At work we did a lot of these tests when R134a first came out. Here's the deal.

Run the compressor at low speed, low blower for a few minutes to return as much oil as possible to the compressor.

Pull the compressor and drain as much oil as possible out.

Replace the leaking hose.

Replace the receiver-drier with a new one.

Reassemble with new o-rings at each joint that was disassembled. Don't mess with the joints that weren't disturbed.

Add a full charge of PAG oil (find out what is the correct oil amount for your particular vehicle).

Pull a vacuum for at least two hours.

Charge with R-134a. Use about 10% less than the factory specified charge of R-12. You can get some indication from the sight glass, but with R134a it may still be "foamy" even when fully charged.

Performance can be improved at idle by fitting a pusher fan as was done by Volvo when they changed to R-134a. The fan probably needs to run off a relay which is energized when the compressor clutch is engaged.

Your performance will never be as good as original with R-12, but the only way to achieve equal performance is to redesign the entire system.

At a minimum you need a set of A/C gauges and a vacuum pump. A REAL vacuum pump, not the el cheapos. Or you can borrow one, or maybe rent one, or pay an A/C shop to pull a vacuum for you. If the A/C shop, maybe you can get them to charge it with charging scales too so the charge amount is accurate.