On some of our trailers, when you apply the park brake on the unit, I can hear all air is dumped out of the chambers on the trailer, but on some other trailers there is no sound from the chambers at all when doing the same thing suggesting that the brakes are only on, on the unit.
Why is this?? It’s the same truck every time just different trailers.
Fuuny the bulkhead on the tesco trailer dont look damaged to me.
I wonder if the “zero pressure” trailer tank had “sucked” the brakes on?
Hell of a good job it wasnt some amatuer like me who serviced them eh!
Even better if an “expert” such as yourself had given them a look over before.Im sure the trailer could have got itself home without the aid of a unit.
Units pump air down the yellow line when you press the foot brake or apply the hand brake.This signal opens the valve in the,for a better word, the valve block in the trailer.
This applies air to the service side “diaphragm” (OAD) .With a 2 line system there is no way that the chambers can dump air when the hand brake is applied,trailer tank pressure applies the pressure.The hand valve will dump pressure from the tractor brakes to apply the spring brake units on the tractor only but feed air to the trailer.
Secondary braking on the tractor,which is part of the mot,Is when you pull the hand lever slowly to give a controlled release of air from the tractor brakes to allow the springs to apply the brakes but at the same time this feeds air via the trailer control valve to the trailer brakes if a trailer is coupled .
The spring brakes on a trailer are under total control of the red line.They are the “mechanical” part of the trailer system,If air pressure is lost these are the “secondary” part of the system that is indepedent of the tractor.At test this system has to be able to apply 16% of GVW without any air supply.
As an aside about the tesco trailer clearly the rear steer bar has snapped,the ring plate has failed or the driver has missed the clam shell block when he has coupled the trailer(most likely) causing the trailer to drop off and as you see the spring brakes have locked the brakes on before it whacked the tree.
FarnboroughBoy11:
On some of our trailers, when you apply the park brake on the unit, I can hear all air is dumped out of the chambers on the trailer, but on some other trailers there is no sound from the chambers at all when doing the same thing suggesting that the brakes are only on, on the unit.
Why is this?? It’s the same truck every time just different trailers.
Silencers on the air exhausts?
Bking:
Lusk what the hell is a “handbrake” on a trailer?When you take a trailer for test and they ask you to apply park brake what the hell do you think they are asking you to apply?
The bloody spring chambers as a secondary brake system doh!
all air is dumped from chambers and the trailer is on stand alone braking.
Know what your talking about before you spout crap,You might end up sounding like your mate OAD
My most sincere apologies Mr Know It All for not adopting your employers terminology and referring to the park brake as a hand brake.
I have to go out shortly but I’ll leave “my mate, OAD”, to explain the operation of the secondary brake to you.
Own Account Driver:
Immediately, we can also see, their sophisticated system of planting trees along the side of the road leaps into action to supplement the insufficient braking force of the springs to prevent further carnage.
That photo must have taken some finding…brilliant one “mate”. 
FarnboroughBoy11:
On some of our trailers, when you apply the park brake on the unit, I can hear all air is dumped out of the chambers on the trailer, but on some other trailers there is no sound from the chambers at all when doing the same thing suggesting that the brakes are only on, on the unit.
Why is this?? It’s the same truck every time just different trailers.
Sorry, I’m not trying to be clever with you, but when you say the park brake are you referring to the normal handbrake? Agree with you that something does not appear to be quite right. How old are the different types of trailers?
Lusk:
FarnboroughBoy11:
On some of our trailers, when you apply the park brake on the unit, I can hear all air is dumped out of the chambers on the trailer, but on some other trailers there is no sound from the chambers at all when doing the same thing suggesting that the brakes are only on, on the unit.
Why is this?? It’s the same truck every time just different trailers.Sorry, I’m not trying to be clever with you, but when you say the park brake are you referring to the normal handbrake? Agree with you that something does not appear to be quite right. How old are the different types of trailers?
Yeah the park brake/hand brake on unit.
All mixed, some new, some old, straight frames, double deckers.
I think bking explained it a few posts up and makes sense to me but I’m staying out of the discussion as I really have no idea
A couple of simple diagrams showing the workings of air brakes. diahttp://uk.images.search.yahoo.com/se … ystemgrams
Lusk:
Mike-C:
FarnboroughBoy11:
how can it jack knife when the braking was done all from the trailer wheels which is trying to pull the unit back, a jack knife is where the trailer pushes the unit round.When the red airline goes the brakes come on in the unit and trailer. I’ll guess the units air tank empties out quicker than the trailers ? Either way the unit will lock up quicker than a loaded trailer which would have more weight to push it.
Trailer only Mike. The multi curcuit protection valve should stop this from happening so in other words, the air tank on the unit which feeds the red air line would deplete but all other tanks on the unit would remain full.
When I dropped a trailer and pulled away without releasing the red air lines, it drained all the air from the tanks on the unit. It was a Premium. It also drained all the bravado from my body. I felt like a right bellend.
