Hi there,
The answer really depends on what refrigerant the car uses. If it is using R12, the neoprene O-rings will be ok, as long as they are an appropriate size. If you have R-134a, they will leak. The molecule size of R-134 is much smaller, and with new O-rings there will be no old oil to plug up the microscopic holes in the O-ring's rubber, allowing the R-134 to pass through. R-134 requires barrier hoses and special O-rings to keep leakage down to an acceptable level, when using new parts. Old hoses seem to work ok without excessive leakage because they are saturated with a surface coating of oil. Most R-134 conversions seem to require yearly "topping off", if they last a year. This is because the molecules of R-134A can and do pass through the rubber of the O-ring seals and hoses.
Richard Wooldridge
'82 300D/4.3L V6
Etc...