C70 S70 V70: a complex problem?ets/cc/hissing

[Redskiies]

Hi guys!

My 2000 c70t5m coupe developed some symptoms and an annoying habit, so it is time to open a thread and start the guesswork, that might help some other people later.

I'll cut to the chase with the symptoms:
-ETS light randomly on/off every couple of 20miles, depending on drive style
-Cruise control won't engage most of the time, when it does, it either way disengages straight away, or works without a problem (kind of a lottery)
-Brake pedal hisses while pushing it in, stop hissing once its pressed in, no hissing when i release the pedal, when the car is off, 3 presses, and the pedal stiffens up.

What is the aim of this thread:
-ETS permanently off
-Cruise control working every time
-hissing can stay, as long as it is not lethal

What we can cross out:
-yellow brand new throttle body in 2015
-no weird idling, no hesitation, drives like a dream, no weird behaviours of the ETM at all
-abs works, even after changing wheelbearing on the front left
-there are no vacuum lines under the battery tray (ye i know, wtf? have they been relocated on the c70s?)

what we can expect knowing the previous owners and their "servicemen's work":
-brake booster leaks somewhere? not the diaphragm, as it does not hisses after the pedal stopped moving either way. I assume something on the cabin side, either way the ring or the cover. It sounds pretty close, not behind the firewall.
-throttle position sensor. I assume, that even they changed the throttle body, they haven't changed the position sensor. Knowing the "mechanic's" work during the last service (in 2015), I can assume, that he cheaped out on essential bits.
-ABS sensors might need a clean? they looked quite clean 2 weeks ago when we changed the wheelbearings.
-I couldn't get a clean picture of the brake position sensor due to having way too many pedals crowded in that small place.

Let the guesses begin, feel free to ask, guess, and pinpoint what else should i check. I tried to look around as much as possible before posting, and crossing out as many stuffs as I could with the informations found on other forums and threads, but it is quite random, most of the people have different kind of hissing problems (the opposite way around, when they release the pedal).

due to the fact of the rarity of the spec of the car /and the problem I face, I am kind of sure, that it worth sorting the car out....after spending all this money getting it back to its former glory.

edit: none of these actually affect the car in daily use, throttle works great, brakes are lovely, the cruise control is annoying during long trips, but this is more like a prevention, than an urgent repair