Wonder if anyone can provide some suggestions for the cause of my suspension problem

I have a C350e which is 4 1/2 years old. DOne just over 60K miles

2 weeks go when drive the children home from school the back end suddenly became very bouncy. No warning message on the dash board. I pulled over and checked and the suspension was right down on both back wheels. Looks fine on the front. I got back in the car and switch the ignition back on and it came up vehicle rising on dash display. Suspension came back up to normal level and drove home fine. I went out it in later in the evening and was all fine.

Next morning, on the way to school, got a dash message saying STOP VEHICLE - VEHICLE TO LOW. Before I had a chance to pull over that disappeared and when I did pull over (maybe a minute later) The rear and front suspension levels looked fine. Even when the message was on the car drove fine leading me to think that it was a false warning but I dont know for sure.

On the way home from school that evening, it did the same thing it did the evening before - back became very bouncy, no warning message but rear suspension low (wheel arch over the tyres). Again came back up when I stopped and started and pressed the suspension up button. I could hear the compressor kick in and bearing in mind that it is right down and the bags are therefore completely empty, it comes back up in a reasonable time (30 secs or so) so that seems to be working.

The car was not used from Thursday to Tuesday due to the issue, during which time the suspension stayed up where it should, so it seems the issue isnt a leak on any of the air springs. Similarly, when I switched the car on on Tuesday to take to get fault codes read, I didnt hear the compressor kick in, suggesting, as far as I thing, that the air reservoirs not leaking, because if they were surely the compressor would kick in to refill them.

On the way to the garage (less than 5 mins from home) the dash warning message to stop again came on and this time the rear suspension definitely dropped right down. Again, though, when I pulled in and pressed the raise suspension button, it came back up and was fine for the remainder of the 8 mile drive to the garage.

The code reading didnt give any errors for the rear which is what had been dropping to that point. Apparentlly there were codes for out of range for the left and right front suspension sensors and also an error for the compressor taking too long to raise the the vehicle.

Based on the symptoms, and that the compressor seems to be working and raising the car, and that it is an intermittent issue, my conclusion at that point (before error codes read) was that it was either a sensor issue - system thinking rear is too high and dropping the rear, but doesnt pick up the change and therefore keeps trying to drop it. However, the fact that the error codes dont indicate a problem with the rear suggests that is not right. I had understood that there is a single sensor for both the back wheeels but have been told on my one there are separate sensors for the left and right wheeels which also makes a sensor issue less likely.The other possibility was that it was a blead valve staying open when it shouldnt - my thinking is that the system was opening a valve to lower the suspension and that was sticking, leading to the read dropping completely. That would then lead to the compressor taking to long fault as it would be trying to bring up but the air put in goes straight out. However, the fact that the car was not warning when the system was too low the first 2 times it happened would suggest otherwise, unless, of course, it is both.

The technical info which the guy at the garage has managed to get apparently points to the compressor leaking somewhere but I cant quite get that unless, I guess, there is a valve sticking open which is allowing air to feed back from the rear springs and causing them to drop, but that would be an issue at the valve manifold rather than the compressor surely.

All the sensor connections etc were check and it was fine for a day or so and then I was going to pick my son up about 18:30 in the evening and I got the message saying STOP Vehicle again. Checked the back and it was fine but this time both the front suspensions were right down. Again, switching the car off and back on the car say vehicle rising on its own and came back up. Checked the fault codes the next morning and the same three were back. The next day it did it twice in the space of 45 minute (separate journeys) - rear dropped on the way out and the front dropped on the way back. No warning message for the rear but there was one for the front.

I am thinking this is one of those issues that if someone has seen the combination of symptoms before they might be able to help.

My fault analysis points to the dropping being caused by the discharge valve staying open when it has been opened. Car says one end it too high so tries to lower by releasing air through the discharge valve, but that is staying open and it is dropping too far. Compressor kicks in to raise back but as the discharge valve is open then it cant do it hence that fault code. Question is, if I am right, is it because the valve is sticking or is it because the controller is holding it open? It is a solenoid valve which will certainly be normally closed (closed when no power). The fact that whenever I switch the ignition off and back on it comes back would be consistent with the controller holding it open and it then closing when the power is cut to the controller??

Anyone come across this issue before or otherwise have any suggestions, in particular how might test the controller and/or the discharge valve?

Thanks