TORONTO, Ont. – If the gauge on the dash reads 90 psi, the alarms aren’t buzzing and you completed your pre-trip inspection at the dawn of a new day, you expect your trailer’s service brakes to be there when you need them.

Not necessarily, says a report issued earlier this year by the Ontario Trucking Association. According to the report, brakes built to federally accepted standards can leave you without stopping power when you need it most.

Compressors can keep ahead of a variety of leaks, and the gauge on your dash will offer little advance warning of a problem.

You could be left with a sickening feeling in your stomach as you push the brake pedal to the mat.

What should you do?

Many truckers have balked at the association’s suggestion to pull the trailer supply valve – the button that looks like a red stop sign, and also called the tractor protection valve control – when the problem occurs, fearing that tractor-trailers will jackknife when spring brakes “dynamite”.

But it looks like both sides of the argument are correct.

On the flip side, trucks with full loads – especially multi-axle Michigan combinations that exceed 100,000 lb. GVWs – may have their best hope in applied trailer spring brakes.

“It depends on where you are, what your situation is, what your equipment is,” says Dale Holman of Truck Watch Services, which specializes in brake designs.

He cites one case in which a 140,000-pound combination was careening down a ramp with no trailer service brakes, and tractor brakes that had faded away from the heat of their stopping efforts.

“The tractor brakes were so hot, they weren’t doing anything,” he says of the brake fade, caused when drums expanded from the heat, beyond the reach of the friction material.

“You pop that valve (to apply the trailer spring brakes) – you’re going to get the equivalent of a 45-psi brake application. You’re not locking anything up here. You may save it from jackknifing by not using it, but it’s not going to help him stop.”

The situation is different for lightly loaded combinations, when tractor brakes alone should be able to stop the load, he says.

“Then you want to gradually bring your speed down,” explains Dwayne MacKenzie, director of operations and training at Fox Professional Driving School in Prince George, B.C. “Use the compression of the engine to do the braking.”

So, too, is it important to keep your hand off the shift lever, which should already be in the appropriate gear to handle the grade of any hill, with available engine brakes barking away.

“If you don’t get that gear, you’ve put yourself in a runaway situation,” he says, referring to attempts to shift.

Terry Braginton, an instructor at the Michigan Center for Decision Driving that offers much of its training on skid pads, agrees. “If you’re going downhill, you don’t want to take it out of gear. Your tractor alone will not stop that load.”

So what if the trailer brakes dynamite and the wheels begin to lock and smoke because you’ve pulled the button at the wrong time?

Driving instructors often repeat the mantra that “locked wheels lead” when describing situations that lead to jackknifes.

Since every trailer travels in a natural dogleg behind its tractor, and the crown of the road has you on an angled surface, the locked wheels begin to push forward.

And the reflection in your West Coast mirror begins to look a lot like the writing on the side of the trailer wall.

“You’ve got to stay in front of that trailer,” Braginton says.

“But if you overpower now, you’ve got two problems.”

One set of wheels is locked. Another spins. The jackknife closes more rapidly.

Once the angle passes 15 degrees, you’ve also passed the point of no return.

“You might be able to go a little past that, but not a whole lot,” he says.

Jackknife situations can be magnified with B-trains, MacKenzie adds.

“Whatever happens to the first trailer is magnified in the second,” he says. “The trailer will go into a whip, and to try to counter-steer, you don’t stand a hope.”

It’s a matter of knowing the equipment that you’re driving, and braking or slowing accordingly.