Lackluster brake performance on my 4-ton motorhome

[roadwarrior]

i have got a 4 ton mercedes sprinter based winchcombe, it has got new disks and pads all round but i have to stand on them if there is a sudden stopping situation, they are bedded in as i have just returned from the pyranees, but they are still lackluster and dont inspire confidence - your thoughts ? they take a couple of seconds to bite and are fine in normal circumstances but.. as they are all round new discs im surprised they are not better.

 

[donkey]

Geoff, I'm going to type this slowly so you understand. Back to school for you I think. Let's pretend I push the pedal at 5psi - that 5psi is distributed as 2.5 psi each way half to the front half to the back, If I have a valve that suddenly allows more fluid IE more pressure to the rear wheels it MUST alter the pressure to say 2 front and 3 rear, that is physical scientific law, you can't give without taking away. See Einstein for details. Something about for every action there must be an an equal and opposite reaction.

I thought you were looking for assistance with your braking issue. I have tried to give that assistance, but obviously, you know more than me when it comes to brakes and suspension systems.
So I am going to leave you to it now.

Geoff

 

[roadwarrior]

This is getting out of hand - thank you gentlemen for all thoughts and advice has been noted.

 

[tacr2man]

By your own reasoning fitting air springs and lifting the ride height would limit the pressure to the rear as its an anti skid measure for rear axle therefore front would get more braking effort !

 

[busbuddy]

Let's pretend I push the pedal at 5psi - that 5psi is distributed as 2.5 psi each way half to the front half to the back,

Technically it's probably 3.5psi to the front and 1.5psi at the most to the back.....rear brakes do minimal braking

But back to the problem....can you lock the brakes up
If the brakes feel that odd...then something is wrong, a 4 ton motorhome can still lock up the front

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[L' Hobo]

Technically it's probably 3.5psi to the front and 1.5psi at the most to the back.....rear brakes do minimal braking

But back to the problem....can you lock the brakes up
If the brakes feel that odd...then something is wrong, a 4 ton motorhome can still lock up the front

Hopefully only for a very short time if it has ABS which, I believe, was mentioned earlier?

 

[roadwarrior]

no gents - i am not able to lock the brakes, no dip on braking , but this is for next year now as i will sorn it soon for winter sadly

 

[pappajohn]

it could be the callipers are worn

How do calipers wear?
If the cylinder/s are worn they'll leak.
If the sliders are worn it won't make any difference to brake efficiency as they are held rigid when brake pressure is applied.

 

[L' Hobo]

How do calipers wear?
If the cylinder/s are worn they'll leak.
If the sliders are worn it won't make any difference to brake efficiency as they are held rigid when brake pressure is applied.

If the sliders are seized, it could be just braking on one pad.

 

[Billy192]

Hello Geoff, your thinking is also flawed, given that the pressure provided by me is finite, ANY pressure allowed at the back MUST be taken from the front, or if you prefer shared. Either way it will reduce front braking power.
Also I have stated already the bags were fitted to help the tired leaf springs and would have lowered the chassis and confused the valve to increase to the back...sooo.. I will raise the body with airbags to rectify that. And before anybody suggests I should have replaced the spring, consider this , costing about the same, I can replace the springs, or I can fit airbags and raise my legal carrying capacity. What would YOU do ?

as a quick test, could you not raise the rear say 4bar and if your theory is correct you front should dip more?

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[roadwarrior]

Technically it's probably 3.5psi to the front and 1.5psi at the most to the back.....rear brakes do minimal braking

But back to the problem....can you lock the brakes up
If the brakes feel that odd...then something is wrong, a 4 ton motorhome can still lock up the front

yes i know its about 70/30 but i was trying to keep it simple

 

[dash300]

Brakes on my S8 Ducato heavy chassis are excellent. What does concern me is that the brake pedal is made of plastic!!!!!

 

[DDJC]

Loads of advice here, but after crapping myself with smoking brakes coming off the Pyrenees this year, I am sacking Fiat standard and putting in Brembo disks and pads. I love my family more than money.

 

[busbuddy]

I thought fiat standard were brembo on the front

 

[1967bowesj]

I thought fiat standard were brembo on the front

Possibly original but they will have been changed and not had Oem put back on?

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[roadwarrior]

as a quick test, could you not raise the rear say 4bar and if your theory is correct you front should dip more?

That's what I intend to do billy, if that shows no improvement I will replace the front pads with pagid. All this of course assumes the valve is even working and not siezed on open which would explain things.

 

[68c]

Do Alko chassis have a rear brake load sensing valve to reduce pressure to rear brakes when braking hard and apparant weight moves forward?

 

[A-Mech]

That's what I intend to do billy, if that shows no improvement I will replace the front pads with pagid. All this of course assumes the valve is even working and not siezed on open which would explain things.

