In a car, the handbrake could be used to stop the car if the regular brakes failed.
With air brakes, if there is a leak, the brakes will fail on rather than fail off. But what happens if the brake pedal jams, so it doesn’t stop and you want to use the park brake to stop? Can you control the stop with the park brake or is it going to lock up all the wheels?
You can stop the truck with GENTLE application of the park brake. Everything will lock if you’re rough with it. But this is not a recommended thing to do unless absolutely last resort.
Pete 
As Pete says, don’t yank it on or you will kiss the screen,
If it’s an Artic, then slowly applying will put your trailer brakes on first, before all the rest lock up,
Munchausen Syndrome by Proxy. Remember the Scania?
Something I’ve always wondered… if truck brakes fail ON when there’s no pressure, how come they also say if there’s no pressure (e.g. you didn’t build it up before moving off) you risk not having enough air to stop again?
SJB:
Something I’ve always wondered… if truck brakes fail ON when there’s no pressure, how come they also say if there’s no pressure (e.g. you didn’t build it up before moving off) you risk not having enough air to stop again?
I doubt very much any modern braking system would completely fail in the event of low air pressure, it would activate the spring brake and stop the vehicle
The sensors in the braking system will not allow the vehicle to move off until the pressure is built up as some of that energy is stored in the vehicle to split the brakes between the normal running system and other items such as power clutch, gearshifts, etc.
Usually there is enough pressure in the system to brake and stop, all other systems will work secondary to the brakes.
Toddy2:
As Pete says, don’t yank it on or you will kiss the screen,
If it’s an Artic, then slowly applying will put your trailer brakes on first, before all the rest lock up,
Is that the same when you use the foot brake - trailer brakes come on a bit before the unit?
Thanks.
Yes you can perform a controlled stop via the parking brake, worth practising gently but only when safe to do so, as said if you yank the thing on the brakes will be applied full, and quite likely without operating the brake lights.
At one time there was often another handle in the cab, the secondary brake or ‘dead man’, these usually applied the steer axle brakes and the trailer only and was meant you could stop fast without braking the drive axle in the event of a jack knife starting…in practice a jack knife is violent and sudden so you’d need to be well practiced in this to make it happen in an emergency stopping the jack knife strightening back up and then re-applying all when closing fast 
, those old systems often operated on a full seconday braking system fed by a third air line (the blue line) so if an air line burst you copuld still stop the vehicle safely.
These were the days before spring brakes, when the actuall parking brake was mechanical because if you lost air the brakes didn’t apply at all and the vehicle could run away, with spring brakes you won’t be able to move the vehicle until you have sufficient air in the system to operate the brakes.
Scanias of old had another handle in the cab, which the driver could control the trailer brakes alone from.
One thing i’d like to know, now we have electric parking brakes appearing, which i distrust completely, is it possible to perform a controlled stop with one or are they an on off switch, in which case where’s the secondary or back up braking system now, ie should the foot valve fail?
edit, i’m going to ask the EPB question on the main forum, bound to get some abuse there 
A Juddian said - we used to have 3 airlines before the days of 300 electric suzies
The dead man handle was great for getting you out of trouble and was easier to control than the 2 stage park brake handle we now have
The new electronic park brakes (like on Volvo) fill me with fear / a completely backward step imo
Toddy2:
The new electronic park brakes (like on Volvo) fill me with fear / a completely backward step imo
Glad its not just me, give a me a bloody proper park brake any time, at a glance you can see if its off or on.
Yes the dead man was easy to control, tended to be a largish lever moving through a quarter turn, had a positive action and wasn’t sprung so would stay in place, we’d use them as a park brake in normal driving, the official park brake on my first artic was a cable/rod linked brake drum on the back of the diff , you can imagine how secure that would be on a steep slippery road trying to perform a hill start
Juddian:
Toddy2:
The new electronic park brakes (like on Volvo) fill me with fear / a completely backward step imoGlad its not just me, give a me a bloody proper park brake any time, at a glance you can see if its off or on.
Yes the dead man was easy to control, tended to be a largish lever moving through a quarter turn, had a positive action and wasn’t sprung so would stay in place, we’d use them as a park brake in normal driving, the official park brake on my first artic was a cable/rod linked brake drum on the back of the diff
Remember the old levers about 3 ft long with a release rod attached to a sprung tooth that engaged in a slotted quadrant on the floor - the aec’s had these and you had to climb over them to get out the cab