1990-1991 STE models could have the PMIII as an option. It was not standard equipment on those cars. If you happen to run across a 1990 STE Turbo, the system did come standard on those.
If the PMIII has been maintained, they are a lot easier to deal with. (Regular brake fluid flushes and accumulator replacements as needed.) That said, most people neglect them and there are a few things to look for on a PMIII car.
First off, when you turn the key on, you should hear the pump motor running to pressurize the system. It should run immediately after turning the key on and continue running for up to 60 seconds. If it runs longer than that, doesn't shut off, or doesn't run at all, there are bigger problems.
Next, check how many presses of the brake pedal it takes before the pump has to run to pressurize the system again. If the accumulator is in good shape, it should take 3 presses before the pump has to cycle again. If the motor cycles every time the pedal is pressed, the accumulator is likely bad.
Look at the dash- are there ANTILOCK or BRAKE lights on? The system will set codes for low accumulator charge, long pump run time, bad wheel speed sensors, etc.
Take a look at the fluid level and condition in the reservoir, also for fluid leaks on the rear pump housing of the PMIII. They can develop cracks over time and leak here. Also look at the rear proportioning valve near the fuel filter, these will leak sometimes as well.
I think I've pretty well covered the common bases. More often than not, a full system flush and a new accumulator will make the PMIII work properly again. The downside of the system is that there isn't a failsafe for it, if it does fail. They are 100% electronic. Others have vacuum brake-swapped their PMIII cars, but it is a very tedious and time consuming task.