When engine is off brake pedal hard no movement start engine with foot still on brake pedal slowly goes to floor
Do you ever have issues with a hard pedal when driving and feeling like you cannot stop? This would indicate brake vacuum was insufficient for what ever reason. I have had this where the brake vacuum pump is old and lazy until servo has gained enough vacuum.
Conversely do you ever come to a junction and find the brake pedal going down under your foot, or when holding it if you release fractionally and then keep the pressure on again it again starts to creep down under your foot, but have no lose of brake fluid at all, this is usually a faulty brake master cylinder.
Incidentally although frightening, on most modern vehicles the brake system is "dual circuit" this means the brake master cylinder will have two pistons in the master cylinder, normally they work as one unit, but if the pressure seals on one fail then the second one will usually work to provide some braking, it can feel as though the pedal is going to the boards, but hopefully it should have some effect.
The only other time I am aware of those symptoms is where a worn wheel bearing allows the brake disc wobbling to push the brake pads back , this means that the first time you press the brake pedal after a period of driving that the pedal goes down a long way, usually after the second "panic pump " the pedal will feel hard again until the next time.
Re single circuit brake systems I did have an issue in a 1955 Series one Land Rover break down vehicle whilst going on a recovery, I had just driven down a very steep hill , got to the broken down vehicle and went to turn around in the wide road, a brake metal pipe split and lost all the braking, the Land Rover went up a grassy bank and stopped luckily with no damage. I fixed the car and drove back to the garage very much slower than the outward journey.
Incidentally just a thought have the brake pads been recently replaced but brake discs with a large wear grove not been changed as that can give similar symptoms until brake pads bedded in to match the wear ridge.
I suspect from your initial post that loss of vacuum overnight is the likely cause as
@jackwhoo suggest checking.
In an ideal World the brake vacuum reservoir should be capable of a few vacuum assisted brake uses even a couple of days later, before the engine starts up.
