Our leyland DAF 45 has a problem with the brakes?

We have a problem with our brakes, when we have been driving for a while the front brakes stick on, sometimes they release and we carry on, recently they have not released and we have to pull over, the front wheel are very hot and smell, we've had a local lorry specialist look at the problem but they cant find anything wrong. the pressure doesn't appear to drop on the gauge when this happens and we've tried laying under the lorry listening for leaky air pipes but nothing conclusive, we're at a loss as what to do next, we can't afford to have garage look at it again only to find nothing and we have given up taking our horses out for fear of breaking down. Leyland DAF 45 R reg. any help or suggestions would be much appreciated.