MOT Failure: 'Service brake efficiency below requirements'

[PhilipHa]

Hi,

My Marco Polo (2017 reg, 32K miles) has just failed its MOT on braking efficiency (it is the first time I have taken it through an MOT).

The brake test results were as follows (vehicle weight 3098kg):

• Front left: 416 kg (28%)
• Front right: 397 kg (27%)
• Rear left: 440 kg (27%)
• Rear right: 445 kg (27%)
• Total left 856 kg, Total right 842 kg, overall 54%

I understand the total efficiency (brake weight/ vehicle weight) needs to be over 58%. Mine is currently 54% ((856 + 842) / 3098).

What I can't work out is why it has failed? The brakes look in good condition, the disks are clean and pads are a reasonable depth. What else can cause the braking efficiency to be too low? Does anyone else have a copy of their brake test results (in kg as above) which you could post on the forum so I can work out whether its the front or rear brakes on mine causing the problem?

I have had this on my car once before - the discs were a bit rusty, the dealership phoned up to say it had failed (no mention of the discs being rusty) and said needed new discs and pads all round. I drove the car around the block braked hard a few times scrubbing off the rust and it passed the retest. It wasn't until 8 year later I replaced the discs and pads. Consequently I never returned to the dealership.

However the discs on my Marco Polo are shiny, flat, no signs of uneven wear and the pads in good condition. Its on all-weather tyres (newish, 2,800 miles of use) but I presume they wouldn't affect the test? And, it passed its MOT 1 year and 2,800 miles ago with the previous owner with no mention of the brakes. Apart from some hard braking to clean the pads and discs and getting it retested somewhere else I am not sure what to do? Given my previous experience its difficult not to be suspicious. Is the test dependent on how hard the tester pushes the brake pedal? Does anyone have any suggestions? The dealership have said they will need to look at the brakes and it could take than more than a day to fix - without specifying what they might want to fix.

Thanks,

Phil

 

[SteveR]

Hi Phil,

Just a thought given your discs and pads seem ok. May be air in the system causing your brakes to be spongey and less efficient. Can you bleed them?

 

[PhilipHa]

Hi Phil,

Just a thought given your discs and pads seem ok. May be air in the system causing your brakes to be spongey and less efficient. Can you bleed them?

They don't appear to be spongey, braking appears to be linear with no obvious 'sponeyness'.

 

[PhilipHa]

I think I have determined the potential issue with the van's braking failure and have asked the garage for comment.

In last year's MOT paperwork which I got from the previous owner, the total service braking was 1617kgf which passed but this year at my local Mercedes garage it was 1698kgf which failed despite being 81 kgf better than last year. Looking through the paperwork the main difference seems to be the van weights which was 2405kg last year, but 3098kg this year (the van was full of diesel plus water - but that would only add up to 70kg + 40kg = 110kg at most). So I am guessing that difference is that the 2405kg is an unladen weight for a Mercedes V-Class (last year's MOT) but the 3098kg (this year) is the actual weight of my Marco Polo (just to be clear this is a vanilla 2017 Mercedes Marco Polo V250 AMG supplied campervan i.e. not a converted van).

It would be greatly appreciated if someone else on this forum could look up their MOT service braking values (kgf) versus their vehicle weight (kg) and let me know, as I am trying to determine whether its the braking which was poor in the last 2 MOTs or the difference in vehicle weights used by the 2 garages?

I have asked my garage for comment by email.

 

[PhilipHa]

The 3098kg weight for this year's failed MOT seems to be very close to the 3100kg maximum 'Max permissible mass' from my V5 registration document which I suspect may be the incorrect value to use for the MOT?