Hauling on the handbrake will never provide much retardation- they only need to be 16 % efficient to pass MoT at gvw 525kg. Once you have clean shoes correctly sprung in drums which are clean and score free all you can do is axjust the cable correctly. I got a marginal improvement by replacing the handbrake cable which had marginally stretched even though only ever used at MoT test. My old tester, now retired, used to want the lever pulled hard from outside the car to get the best result - hardly practical but never an issue at MoT on mine.