Parking break ?

I would have to disagree with you. I do not believe it would be anything other then as if you simply push hard on the brakes. You have anti lock brake systems that take over. I have had an air hose break and that is the same thing. I have run over a piece of tire in the road way that took out the brake lines. No it is a very controlled slowing of the vehicle.

The system we using in trucking is very similar to what the railroads use. Those brakes go into emergency when the airlines break. The lose of pressure forces the brake shoes to apply by way of a piston pushing against a brake lever. Truck systems have a spring that keeps the brakes from fulling applying in a rapid air loss situation. However, just to be sure, try it and let us all know how things are going in your new career.

 

For those of us who have been doing this for some years and have blown an air line defending a grade here is what can happen.............
First the low air buzzer goes off and you have a holy $$$$ moment.
second you try to shift down as quickly as you can with out much success.
Third you watch the buttons pop out on the dash and you get about 8 seconds of semi comfort as the spring brakes apply and you start to slow slightly.
Then the real fun begins as you look in the mirror and watch your brakes go up in a cloud of smoke and next you are just hanging on for the ride going faster and faster.
If you are as lucky as I was there is one last up hill grade before you hit town and you can down shift some as you go flyin up the grade.
You mange to get shifted down but still in the high side as come to a stop just before the crest of the hill.
You thank god you made it without killing anyone or your self and then you realize the ride isn't over yet.
Now you are headed backwards down the grade and you pop the clutch on the old 79 kW with a 435 Detroit that you are driving and you are descending the grade backwards in the high side with smoke coming out both chrome air cleaners as the old Detroit is now running backwards.
Now you get an education on straight backing in the high side at a high rate of speed.
Finally you hold it straight enough and come to an awkward stop in the valley between the two hills thanking got ther was no traffic coming when you came down the hill backwards grossing 80,000#.
This is an absolute true story that I don't ever want to repeat.

 

Blowing out airlines will rapidly release the air. But pulling out the knobs does it quicker. I would expect a total lock up without abs working. At 55mph, total loss of control, maybe a roll over. Depending on which brakes work the best, it will pull on one side, causing to ditch the rig, hit the grass, and that will start the roll. Most likely death, flatbed driver with a load, that will increase the chances

 

ABS works on the service side, the modulator valves are located between the service relay and the service side of the brake chambers. They have no effect on the parking circuit. You dump the air holding those big springs back and those brakes are coming on hard. Blowing the trailer supply line will have the same effect as pulling the red knob. Trailer brakes are going to come on and there isn't a thing you can do about it.

 

The times I would have like to been a fly on the window....

 

Locked trailer brakes wouldn't stop a truck doing 55. I tried it once at 50 and the truck barely lost speed.

 

I don't think the brakes would be a problem I would probably wreck getting my tools out of my side box to pull out the tractor protection valve .

 

The excitement level would ddepend entirely upon whether or not you are loaded, and how many axles have spring brakes. Most road tractors (especially "fleet-spec" trucks bought with cost and weight savings in mind) only have spring brakes on 1 drive axle, and so even if you're empty, it won't really be very exciting.

It only takes 40-60 psi to overcome the spring in the parking brake chambers, so in essence, that is all the more application pressure they can provide. It isn't like the service brakes where you can put 100+ psi of pressure into the can...just that little 40-60 psi spring. In other words, popping the valve is no different than applying 40-60 psi on the service brakes to both trailer axles and 1 (MAYBE 2, if your truck has them) drive axles. If you're empty, those axles MIGHT lock up. Loaded? You'll just ease to a stop...eventually...

 

That won't happen,both brks are far enough from your seat and they take a little force to pull them.