Take to MOT garage as they will have rolling road. You can see what kind of braking effort you have currently.
Dump all air which should increase braking effort.
Inflation of bags should decrease rear brake effort as the van things there is no load.
This exercise will also prove your load sense valve operation. (general rule of thumb ls, all cars, vans, trucks have LSV.
After you know this information on both axles braking force, then start spending money on parts, etc.

 

[roadwarrior]

Take to MOT garage as they will have rolling road. You can see what kind of braking effort you have currently.
Dump all air which should increase braking effort.
Inflation of bags should decrease rear brake effort as the van things there is no load.
This exercise will also prove your load sense valve operation. (general rule of thumb ls, all cars, vans, trucks have LSV.
After you know this information on both axles braking force, then start spending money on parts, etc.

that is exactly what i intend to do, i agree completely. Mot man should also give me an idea on where the valve is, if it exists and is it working based on results. sadly winter has come so i will be sorning it till march pretty soon.

 

[roadwarrior]

Yes poppy, I do in fact trust the mechanic at the motorhome sales place, it's all they do all day is fix motorhomes and caravans and they have an excellent reputation. Frankly my suspicion is more likely to be budget disks and or pads were fitted.

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[Guigsy]

Frankly my suspicion is more likely to be budget disks and or pads were fitted.

Or they were fitted badly and got contaminated?

 

[CORNHYM]

I also have a 3.9t Mercedes Sprinter based motorhome and the brakes felt spongy when I purchased it. After a couple of months I noticed that there was a Commercial Mercedes dealership that serviced and repaired dozens of DPD Sprinter vans so thinking they would surely know if there is any problem booked it in. After tests and getting it on the rolling road I was informed that the spongy brakes were an anomaly to this model and they were working to the standard.
Following a MOT advisory a year later stating that the brake discs were now getting too thin and near to being out of spec I had the discs and pads renewed all round (at non Mercedes garage) and also hoping that this would make the braking much sharper. It did improve the pedal having less travel but brakes were still having that spongy feel but did pull up better. I guess I need to speak to a DPD driver to see what they think !

 

[Spriddler]

When the brakes seemed to have become progressively less effective on my m/h I got my MOT chap to put it on their rolling road. He said it would pass as the balance between them all was fine but the pedal pressure needed was extremely high.
Upon close investigation I found a barely visible 1/2" long split in the hard rubber pipe between the vacuum pump and servo unit. A bit of heat-shrink over the split and I had great braking again with just the lightest touch on the pedal.

 

[Carpmart]

The reality is, it could be a multitude of things giving the lacklustre brake performance..

Manufacturers source pads and discs from a number of suppliers. Then there is the aftermarket or patten part suppliers. The OEM brake components, if you know the original manufacturer, can be sourced from GSF and Euro for sensible money, so OEM quality for reasonable money!

Someone else doing the job may just buy the lowest cost pattern parts for the vehicle and IMHO, there are some poorer quality parts out there as it’s a very price sensitive market.

My guess is poorer quality pads is the issue.

 

[L' Hobo]

Has the vehicle got a Servo unit?

No-one has mentioned this as a possible cause?

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[roadwarrior]

i was under the impression all brakes have servo's

 

[roadwarrior]

no, i have no concerns there were fitted badly

 

[roadwarrior]

my feeling is EITHER lacklustre front pads OR the brake balance valve , either way it goes in for an mot friday 12th july with my freindly mot man who i have asked to give the brakes particular attention, it pulls up straight so will pass the mot ok but i have explained my concerns and feel sure he will have a good look for me, and the following friday it goes to mertrux for an engine and gearbox oil change for an eye watering 800 quid, but considering a sprinter engine takes 13.5 litres of oil its acceptable perhaps. its goes to europe for 9 weeks after that so i want it right

Of course it could be anything, upto and including a leaking hose.

 

[Daffy64]

I’ve said it before but carefully check the any rubber hoses either side of the vacuum pump, it wouldn’t be the first time that one had become contaminated with oil and collapsed internally, it gives the impression that the pump is working ok but the vacuum fades as the brakes are used.

 

[DJT]

my feeling is EITHER lacklustre front pads OR the brake balance valve , either way it goes in for an mot friday 12th july with my freindly mot man who i have asked to give the brakes particular attention, it pulls up straight so will pass the mot ok but i have explained my concerns and feel sure he will have a good look for me, and the following friday it goes to mertrux for an engine and gearbox oil change for an eye watering 800 quid, but considering a sprinter engine takes 13.5 litres of oil its acceptable perhaps. its goes to europe for 9 weeks after that so i want it right

Of course it could be anything, upto and including a leaking hose.

Is it manual or auto gear box?

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[roadwarrior]

Has the vehicle got a Servo unit?

No-one has mentioned this as a possible cause?

yes and i have tested it all fine