during the 80,s mercedes started using aluminum evaporators with copper u tubes epoxied in at the ends.at that time they knew that this epoxy would not present a problem when used with r12[large molecules]now here comes 134a and all the exotic derivatives[e.g.fr12,freeze 12 etc]which contain a majority of 134a with something else to carry the mineral oil.all of these refrigerants have common problems when used with r12 systems.firstly just from a performance point of view 134a requires a 50% bigger condenser area just to work and then it will only be 75% as efficient as r12[r12 is an extremely efficient cooler and for many years we were all spoiled because there was no need to apply technology to r12 refrigeration it worked so well]imagine if you will that we still had only 4 cylinder engines but that we used rocket fuel and all you had to do to get more power was push in more fuel.engine technology would still be back in the 50's because no need to make it better. secondly,134a and all it's brothers have very small molecules and when used with r12 hoses it will leech out.now remember the evaporators with the epoxy it will make a hole thru epoxy and then make hole bigger as it leaks out.if your evaporator has been replaced with all copper you have nothing to worry about but replacing it is not a diy job.i am speaking from experience.this happened to me on my 88 300te.david poole european performance dallas texas.