We had that problem years ago and on advice, replaced the spring in the cylinder...didn't make any difference.
In our case it was because the pads were worn and the piston was at full excursion.
I did the washer trick and that worked for about two years and it started to slip again - more pad wear.
There is still plenty of meat on the pads so what I did was take off the brake mechanism, take off the front caliper and drill a hole in it in the direction of the piston thrust, threaded the hole and put in a bolt with a lock nut.
Now I can adjust for pad wear !
I really need new pads but Maud doesn't have pads that look like ours. One day I will get them relined at a brake shop.