I have searched on this and haven't found an exact answer or an issue similar to this.

94 accord EX 5spd swap
Car is warmed up and at normal operating temp: While driving the idle is fine and motor runs smooth. Intermittently, with clutch pressed or in neutral(no load on motor) while under light but consistent braking pressure the pedal will slightly sink(slowly) and start to feel mushy. A few pumps remedies the sinking for a second. When at a stop and idling the peddle sinks further but still slowly. WHEN EITHER OF THESE BRAKE ISSUES ARE FLARING UP: the idle will bounce up and back down from approx 800 - 1200 rpms. After about 8 seconds the idle returns to normal. Pumping the pedal causes the idle to raise higher and drop faster.

The ABS light is on. I have bleed the brakes and fluid level is correct. My guess is master cylinder (because of the sinking) but the booster would make more sense because of the associated vacuum and idle.

Any help in in determining whether this is booster, cylinder or abs related would be greatly appreciated.