Hi all. Anyone experience this or have an idea about what may be going on?

When I get in my '93 525 the next day my pedal is as tight as teak wood...like someone just pumped the hell out of the brakes w/ the car off. Car sit's overnight and this has just started happening.

When I turn the car on the system does it's thing and I get the right pedal feel again, throughout it's use. Then I park it, and when I get back in the next day they're hard/tight.

Any idea on what could be happening? Huge thanks-

Eric