In article , stickit@up- uranus.com spouted forth into uk.rec.cars.modifications...
If the overboost, or pressure transducer is leaking, you will hear a hiss coming from the passenger/left hand speaker grille, as they are both located below the speaker.
Some vacuum leaks don't show up as noise. So replace the hoses with decent silicon till you find it, then replace the rest to match (do a different colour to black, and then you know which you have done.
Also the Bosch recirc dump valve (if fitted to the late 8v, should be on APC cars I think) tended to rip their diaphrams and leak under boost pressure. They ar eabout £30 to replace, or bailey do an alloy one for about £90.
Standard boost should be about 2/3 into the orange section, on the way to red. It will spike to about 3/4, then drop back. Once that is sorted, you can start tweaking the APC.
BTW, do you have cruise control? If it is set to on, you will only get Base Boost (what you are describing). Like wise, if the brake and clutch switches for clutch are faulty and making a connection boost will be limited to base too. With these kind of faults it is more tweaks and scrap yard parts than seriosu rebuilds. I spent months trying to work out what was causing my base boost. Turned out the overboost protection was leaking vacuum. Disconnected and bridged the hoses, and normal service has been resumed.