Press brake pedal twice for good pressure

[Number4]

Yesterday I noticed that after driving for some time (not using brakes) and coming to a stop or slowing down, that the brake pedal was real soft. Impacting the stopping ability. However, if I immediately let off and re-apply, I have firm braking pressure.

Also, once I have firm pressure, the pedal does not lower any further, as it seems to when it's soft.
No evidence of fluid leaking out. Is this a master cylinder issue? Brake booster?

 

[boominXplorer]

I'd first suspect a leak. Mark the master cylinder resivior with a sharpie to monitor level. It could be in the early stages where it's not dripping out yet. I've seen a caliper boot fill up around the Piston and not leak. Had a good bit of fluid in it too, there should be none.

 

[imp]

Yesterday I noticed that after driving for some time (not using brakes) and coming to a stop or slowing down, that the brake pedal was real soft. Impacting the stopping ability. However, if I immediately let off and re-apply, I have firm braking pressure.

Also, once I have firm pressure, the pedal does not lower any further, as it seems to when it's soft.
No evidence of fluid leaking out. Is this a master cylinder issue? Brake booster?

@Number4
The master cylinder has two separate pistons in it, one feeds the front brakes, the other the rear. This change came as a safety consideration many years ago, across the board.

If the seal on the rear piston in the master cylinder begins to allow fluid past it, that fluid goes into the front piston's part of the cylinder, without any leakage occurring. This shows up as a "squishy" pedal for one push, sometimes a few pushes when real bad, then releasing the brake pedal immediately fills up the void, resulting in normal pedal "feel".

The fluid bleeding by the leaking piston seal winds up in the reservoir. If leakage is occurring at a brake caliper, into it's seal boot, that will show up as a loss in the M.C. reservoir. Check for that first. If fluid loss is not seen, consider the likelihood of master cylinder rebuild/replace. imp

 

[TechGuru]

Could also be a warped rotor causing a gap to widen as you drive then you have to press it twice to close the gap...