Did a bunch of searches, but couldn't find any clear answers, so here goes.

Have been getting weird issues on my EF XR6 wagon over the past week:
- Have minimal braking when I first start the car from cold, takes a second or two to start slowing down, no matter how hard I press the pedal, then it stops great. Feels like no power assist when I first hit the pedal. It gets progressively better over the first few stops, then braking is back to normal as long as I drive casually. If I put the boot in, then try to stop, it goes away for a second or two again - not fun coming into a tight corner . Pumping the pedal multiple times before moving off makes no difference.
- Am getting a constant light hissing sound when I hold the brake pedal down.
- This may not be related, but have been getting what sounds like a repetitive low-frequency light rubbing/scraping or whooshing sound that increases in frequency with road speed, not rpm. Looked for anything rubbing against the tyres or similar, but found nothing. It seems louder when braking, but that could be my imagination, as it will sound louder when not competing with engine/exhaust noise.

I'm suspecting the brake booster or check valve or vacuum hose leading to it. Maybe seized brake caliper, but then the other three calipers should slow the car down just fine. Will take a good look in the morning, but wanted some ideas of what to look for. Thanks for the help.

Well, swapped the brake booster this morning, and braking issue seems to be fixed. The guy at the Ford wrecker used to race a full-competition EF Falcon; says he had to replace the brake booster every 4-5 meets. The heat from the headers would melt the rubber diaphragm in the booster. That explains why I couldn't find a vacuum leak in the booster: the atmospheric pressure side get its air from vents around the shaft - under the dash in the passenger compartment.

The other low-frequency sound is still there. It's proportional to road speed, with a frequency about once per two tyre rotations. I don't know of anything that cycles that slowly in the car. Weird!

You can disable your ABS by pulling the ALB fusible link.
Take it for a run and see if your frequency thing disappears.