Restrictors in brake lines - what for?

[NickJ67]

I was recently working on a '96 Audi A6 Avant 2WD, ABS equipped. This has in-line restriction orifices located in each rear brake line where the caliper hose meets the hard lines. Being made from aluminium and located very much in the splash zone, these dissolve away quietly and eventually fail catastrophically - as in this case. They are also small and well hidden, so unless you know to look out for them, you'll not find the fault until the pedal hits the floor. Roads have been very heavily salted this winter which won't have helped matters. Perhaps surprisingly, replacements are also aluminium.

Question is, what function do they have? According to Audi they are only fitted to ABS equipped Avants (station wagon). The saloons don't have them. They can't have any true load proportioning action and the ABS effectively does this anyway. As far as I can work out, all they can do is delay the response of the rear brakes - maybe giving a damping effect under ABS deployment conditions?

Seems strange that the saloon, which is very close mechanically (I imagine spring and damper ratings are different, but other wise the same) is not thought to need them.

The car is currently running without them (on back order from Germany!) and both braking and ABS action seem normal. I'd prefer not to refit these parts (in aluminium at least), but would like to know what effect they are supposed to have before making a final decision.

Picture shows the opposite side, which hadn't yet failed - it's buried under the mud in that clip and looks just like the normal (plated steel) hose end.

It was pulled off using two fingers only, becoming this....

and the orifice looks like this - approx. 0.6mm

Another worrying thing is that although the car has diagonal split line dual circuit, only one circuit was punctured, and the reservoir was not empty - this single fault caused total hydraulic failure. Surely this should not be the case?

If you happen to have one of these cars I'd suggest checking these parts out as a matter of urgency.

Regards

Nick

 

[MikeHalloran]

My Z28 has a restrictor in the hydraulic clutch circuit, alleged to be there so you can't shock the driveline by sidestepping the clutch.

So, I'm guessing the restrictors are present to limit the violence of brake apply/release cycles while the ABS is hammering away.



Mike Halloran
Pembroke Pines, FL, USA

 

[MintJulep]

I'd bet they are there as an attempt to prevent the ABS from pumping all the fluid out in the event of a hose failure.

Ironic, isn't it.

 

[MikeHalloran]

Maybe they don't salt the roads in Europe?
Maybe Audi expects you to replace the entire brake system before it corrodes away?
I'd get rid of the aluminum for sure.

Old-fashioned split systems allowed the pedal to go to the floor when a single circuit failed. You were expected to release and reapply the pedal, at which time the pedal would go halfway down, but the brakes would sorta work with the remaining circuit. When ABS arrived, drivers were re-trained to not release the pedal, ever. I wonder if the owners manual covers that exigency.






Mike Halloran
Pembroke Pines, FL, USA

 

[SomptingGuy]

Restrictor in a hydraulic circuit equals low-pass filter perhaps? Possibly a response to a problem rather than part of something designed properly.

- Steve

 

[Warpspeed]

Steve might be onto something here.

It is probably a band aid cure for some dynamic manifestation discovered during testing.

 

[patprimmer]

My guess is it slows down response rate of rear vs front so the rar end behaves itself under maximum braking effort, maybe only on slippery or bumpy surfaces.

The wagon will have somewhat more weight at the rear and the CG will be higher. This will have some influence on brake balance.

Also the difference between fully laden and lightly laden is typically greater in a wagon than a sedan, so the wagon should be more forgiving at the extremes even at a little cost to median conditions.

Regards
Pat
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[Tmoose]

I was struck by Volvo's use of 3/16 inch lines on our 1975 240. I was used to looking at relatively hugh-er brake lines on GM vehicles' front disk brake circuits.

 

[thruthefence]

I suspect it has something to do with the anti lock function under heavy vehicle loads, ie, a station wagon may be loaded in the rear much more then a sedan, under certain circumstances. Sorta like a pick up truck; has to work laden, and un-laden.

 

[NickJ67]

Thanks for your thoughts.

They certainly do salt the roads in Europe! Lots this winter as it's been cold and snowy. Aluminium is a poor material choice for this application!

Spoke with the dealership again today. They confirmed these are used on the ABS equipped station wagons only. No-one there, even the "techs", has any idea what they are for! Disappointing, but not unexpected.

We all seem fairly well agreed that they are most likely dampers for ABS function on the rear. One thing bothers me though - I agree that the wagon could be more heavily loaded than the sedan, although oddly enough the vehicle handbook doesn't differentiate between the wagon and sedan for either vehicle weight or load capacity. However, I would have thought that early locking up of the rear and any ABS instability would be more likely under lightly laden conditions?

The vehicle handbooks says that ABS operation is made known to the driver by a "pulsating brake pedal and audible noise. This is an intentional warning to the driver that one or more of the wheels are tending to lock up. In this situation it is important to keep the brake pedal fully depressed so the ABS system can regulate the brake application. On no account 'pump' the brake pedal".

In fact, pumping the pedal did not restore any hydraulic function at all in this case. I don't understand why. I'll be bleeding the brakes once more in the morning, then taking the car to my local test station for a ride on their brake rollers to make sure both circuits are working properly.

A bit of surfing on the Audi forums suggests that this is quite common failure at the moment and there have been a number of close calls. Several are now running without the restrictors and are reporting no adverse effects detected.

Thanks for your time

Nick

 

[MacGyverS2000]

Have those running without the restrictors hammered at the ABS system for a while? Taking them off but not testing ABS is akin to replacing your parachute with an anvil but never jumping out of the airplane.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[NickJ67]

Well, I don't know about the others, but I did give the brakes and ABS a pretty thorough work-out after renewing the brake pipes. All seems to work as before although as I don't regularly hammer the ABS I might not notice subtle changes. Plenty of other things to have an effect like road surface, tyres etc.

Parts-computer surfing at the dealer today shows ABS pump and ECU are the same between wagon and sedan as are calipers and discs. These restrictors seem to be the only system difference. Most odd.

There's now a fellow victim on one of the Audi forums who has a machine shop and is considering making a run of these parts in a stainless steel. I like the material change, some reservations about possible dimensional/QC issues.

Nick

 

[modesign]

In the Porsche Boxster a pressure limit valve s fitted to restrict the line pressure to the rear brakes. Under IVA regulation the rear brakes must not lock before the fronts. I understand that this valve stops any chance of rear brake lock.

 

[patprimmer]

By Pascals Law, a restriction that is less than a total blockage cannot stop the brakes from locking, it can only delay the level of application of the rear brakes as the pressure in the fluid will equalise given time.

Regards
Pat
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[modesign]

Further clarification of the Boxster brake system from the manual.

"To reduce the brake pressure at the rear axle and to adapt the brake pressure to the wheel load distribution a brake proportioning valve with a switch over pressure of 25 bar and a reduction factor of 0.46 has been fitted."

Regards,

Sandy