Re: Soft Brake Pedal
The brake fluid should be completely changed every few years as it's hygroscopic and does absorb moisture.
I would have the break system completely bleed with new break fluid and see how it is then.
The seals in the master cylinder could now be starting to leak? Without the engine running the brake servo wont be working and therefore the amount of force on the brake pedal is increased.
With the engine running and the brake servo working and the increased force on the master cylinder a slight seal (internal) past the master cylinder seal would cause the pedal to sink.