This just stated happening a few days ago. The brake pedal is so stiff that it takes a much more noticeable effort to press the pedal down far enough to be able to start the car. Its only happening during prolonged engine off periods, like over night and while I'm at work for my normal 10 hr shift. The pedal feels normal after the engine starts and during normal driving. No evidence of brake fluid leaking from anywhere.

Did a search and found this entry on the X type forum:

"I'd place money on the brake booster pipe check valve deteriorated to the point it allows vacuum to bleed off on accel and loss of brake booster assist. Big vac leaks also creates lean engine codes which will give you the "engine damage result" flashing check engine light. Check and replace the pipe that goes from the top of the intake to the brake booster.Might as well replace the imt orings too with the self sealing green ones too "

Considering the amount of rubber hoses that have had to be replaced this calender year, this sounds like another hose issue. I traveled back from the states with a couple of coolant hose assembly's in my suitcase.

TIA, Ish

 

It would appear that you are losing the vaccum in the booster due to the same or a similar problem.