new air springs and sensors,still dropping

I recently changed out both rear air springs and sensors at the same time on my 03 D2 as they were around five years old and I had a spring overinflate due to a sensor going faulty...All was well and I did a few long trips no problems but a couple of weeks ago I came out one morning and both sides had dropped right down.

Started the car and nothing happened,all doors closed ,then I disconnected the battery and restarted the engine and the springs came up ok...was fine again until today when I got home,opened rear door to take out some stuff,heard the usual "hiss" thinking the it was just going to settle ,but it dropped right down again. Again they wouldn't come up until I disconnected the battery...Any ideas.. as I thought changing both springs and sensors I would be trouble free for another few years..

If one sensor was bad at a time usually a target height recalibration with tester is needed and to erase the stored fault codes from ECU otherwise the system might be mixed up and make all kind of tricks...so if this was ruled out and still no joy we'll speak then

Thanks Sierafery,as it happens the ride height had been a tad uneven so I had planned to leave it into a specialist I use periodically to have it recallibrated as well as recharging a/c ..So lets see what happens..Btw I didnt go near the compressor side when changing the springs this time,just cut the pipes at the springs with a sharp knife.However last time I did the same job I opened the compressor nuts and had some problems later on,until I eventually tightened up the nuts further...
While I have you (and if you dont mind) am also thinking of getting brake fluid changed as I have newish disks on the front and new brake pads front and back but they are a bit spongey and not as good as they should be.(2003/130kmiles).So my question is do you need Hawkeye to do it properly or is it just a case of keep topping up and bleeding in order .I do a lot myself but haven't done this before(on this car)..