You know how cars like the Octavia have dual braking systems so failure of one component should never cause total brake failure?

 

NOT SO!

 

A blown rubber brake hose on the rear passenger brake of my '03 Octy diesel estate meant complete and total loss of brakes. Pedal to the floor. No backpressure. Nothing. Nada. Nowt.

 

I'd just set off through the car park of our local Moggisons at just over tickover in 2nd gear and was heading for the exit. Following my advanced motorcycle training of all those years back I tested my brakes on an open and unimpeded lane in the car park only to find there was absolutely nothing there. I tried pumping the pedal to no effect whilst checking around to see how closely I was being followed by a shopping moron and then pulled over, declutching and slowing the car with the handbrake. Oh, did mention the icy surface? Cadence braking with the handbrake was a new experience ...

 

After putting on the hazards and handbrake I sat for a minute to allow heartrate, blood pressure and sphincter to return to a more normal state, ignoring the irritated blast on the horn from the aforementioned shopping moron as s/he accelerated past me, only to engage in some spirited maneuvering whilst attempting to slow again for his/her approach to the exit ahead.

 

To paraphrase a Texan friend of mine, I unassed the car and had a look around the underside for evidence of leaks. Nothing. Popped the bonnet to investigate brake fluid level. None. Interesting.

 

I debated a call to the AA for a tow home but decided as it was a quiet time of the day and the 5 miles to home could be achieved over relatively quiet roads I limp home at a fast walk with hazards on and judicious use of the handbrake. It's worth noting that in a diesel at those speeds engine braking doesn't help much :( 

 

Once home safely, with only a handful of rude or passive aggressive greetings from fellow motorists for having the nerve to attempt to travel at less that 10mph over the posted limit, I dropped the car at our local garage and waited on the autopsy results.

 

Flexible coupling to nearside rear brake split.

Master cylinder failure (couldn't pump fluid)

Also, a leak in the reservoir about half way up.

 

OK, so I understand the split coupling. It happens,  and it would have depressurised that braķe circuit.

 

But why did BOTH circuits fail simultaneously?

 

Is this an unplanned failure mode that represents a risk to all such systems or just a one in a million coincidence?

 

Any ideas, anyone.

 

Regards, Mike

 

PS Yes, I have now changed my underwear and am also in need of a brake reservoir - anyone?

Edited 2 March, 20188 yr by StickyMicky
typos

5 minutes ago, StickyMicky said:

I unassed the car

Love that!

5 minutes ago, StickyMicky said:

But why did BOTH circuits fail simultaneously?

Are you certain that you weren't already down to one circuit, before the problem became so obvious?

9 minutes ago, Wino said:

Are you certain that you weren't already down to one circuit, before the problem became so obvious?

 

One of the habits I developed as a motorcyclist is a good "vehicle sense" - also known as "seat of the pants", although in view of my earlier comments post-incident we should perhaps avoid the seat of my pants for now! - so whilst I cannot absolutely guarantee there was no prior problem I didn't sense anything untoward about the car before the incident.

 

The garage autopsy checked all the braking components and no other issues were noted.

 

Something I'm not clear on is whether the master cylinder is one piston feeding two circuits or two pistons feeding parallel circuits?

Well, this prompted me to do some reading. A dual circuit master cylinder has 2 pistons moving in a single bore, each of which pressurise some of the wheel cylinders (On a VAG car normally X-split so OSF+NSR and NSF+OSR). So far so good, but those 2 circuits have a common hydraulic reservoir which does suggest a failure mode where you pump all the fluid out of one circuit and the reservoir, and then if there's any leakage past the central piston you could pump the master cylinder totally dry.

4 hours ago, StickyMicky said:

Something I'm not clear on is whether the master cylinder is one piston feeding two circuits or two pistons feeding parallel circuits?

 

4 hours ago, KenONeill said:

those 2 circuits have a common hydraulic reservoir which does suggest a failure mode where you pump all the fluid out of one circuit and the reservoir, and then if there's any leakage past the central piston you could pump the master cylinder totally dry.

 

Or if the unsuspecting driver (me) attempts to pump the brake pedal looking for some pressure - but in fact simply empties out the reservoir via the breached circuit.

 

I'm not comforted to know that total brake failure is possible but now I do at least understand how it could happen and can be ready for the possibility.

 

BTW is there a true 2-zone reservoir that would fit my car?

Did you not get a low brake fluid warning light come up?

What can happen if you bleed the brakes the traditional way (ie someone opening the bleed valve while you push the pedal to the floor),

Over a period of time the seals in the master cylinder create a slight wear ridge in the bore, usually at the point at which brake cylinder piston

travel usually stops.

What then can happen is when pushing the brake pedal to the floor is that the piston seals travel past this normally stopping point and the seal lip

catch on the ridge in the bore and turn inside out partly or all of the lip invert, thereby allowing all or partial pressure to be lost internally.

This happened to me many years ago, there was no brake fluid leaks and the reserviour level didnt drop but there was no pressure resistance at the pedal, it went

down to the floor pan!!!.

Now I alway use a pressure bleed system like Gunsen Eezibleed (about £20)

Edited 9 March, 20188 yr by Silver Bullet

Interesting. 

 

Years ago I had a Datsun 100A that had a dual circuit mastercylinder but was fitted with seperate reservoirs per circuit

 

In fact it did develop a fault with the seals where the effect was to transfer brake fluid from one reservoir to the other - fixed with a repair kit (these days I'd buy a new mastercylinder). However it does demonstrate an issue with shared reservoirs - you'd never know there was a potential serious brake issue looming!

 

 

 

Another thing to remember  - manual cars share the same reservoir heading the clutch hydraulics.

 

Whilst you are at it - change all the other flexi hoses as they may all be the same age!

 

 

 

 

Edited 10 March, 20188 yr by bigjohn
Photo

I had a similar trouser ruining moment a couple of weeks ago returning between Alston & Hexham.

Ever so gentle 'p' sound followed by brake pedal hitting floor. luckily I was on an empty road slowing to turn into a T-junction and could eventually stop on the hand brake, pumping the pedal had zero effect. 

 

The beast had thrown the leading piston from the (original) O-S-R cylinder with the (again original) shoes nearly down to metal.

Normally I do all of my own fettling but a couple of years ago in January I had the rear Drums & shoes (allegedly) replaced at a garage I'd used before (no names. no defending a libel case).

 

After a complete rear brake rebuild (both sides) I hit the ABS bleeding problem.

Using a diagnostics system to force the ABS pump 3-4 times sorted the issue in a few mins, I wouldn't advise any driving with known faulty brakes, Prosecution following any injury causing accident would be a slam dunk for jail time.

 

This is my second Octy Mk1, bought as a 1-owner,12 yr old with 110k miles. My first was bought new and provided a very reliable 250k miles (150k before anything non-wear packed in).