How do i adjust rear brake compensator on vauxhall movano?

Hi, i've had my vauxhall movano for 4 years and it's always had an intermittent brake problem- when it's not loaded every few months the rear brakes will start squealing, snatching and then locking up, particularly in rain. I've had the rear brakes checked and the master cylinder replaced, to no avail. Finally someone tried adjusting the rear brake compensator and that seems to sort it every time. Apparently it was used to transport very heavy loads and now i've converted it into a campervan the weight difference causes the problem. But it would be great if i didn't have to take it to a mechanic to do it every time. I know where it is but how do i adjust it?? Many thanks, julie