What about my AC high pressure cut of switch?

Dear list-members,

I'm lost here. I have AC troubles and I can't find my answers in the archive. I have had my system converted to 134a by a specialised company. (new compressor, new orrifice, new accumulator, flushed etc.) by the book I would say. It worked for over a year and then all of a sudden it stopped cooling after getting very cold. The season was over, so this spring I had a new fill (with lots of that stuff wich lights up with Ultra violet-light) and after 3 weeks it was down again.
I have checked the system completely. I have had the condesor out of the car and checked it thoroughly (with UV and glasses). I even got the evaporator out of the dash, wich showed no leak at all.
It seems that the low pressure switch is broken because the clutch from thecompressor still engaged after the system was drawn empty by a "AC-machine" 

But the strange thing is, and this is my question to you all; I could not find a high pressure switch and safety valve! WHERE ARE THEY? And if compressors should cycle on and off all the time, which component is controling this?
The workshoop manual is not very clear at this specific point. The switch is mentioned but there is not one picture showing the switch in the entire book (except fig 22 at N:03:02). Take for instance pic 55. at N:08:18, no switch where it should be. The switch is present at the wiring diagram at N:10:13 but the wiring diagram in my garage (uge poster) has none!
What is going on here?

I'm gonna have the components tested individually. There shlould be a way tho do a leak test with pressurisation. I'm pretty sure they won't find a leak. The next thing I could do is replace all the hoses. Once I heard that they can be porous. Do I realy have to split chassis and body for this?

I hope you can help me out here. An answer to my questions could really help me.

Regards,

Michiel Bohmer
(VIN #2998)