Rear brakes trouble

Suspension; Gearbox, Clutch and Driveshafts; Brakes; Steering; Throttle and clutch cable issues
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rosie
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Rear brakes trouble

Post by rosie »

Failed my MOT due to rear brakes on both sides. (‘Service brake effort inadequate at a wheel’)
Mechanic replaced the hydraulic brake cylinders.
Brakes are now improved, but still not good enough for the MOT.

What’s the likely culprit(s) please? Where do I start?


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GT_II
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Re: Rear brakes trouble

Post by GT_II »

Have the brakes been retested with the new cylinders? Unlike modern cars with ABS, the rear brakes are designed to minimise the risk the wheels locking, so they will never be as powerful as on a modern car.

Did your mechanic fit the later type 17.5mm diameter cylinders or the earlier 14.3mm type? I'm pretty sure your Scirocco should be fitted with the larger diameter cylinders which are for later models with pressure regulators for the rear brakes attached to the master cylinder. If they have fitted the smaller diameter ones by mistake, the braking effort at the rear wheels will be 33% lower than it should be.

Has the braking system been thoroughly bled, preferably with a pressure bleeding system? Any air left in the system will massively reduce braking effect.

Finally, does the MOT test bay have an automated roller brake tester, based on the measured weight of the car? If so, make sure you aren't carrying excess weight in the car, as this would lower the efficiency calculation.

Alternatively, if the roller brake tester is a manual system, where the tester manually enters the gauge readings on the MOT system, check what figure for test weight was used in the calculation - I found the test weight for my Scirocco had been incorrectly recorded on system as 1230 kg, but it should have been 1040 kg. Correcting this figure increased the calculated efficiency by 18%. See: viewtopic.php?t=22731


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Re: Rear brakes trouble

Post by rosie »

The MoT tester said that initially the brakes were recording something like 13 and 19, when he would want them to be around 100.
At the retest they were 48 and 91.

I was initially sold these, based on the year of the car
https://www.eurocarparts.com/p/pagid-wh ... -134440318

When my mechanic tried to fit them he said they were wrong and should have been the cast iron ones (from an earlier version possibly?), so he then ordered and fitted the replacements, but I don’t know what size they were as I didn’t deal with the ordering or fitting myself.

When I initially ordered the cylinders from Eurocarparts, I queried whether there were multiple versions available, and was told there were two - aluminium or cast iron - and nothing was said about different sizes in addition to this.

The brakes were apparently bled. However, the mechanic I have used no longer has a fully operational workshop (this is a very long complicated story, my usual mechanic died, and his son has taken over, but I don’t know how reliable he is. I’m looking for a new mechanic) so I don’t know if they would have been done with a pressure bleeding system. I’m guessing probably not.

There was no additional weight in the car at the test but I will read the thread you’ve linked to and query the recorded weight with the MOT station.

Thank you for all of this info.


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rosie
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Re: Rear brakes trouble

Post by rosie »

Just spoke to the MOT tester. He says he used a manual roller system and inputs the weight from the DVSA system. I asked him about the possibility of the DVSA weight being incorrect, and he said that even if it was miles out my brake effort was still incredibly low, so would still be too low for the MoT.

I got the figures wrong in my previous post.
One side is 48 and the other is 191. He was happy with the 191, but not the 48.

I saw the mechanic sanding the brake shoes to remove rust, and said it was all on the surface.

I wonder if I need to start thinking about replacing other bits of the braking system? Hoses maybe? Although I thought that that would have been flagged up in the MoT.


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Re: Rear brakes trouble

Post by GT_II »

The fault is clearly only on one side, so it is unlikely to be related to the type of cylinder. It might be worth a quick visual check of the rear brake pipe in case it has accidentally been crushed or kinked at some point.

If I was investigating it myself, I would probably first bleed at least quarter of a litre of brake fluid through that brake cylinder, to clear out any muck and make sure there is no air in the line. If that didn't work, I would dismantle that drum and check all the contact points are free and adequately lubricated with fresh lithium brake grease.

If those things didn't solve the problem, I guess that would leave either a restricted brake hose or a fault in the pressure reduction valve attached to the master cylinder, but I don't know how likely any of these would be.


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rosie
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Re: Rear brakes trouble

Post by rosie »

Thanks for your help. I’ll have a look into all that.
The Mot guy has offered to let me unofficially retest them once I’ve had a bit of a fiddle with them.


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Re: Rear brakes trouble

Post by mark1gls »

Did your mechanic change both wheel cylinders? You should be able to see them from the back off the backplate so you don’t need to take the drums off to check.

Could be the automatic brake adjuster is not working very well

Have a read of adjusting the brake adjustment wedge that I mention in the link GT II posted above.

Video of where the automatic adjuster is (video a bit rubbish and it’s the same drums as the Mk2 Scirocco)

https://www.youtube.com/watch?app=desktop&v=IPIPDsawQp8

If the wheel cylinders where leaking brake fluid and any got onto the brake shoes then the shoes are no good and need replacing as the brake fluid will stop the shoes working properly, you can clean up the drums with brake cleaner spray.


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james butler
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Re: Rear brakes trouble

Post by james butler »

does sound like either air still in the one side or alternatively something mechanical
daft as it sounds if the old ones had leaked at all and gotten onto the shoes that really wouldnt help either
fyi
the larger slave cylinder should have this part number
https://www.classic-vw.co.uk/rear-brake ... 8116-p.asp


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james butler
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Re: Rear brakes trouble

Post by james butler »

what was the handbrake figures?
on the test there should be parking brake figure and the foot brake figure, wondered if both tests highlighted an issue on one side or only on one part of the test?


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Re: Rear brakes trouble

Post by GT_II »

james butler wrote: Sun Oct 01, 2023 5:31 pm what was the handbrake figures?
on the test there should be parking brake figure and the foot brake figure, wondered if both tests highlighted an issue on one side or only on one part of the test?
Good point, if the parking brake figures are reasonably balanced, that would suggest a hydraulic fault. If the parking brake balance mirrors the rear footbrake figures, that would suggest a problem with the mechanical brake set.


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rosie
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Re: Rear brakes trouble

Post by rosie »

Thanks for all the ideas.
The brakes are on hold for now while I deal with carb issues and some other stuff, but I’ll come back with an update soon. At the moment I’m off the road.


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