Baffling rear brake efficiency problem

[Grubtraw]

My 1998 Octavia SLX TDI 1.9 failed its MoT test recently on what I thought were fairly minor problems, so I authorised my tame mechanic to go ahead with repairs.  Although the car is getting on a bit I like it a lot, it has given me very little trouble in 14 years of ownership and I don't know what I would buy to replace it as my daily driver.

 

However one failure item has us beaten and I would like to ask whether anyone has had a similar problem.  The rear brake efficiency was only about 40% of what it should be on both sides but the front brakes were fine, in fact better than they need to be.  The rear calipers are fairly new (about 5 years old) and the discs and pads were replaced more recently.  The calipers have been removed, cleaned and checked and the pistons are moving freely.  The system was pressure-bled and fluid seemed to run through normally, however the problem did not go away.  The car has now had a replacement ABS hydraulic unit (part number 1J0 614 217C) from a breaker, followed by a new brake master cylinder but neither has cured the problem.  As it's starting to get expensive my mechanic has asked me to take it away until we can get some expert advice.

 

Current thoughts are that the "new" ABS pump could have the same fault as the old one (if it was faulty), the ABS pump isn't bleeding properly inside or both rear brake hoses have failed internally at the same time, which seems unlikely but it's just possible.  Any advice would be very welcome.  At least I'm getting to drive my 1971 Wartburg daily just now 🙂

[KenONeill]

51 minutes ago, Grubtraw said:

The rear brake efficiency was only about 40% of what it should be on both sides

OK, suggestions:-

1. Check the handbrake adjustment is good.
2. Not sure on this, but a 1998 car may have a line limiter in the rear circuit, even with ABS. If so, is this working right?
3. You don't say who your specialist is. Have you tried "A for Audi" over in Clydebank (handy for Clydebank railway station if you need to get into Glasgow).

[Grubtraw]

Many thanks Ken for your reply.  Handbrake adjustment was checked and seems good.  There doesn't appear to be a limiter in the rear brake circuit and each rear brake has its own pipe coming from the ABS unit.  I'm using a local mechanic in Greenock who isn't a VAG specialist but he's usually very good.  Haven't tried A for Audi, thank you for the suggestion.

 

Cheers, John.

[J.R.]

The obvious question is what is the handbrake efficiency on the test rollers, is it what it should be, at the lower end of the pass figure or below?

 

Does the graph for the footbrake (watch while they are testing) show the brake reaction force gradually rising with the applied pedal pressure before stopping dead despite further pedal pressure and does the pedal feel harder after that point?

Edited 28 May, 2022 by J.R.

[Warrior193]

5 hours ago, KenONeill said:

OK, suggestions:-

1. Check the handbrake adjustment is good.
2. Not sure on this, but a 1998 car may have a line limiter in the rear circuit, even with ABS. If so, is this working right?
3. You don't say who your specialist is. Have you tried "A for Audi" over in Clydebank (handy for Clydebank railway station if you need to get into Glasgow).

+1 for point 2 ^^^^^

[Grubtraw]

Many thanks for the replies.  I didn't watch the tests (embarrassingly it failed a re-test!) and the paperwork is still at the MoT station, so I won't know any more until Monday at the earliest.  I've been put in touch with a chap who services and repairs many of the local Skoda taxis and I'm hoping that he can sort it out next week.  A mechanic friend told me this morning that the pressure limiter is built into the master cylinder.  The guy who bled the brakes knows about the bleed screws in the cylinder.

[Silver Bullet]

Hi Grubtraw

Interesting problem, did you find the cause and what was the solution?.

I replaced both my rear calipers last year and RNS was a b*** to get the air out.

Cheers

Edited 1 July, 2022 by Silver Bullet

[Grubtraw]

Hi Silver Bullet, three months and three mechanics later the car finally passed its MoT test on 11 August.  After replacing most of the hydraulic system the culprit turned out to be the rear discs, which were brand new in April 2021!  The first mechanic had given them a good clean so initially we ruled them out as the cause.  The second guy (Skoda specialist) was on holiday for a month and had a big backlog when he came back so the car ended up going to a friend who has a brake tester.  He tried cleaning the discs again and the efficiency improved, so he repeated the process until it was good enough to pass the test.  So now I have a spare ABS pump which might come in handy some day.  Hat off to good old Warty which gave me three months of trouble-free daily use 🙂

[Silver Bullet]

Glad you got it sorted👍,

I decided to replace the RNS brake  brake line with cupra-nickel from ABS to the brake caliper and the flexable hoses on that line. Before I started I pressure bled that line together with a vacuum brake

bleeder at the caliper end. A  load of small particles came out with the fluid, I think the U shaped hose was breaking up internally and bits were blocking the line. That hose placement is a very

bad design it, it could have been mounted differently with just a gentle swept curve not so extreme as a 180 deg bend. I plan to replace the offside one later on, though that seems ok at the moment.