The aircon in my T4 failed first at the O-rings where the engine bay pipework goes through the firewall. The next failure was in the compressor. The o-rings on the shaft behind the pulley failed big time and the dye that was already in the system ended up splattered all over the engine (looked spectacular in UV).

Basically all of the fails were gradual and mechanical and very straightforward for an air-con specialist to diagnose and repair. Not cheap for sure but not complicated either.