“One of those handbrakes” says to me it might be an ARP4 which I seem to recall had handbrake issues from new? If so, advice may be along soon as certain procedures or mods were necessary.
Apart from that, could it just be that the cable has stretched? Apply the handbrake and check if there is any gap remaining in the handbrake slot on the gearbox tunnel. If the lever is hard up against the slot and you can’t squeeze a small finger in, you can easily take up some slack by adjusting the cable at the rear, beneath the tool tray. Handbrake cables are also well known to seize and break where the flexible outer covering breaks away and lets water in. Typically on tight curves where it turns down to approach the rear wheels. I wrap mine with Denso as a precaution.
