Jamiegreen

Jamiegreen

Well-Known Member
Hey, work truck has come in with the problem of a low brake pedal when towing. Have taken it for a drive without a trailer and it brakes perfect, pedal is not too low, and "locks" the wheels no problem. But I've put 3.5 tons on and the brake pedal drops right down near the floor, but then works fine and "locks" the wheels. Obviously not ideal even if they are effective at the bottom of the pedal.
Had new callipers and brakes a week ago. And a new master cylinder a couple weeks before that (although britpart)
Have manually and power bled them multiple times with Hawkeye and can't see any air coming out.. any ideas? Tia
 
You seem to be exceeding the braking capacity of the vehicle. There's a lot of energy in an additional 3.5 tons hurtling down the road and trying to stop that with standard brakes is no mean feat. Perhaps you should check the local regulations as there are limits on how much you can safely tow without additional brakes. The last I checked, it was 750kg and that includes the weight of the trailer. Anything above that would require the trailer to have service brakes, parking brakes and breakaway brake.
 
Sound like trailer brakes to me.
I regularly tow with my disco 2, with nearly this amount on and sometimes with this amount on. It IS legal provided the trailer is braked properly, and not normally a problem, so I'd take a good long look at the trailer's brakes.
 
D2 brakes are **** stock.

BRAKE FADE!!

U have to plan a mile from a junction lol ;)

Especially when towing or you'll sail right through!

Worse on an Auto though, on a manual you can use the box to slow it down ;)
 
Henry_b said:
Extra weight

Extra Heat = Brake fade.
Click to expand...
Only if the weight is all taken by the braking system on the towing vehicle.
I have towed where the trailer brakes were so sensitive it felt like the towing vehicle was being slowed by the trailer.
My huge box trailer is a twin axle with brakes on all four wheels. Admittedly they are not disc brakes but I keep them really well adjusted and they work absolutely fine.
The braking mechanism on the towbar also has to be serviced and properly greased, but it isn't rocket science.
But if they are maladjusted, or the tow bar is locking before actioning the brakes then the towing vehicle has to do more of the braking, simple really.
 
Stanleysteamer said:
Only if the weight is all taken by the braking system on the towing vehicle.
I have towed where the trailer brakes were so sensitive it felt like the towing vehicle was being slowed by the trailer.
My huge box trailer is a twin axle with brakes on all four wheels. Admittedly they are not disc brakes but I keep them really well adjusted and they work absolutely fine.
The braking mechanism on the towbar also has to be serviced and properly greased, but it isn't rocket science.
But if they are maladjusted, or the tow bar is locking before actioning the brakes then the towing vehicle has to do more of the braking, simple really.
Click to expand...

Doesn't explain the soft pedal though
 
Interceptorxj said:
Yes but brake fade doesn't cause the pedal to travel further , only air in the system can cause the pedal to travel further as air compresses where fluid doesn't.
Click to expand...
Brake fade is caused where so much heat is generated in the disc and pads that the heat transmitted to the fluid makes it boil, when that happens the pedal and cylinders finds itself having to try and compress a gaseous fluid rather than a straight fluid. This is not really possible, so the brakes become largely ineffective except for those where the fluid does not boil, i.e. maybe on the rears.
As OP says they work, but just further down, I expect they are just haing to work harder, and that if he pumped them the pedal would come higher.
So I still say, check out the entire system on the trailer.
 
Stanleysteamer said:
Brake fade is caused where so much heat is generated in the disc and pads that the heat transmitted to the fluid makes it boil, when that happens the pedal and cylinders finds itself having to try and compress a gaseous fluid rather than a straight fluid. This is not really possible, so the brakes become largely ineffective except for those where the fluid does not boil, i.e. maybe on the rears.
As OP says they work, but just further down, I expect they are just haing to work harder, and that if he pumped them the pedal would come higher.
So I still say, check out the entire system on the trailer.
Click to expand...

That is call brake fluid vapour lock.

Brake fade is Brake fade when the brake pad and the brake disc no longer generate sufficient mutual friction to slow/stop the vehicle

He could have both
 
The OP needs to clarify how soon the pedal drops, from his post I read it as the pedal goes to the floor when he hooks up a trailer and goes back up when he uncouples the trailer???
 
