Back in the spring I replaced a leaking compressor and the dryer. I then had a very good independent MB shop double check the system, put a vacuum on it, and recharge. It's worked great for several months. 3 weeks ago it was 100 degrees in Chicago and I was on the highway going about 65 - air blowing nice and cold. All of a sudden I heard a belt squeal followed by steam/vapor/gas coming out both sides of the car. Initially I thought the cooling system blew (also rebuilt at same time) but a quick glance at the temp gauge showed the engine was fine. I then noticed that the air was no longer blowing cold. What I saw was the freon escaping.

I've checked the codes in the HVAC display - none. I've also checked the pressure - 6.6 - so clearly low. I can't find anything obviously wrong with the hoses nor the condenser w/ a rock strike or crack. They all look good. Normally when freon is leaking there would be a wet spot from the oil. I'd also think with the gas venting out in such a spectacular fashion a breach would be obvious. I can't find anything wrong on a visual inspection - but admittedly haven't put the car up on a lift and removed the underbody tray to inspect the compressor closer.

So my long winded lead-up.....does this system have a "blow off" valve if the system pressure is too high? I'm wondering it maybe it was over-charged and with the 100 degree temp and highway speeds the system vented itself??

I highly doubt Mercedes manufactured a vehicle that intentionally releases fugitive emissions in the form of freon. I've owned my C43 for over 8 years and have never seen anything like you are describing. There must be a ruptured hose somewhere

Lucky you! I have to pressurise each year and on the vacuum test before filling it holds When working on my engine mounts, I did see some yellowish liquid hanging on the end of a compressor bracket. It's not coolant as that is now blue after last year's flush and replace, so could be HEVAC oil from the A/C gas mixture

Some systems do in fact have a vent/blow-off feature - sometimes it is even part of the compressor. I just don't know if the 202 does. I have a call out to Behr to try and figure out if this particular compressor does but that won't cover the possibility one exists somewhere on the rest of the system. This is actually pretty smart - the engineers know at what pressure the system will fail - a pressure release that prevents catastrophic damage is better then the damage + the gas release. My hoses are all fine as well as the visible part of the condenser. The venting happened under the engine bay - so it isn't the evaporator either. What I haven't had the time to do is remove the under-body tray to inspect the compressor nor remove the fans and possibly the radiator to get a better look and the condenser. If I knew the system had this feature and simply vented itself the repair may be as simple as a recharge.

Your best bet is to refill it with UV die and refrigerant and run it again and use the UV light to detect the leak point.

Fully agree and refill with the specified quantity only. The quantity is noted on the cover of the electronics box of tricks

Step 1 is to confirm or eliminate the possibility the system has a pressure release feature. No reason to refill the system until I know why/how it lost freon in that sudden of a manner. It's not a small leak we are looking for and the system is still holding pressure at 6.6 over a month later. I'm betting it does....just need to figure out if it's on the compressor like some cars have or elsewhere on the system. Maybe one of the ports on the dryer? If the system does have this feature - will likely replace it as it could have been the problem (vs. too high of pressure). Surprisingly little info on the details of the system. But since it's now fall here and I'm considering a 55 swap this job is better to delay. Don't want to evacuate/capture the remaining freon, disconnect, reconnect, place under vacuum, and re-fill twice. Not a DIY task b/c of the equipment required.