You can't go to a garage and buy a set of mechanical brakes for a horse.
A schoolmaster horse potentially offers many advantages to a rider but they mostly come with the problem of a hard mouth and insensitivety to the aids. Any experienced rider will know this - a novice rider might not. Some folks believe a schoolmaster is a good choice for novices - that very often is not the case because most school masters have never left the training arena. They are very often herd bound and reluctant to go out alone.
But let us assume we buy a good specimen.
I assume we talk about riding English.
I'd seek to find out why the training centre let the horse go. A good school master is worth a fortune.
I'd make a video of the horse being lunged in the arena.
I'd have the back checked by a horse physio.
I'd have the teeth rasped.
I'd initially fit a stronger bit than a snaffle - a Waterford would be fine.
I'd fit a running martingale - preferably a hunting design which fixes to the rings of the saddle and the girth.
I'd make sure the saddle fitted - I might even buy a new one.
I'd watch the diet - no heating feed. I'd watch the weight carefully.
I'd go back to the training arena and start again as with a young horse.
I'd start with in hand work - my head alongside his head.
I'd introduce it to horse bikkies
I'd do lots of pole work, lots of tight turns. Lots of starting and stopping. Lots of standing four square. We'd go up and down the paces.
There would be short lessons every day.
I'd teach the horse to respond to the legs and the seat.
I'd ride collected. I'd bring the horse's head down - slowly over months.
I'd gabble away to the horse all the time.
I'd stroke the horse - all over. I'd deliberately invade its space.
I'd keep my legs off the flanks.
I'd sit upright and tall - I'd keep my weight off the bars of the sitrrup irons.
I'd avoid going out in groups - I'd go out only with one or two carefully selected horses.
I'd make sure that the horse learned all the local routes.
I'd submit it to all sorts of frightening spooky things in the arena
Etc etc etc.
And when I had finished (if I ever did finish) - the last thing I would do, would be to sell this paragon of virtue.
Such school master horses need to learn to belong to one owner. Much depends on how long they have worked in the school as to whether they ever will. Every horse will be different.
Each will present a different set of problems for which solutions will have to be found.
As for brakes (breaks) - well the risk is always that such a horse will bolt when excited especially when in the company of other horses. It is unlikely one will ever stop the urge to run - it will become a question of whether the rider can control the horse at the gallop.
There is no standard process for converting a professional working horse into a private owner's pet. But take such a horse away from its training centre environment then for a time it will be missing its mates. It will need time to settle. Some horses will respond to a softer life, others will try to take advantage of their new owner. Many will miss the heavier work load in the riding school.
As with all horses - the rider's ability has to match the horse's capability.
If this Tucker is a character horse with potential - then keep him.