bad MC, or air in the lines? 4-wheel drum '68 coupe. Dual MC, non-powerbrakes.

ORIGINAL: JMD

Unless you are leaking fluid somewhere, I think it is about a 95% chance it is the master cylinder.

I imagine that when you hit the brakes hard, the quick action of the fluid against the piston seal in the MC, pushes the edges of the seal firmly against the cylinder walls, thereby causing a better seal between the piston and cylinder wall than the "gentle" application.

So in other words, I think that the seal(s) in the MC are either worn or hard, and fluid leaks past the seal when you apply the brakes gently.

I can "see" what I am trying to say here, but I don't know if I am explaining this well enough for others to see, but if not, the simple answer is that I think it is the MC.