Noticed a funny trait the other day in my 1987 Jag xj. I was waiting in a long drive through line, and after sitting in drive for say 45 seconds or so (on the brakes so not moving), the brake pedal would get a bit softer, and the car would start inching forward. Like a loss of braking pressure. If I push down further, or lift my foot and push again, the car would stop no problem. When sitting stopped, I don't sit hard on the pedal, more like just hard enough to get the car to stay still.
Now that I am paying attention to it, the brake pedal does seem a bit 'under assisted' during normal driving, meaning the pedal is harder to push than I would expect, though all the brakes work well, and the car stops great if you jump on em.
No leaks I can see, and the brake fluid level is good.
I'm thinking vacuum leak or something internal in the booster? Other ideas? Idling in drive while stopped would be a very 'high vacuum' condition, so I wonder if the booster is leaking vacuum but occasionally during high vacuum conditions the booster works?
Textbook case of failing master cylinder. The seals seal in a positive feedback loop when stomping the pedal, but bleed past the seals when holding gently.
Master cylinder like Pete says.
Maybe your bork booster is braked, instead. 
Tom Suddard
Director of Marketing & Digital Assets
10/31/20 3:46 p.m.
Yep, needs a new master. And if the master was leaking fluid into the booster, it needs one of those, too.
Has anyone mentioned that you have classic symptoms of a master cylinder failure yet?
Just asking.
As mentioned above, internal master cylinder failure. Do not continue to drive it. I had a customer with this condition and he could not afford to have it repaired. I told him to take it directly home and park it until he had the funds. He did, however he hung the keys in normal place, his sister came home, took the car and crashed it.
I once put a buick up a runway truck ramp.
It had the same symptoms for a few weeks before that.
Put a master cylinder in it, instead of the alternative.....
I'm glad I read this discussion. I didn't know.
As the resident brake guy, I've got nothing to add.
Streetwiseguy said:
Has anyone mentioned that you have classic symptoms of a master cylinder failure yet?
Just asking.
Nah... my guess is the brake master cylinder.
Well i'm glad you posted here... I thought i was noticing this, but assumed i was paranoid after the events at road america since i wasn't seeing fuid loss.
New master cyl on the way for my fit..
Well then! Glad it was easy.
I've been meaning to change out my soft lines too so I guess this is my opportunity.