FarnboroughBoy11:
Lusk:
FarnboroughBoy11:
On some of our trailers, when you apply the park brake on the unit, I can hear all air is dumped out of the chambers on the trailer, but on some other trailers there is no sound from the chambers at all when doing the same thing suggesting that the brakes are only on, on the unit.
Why is this?? It’s the same truck every time just different trailers.Sorry, I’m not trying to be clever with you, but when you say the park brake are you referring to the normal handbrake? Agree with you that something does not appear to be quite right. How old are the different types of trailers?
Yeah the park brake/hand brake on unit.
All mixed, some new, some old, straight frames, double deckers.
I think bking explained it a few posts up and makes sense to me but I’m staying out of the discussion as I really have no idea
I think it is either, as has been said, silencers aren’t fitted or what you’re hearing isn’t what you think. If you want to get a feel to what the air being dumped from the spring part of the chamber sounds like it’s the noise you get when you fill the trailer with air with the red line and pull the park knob out on the trailer.
Bking:
Units pump air down the yellow line when you press the foot brake or apply the hand brake.This signal opens the valve in the,for a better word, the valve block in the trailer.
This applies air to the service side “diaphragm” (OAD) .With a 2 line system there is no way that the chambers can dump air when the hand brake is applied,trailer tank pressure applies the pressure.The hand valve will dump pressure from the tractor brakes to apply the spring brake units on the tractor only but feed air to the trailer.
Secondary braking on the tractor,which is part of the mot,Is when you pull the hand lever slowly to give a controlled release of air from the tractor brakes to allow the springs to apply the brakes but at the same time this feeds air via the trailer control valve to the trailer brakes if a trailer is coupled .
The spring brakes on a trailer are under total control of the red line.They are the “mechanical” part of the trailer system,If air pressure is lost these are the “secondary” part of the system that is indepedent of the tractor.At test this system has to be able to apply 16% of GVW without any air supply.
Ok, you’re sort of learning some things. You do need to qualify this though that this really applies to a trailer with a system where the emergency relay valve and load sensing valve function are not included as separate components in the system any longer because they’ve either been included within the EBS modulator or park/shunt valve.
You’ve gone way off track on the secondary. It is the first part throw of the parking brake lever in the unit and is analogous to a lever version of the pedal and is doing exactly the same thing until you click it on and only then does it become the spring park brake, prior to this it is the service portion of the chamber, on not just the drive axle, that is being applied gradually as the lever is pulled.
The clue is the secondary braking system needs to be a genuine alternative to service brakes, allowing you to stop in a controlled manner if the pedal has failed. If it was just the parking spring on the drive axle the drive axle would lock and you could jack knife. The spring brake system on unit and trailer was only ever designed for the scenario of simply to continue holding a vehicle or trailer in a stationery position once it has already got into that stationery position by other means. Now, unwisely in my view, trailer brake system manufacturers have started to use springs, as dynamic brake actuators because it suits an agenda, with the advent of EBS, of just having one big modulator block component on trailers that does everything.
I haven’t been asked to operate the ‘hand control valve’ to check secondary brake performance at MOT for a long time now. I have been asked to fit the adapter to the trailer third air line to check the trailer secondary brake however.
cav551:
I haven’t been asked to operate the ‘hand control valve’ to check secondary brake performance at MOT for a long time now. I have been asked to fit the adapter to the trailer third air line to check the trailer secondary brake however.
That must be an old trailer?
Yes it is getting on a bit. A 28 tonne Southfields 10 metre tandem - on Hendrickson running gear if I remember rightly.
One last time oad When you move the hand brake lever from “off” to max just before it clicks into park look at the dash you will see the park brake warning light illuminate.
This park brake light has a switch usually mounted on the rear axle relay valve.When full air pressure is supplied this switch is turned off extinguishing the light.Tractors do not use quick release valves for the spring brakes they use relay valves as they have a much shorter operating time.They also use relay valves on the service brake circuit.
Back to the switch,If you jump in a unit with low air pressure and let the hand brake off the park brake light will stay illuminated till enough air is fed to the relay valve to lift the springs in the chambers.So doesent it stand to reason that using the “secondary” part of the hand valve which lights the bloody park light signifies a loss of pressure to the spring chambers.
Enough of this anyway there are none so blind as those that will not see.
Bking:
One last time oad When you move the hand brake lever from “off” to max just before it clicks into park look at the dash you will see the park brake warning light illuminate.This park brake light has a switch usually mounted on the rear axle relay valve.When full air pressure is supplied this switch is turned off extinguishing the light.Tractors do not use quick release valves for the spring brakes they use relay valves as they have a much shorter operating time.They also use relay valves on the service brake circuit.
Back to the switch,If you jump in a unit with low air pressure and let the hand brake off the park brake light will stay illuminated till enough air is fed to the relay valve to lift the springs in the chambers.So doesent it stand to reason that using the “secondary” part of the hand valve which lights the bloody park light signifies a loss of pressure to the spring chambers.
Enough of this anyway there are none so blind as those that will not see.
Yes, but that’s not only the drive axle the secondary is acting on it’s operating the service portion of the chamber on the other axles.
As has been said, secondary (the clue is in the name) acts on all service brake points.