R12 is coming down in price while R134 is going up fast. There is just no valid reason for converting an R12 system to 134 given todays refrigerant prices.

If you have a leak that is so bad that the refrigerant is going to come right out, then it is enough of a leak to find with an electronic leak detector. If you don't have one, a shop will check for you for a pretty low cost.

Statistically a converted system lasts about two years. A few years ago when R12 cost as much as $50 a pound, it seemed to make sense to convert. Now with 134 at $8 or $9 a pound and R12 at $14 a pound, where is the logic in spending a bunch of time and money to convert?

Best of luck with it,