yes there is a low pressure switch that prevents the compressor clutch from pulling in when its low on refrigerant.
having it charged "every so often" is a sure sign there is a leak somewhere. It could be a simple hose or, (if you're real lucky!) just a leaky Shrader valve.
Using a dye test or a Halogen sensing device (beeper) a leak can usually be spotted right away. It's what is causing the leak that will determine the next step and cost.
But WHATEVER you do, don't convert it to R134A

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!