The Brake Pad warning light system uses a four
plastic sensors one of which inserts into each brake caliper.
When the pads wear to a certain thickness, the plastic is worn away by the disc, eventually exposing a wire, which triggers the warning light. These give you plenty of warning before the pads wear to an unsafe thickness, at which the disc could be damaged, brakes become susceptible to overheating or even failure. Nearly every other car uses the pad backing plate as a warning; when you have no pad left, the screeching of metal on metal alerts most people to the fact that something is wrong with their brakes. (This often doesn't work; I have seen one side of a ventilated brake disc worn away!!)
The sensors are not cheap.
I suggest that you remove them now, to avoid them wearing to the point that they will not stay in the caliper ie necessitating replacement, if they haven't already. Mine had been worn away and tied up, out of the way, when I got the car, and I have not replaced them. I have the wheels off regularly, anyway (what a sicko!).
Porsche pads cost about AUD$250 for the set.
They are great pads for street use, though they tend to produce a fair amount of brake dust. They are a worthwhile investment. Their replacement is easy, provided that you own a 7mm Allen key (fronts; I haven't touched the rears, which are different) ... though I would be inclined to have the front wheel bearings cleaned, inspected and repacked/replaced if necessary, since the caliper will be coming off ...
You should top up the brake fluid, as Otto suggests, and see whether the 'Brake Pressure' and 'Brake Fluid' warning lights go out.
I use Shell DOT 4 Ultra Stop, which has a 280 degrees C boiling point. You
can buy it at your Shell servo, but I always look for the latest date stamp
on the side (I think that it is the date of manufacture, rather than use-by).
Ensure that only new fluid is used to flush your system and refill it.
You shouldn't use brake fluid between changes; if you do you
probably have a leak.
These are all simple jobs, which I do myself because I can (communing with
the car as I do so, though it often gets to hear a wide variety of swear
I'll stop now before I depress you further .. Just remember that
928 ownership is worth it (at least, we all think so!) Also, any
low-volume car is expensive to maintain, once it is no longer new!
Even things like 300ZXs, according to friends of mine ... the owners
of Falcadores and Excollas don't know how much they benefit from
economies of scale, and the fact that ownership costs drive the design
of these cars, if cost is all they care about! - Glenn Evans, '80
928 Petrolblaumetallic 'S' look 5 speed
I haven't had the pleasure on the shark.
Squealing is caused by the pads vibrating. Don't omit the shims on the back of some types of pads when you replace them - they help stop squeal. Sometimes they come off stuck to the old pad and get chucked by an unsuspecting changer. If you have a squealer try using pad glue - you glue the pads to the pistons so they don't vibrate. Its just a silicon type gooey compound and does not cause any removal problems. It just stops the vibration. Worth trying before you chuck the pads. It doesn't last for the life of the pads generally, but its not too hard to do. - Jon (silence is bliss) Riddett