Henry_b said:
Doesn't explain the soft pedal though
Click to expand...
Didn't say it was "soft" or "springy" which are usual symptoms of air in the system, just said it all worked but lower down.
Have you ever driven a car where half of the system has failed on a split braking system? I have, it frightens the Ferk out of you cos the brakes work but much further down.
What OP describes is like this. So I think the brakes on towing vehicle are having to work much harder, OR half the system IS experiencing fade, and the other half isn't.
Whichever, it is, as the brakes work fine when the vehicle is not towing, it stands to reason the trailer system needs looking at. Perhaps he can put the same weight in another trailer and see if it brakes the same. Mind you, you can tow up to 3 tonnes and not really feel it, it is the last 1/2 ton that seems to make the difference.
 
"The reduction of friction termed brake fade is caused when the temperature reaches the "kneepoint" on the temperature-friction curve and gas builds up between disc and pad. All brake linings are cured under mechanical pressure following a heating and cooling curve backstroke, heating the friction material up to 232 °C (450 °F) to "cure" (cross-link) the phenolic resinthermoset polymers: There is no melting of the binding resins, because phenolic resins are thermoset, not thermoplastic. In this form of fade, the brake pedal feels firm but there is reduced stopping ability."
 
Stanleysteamer said:
Didn't say it was "soft" or "springy" which are usual symptoms of air in the system, just said it all worked but lower down.
Have you ever driven a car where half of the system has failed on a split braking system? I have, it frightens the Ferk out of you cos the brakes work but much further down.
What OP describes is like this. So I think the brakes on towing vehicle are having to work much harder, OR half the system IS experiencing fade, and the other half isn't.
Whichever, it is, as the brakes work fine when the vehicle is not towing, it stands to reason the trailer system needs looking at. Perhaps he can put the same weight in another trailer and see if it brakes the same. Mind you, you can tow up to 3 tonnes and not really feel it, it is the last 1/2 ton that seems to make the difference.
Click to expand...

one of his main concerns is the soft pedal.

Sure he should look at the trailer brakes, but his soft pedal is one of the key points.

Either Air in the system
or
Brake fluid vapour lock
 
Interceptorxj said:
He didn't mention any prolonged braking to cause fluid boiling.
Click to expand...
Well there is no way it's "ok" when there is no trailer.

Then its magically soft just from attaching the trailer :rolleyes::D

He would of had to drive it before the pedal started acting up/
 
Interceptorxj said:
"The reduction of friction termed brake fade is caused when the temperature reaches the "kneepoint" on the temperature-friction curve and gas builds up between disc and pad. All brake linings are cured under mechanical pressure following a heating and cooling curve backstroke, heating the friction material up to 232 °C (450 °F) to "cure" (cross-link) the phenolic resinthermoset polymers: There is no melting of the binding resins, because phenolic resins are thermoset, not thermoplastic. In this form of fade, the brake pedal feels firm but there is reduced stopping ability."
Click to expand...
Right, OK, so there are two forms, maybe more, of brake fade, I don't think the person suffering it gives a tinkers sh1t which form it is when it happens he/she just wants to be able to slow down and maybe stop! The between the pads and disc type of fade is usually sorted out on high performance cars by having vented discs, cross-drilling and grooving, the other type is improved by always having new fluid in the system to ensure minimal presence of absorbed water in the system, or by using silicone systems, although they have more compressibilty in them anyway!!!
But what you quote is quite interesting as it implies whis is what he had rather than the type I described.
BUT, op said he had
Jamiegreen said:
new callipers and brakes a week ago.
Click to expand...

Jamiegreen said:
Have manually and power bled them multiple times
Click to expand...
So unless the disc is shot to beggery, which seems unlikely as if he has the nouse to change pads and calipers he ought to have the nouse to change the discs as well, and if he has done that much bleeding then he probably has renewed the fluid more or less too.
So I am awaitng a response as to the state of the trailer brakes.
But I do appreciate your looking for and finding a description of brake fade which is different to mine.
Question, did you find one like mine while you were looking around?
 
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