[s-cars] Dash Brake Light On

[Tom Green]

> On Sep 3, 2019, at 14:20:19, Douglas Fifield <douglas.fifield at gmail.com> wrote:

> Hedz,
> 
> Patient is a 95.5 S6 Avant who has been hibernating. I have resurrected it and am trying to fix all the little issues that go along with old sleeping cars.
> 
> The issue I need help with now is the resolution of the dash brake light being on. When I first started it up the light stayed on for a little while and eventually went off. Today, I took it out for some errand running and at first the light went off after starting the engine - until it did not. 
> 
> I need a quick refresher on what triggers that light. I have not touched the parking brake. That at least should not be an issue.
> 
> As always, thanks in advance to the collective mind out there.
> 
> Douglas in Duluth

This help list has been sleeping along with your vehicle, Douglas, which is not good for either.  Just a year or two of non-use usually spells the doom of most s-cars at this stage in their life, since they are usually parked without much fanfare or preparation.  It then takes an extraordinary amount of effort to coax all the systems back into proper operation.

Stott was kind enough to forward some advice I gave the s-car forum on Quattroworld about finding power brake issues.  You might digest some of that which applies to your situation.  The owners manual information was mentioned so you can pull that book out to find all the sources for this brake light. It might well be a simple low fluid quantity in the brake fluid or the hydraulic fluid reservoirs, although if sufficient time has passed, it may be advisable to drain and change both fluids.
I can provide one additional hint to initiate troubleshooting this system.  The power brake hydraulic servo just behind the brake master cylinder has a pressure sensor easily found by the electrical wire attached.  You can disconnect the wire with the engine running and brake light on.  If the light extinguishes, the sensor is the cause of the brake light.  The pilot types that need help starting the engines may chime in right away that it is probably a bad sensor.  Not likely-this sensor hardly ever fails, and that folklore from 5K models should be left there.  

You should look for any leaks in the brake side of the system, since the high pressure piston pump provides a very small amount of fluid compared to the vane end providing steering rack power assist.  A significant hose leak or the check valve in the accumulator stuck open can negate the pump attempts to maintain system pressure.  Some seepage from the J hose ends can usually be overcome, only causing the light to be on for 20 sec or so while replenishing bomb pressure lost overnight. Significant leaks are usually accompanied by large expenditures of cash for hydraulic fluid.  The unforgivable action is to operate the pump without a lubricating fluid supply.

I might agree that leaving the parking brake set for a year or so is a bad practice, but the advice not to use the system regularly is bad advice and has no place in an enthusiast car forum.  It is more properly called the emergency brake and if you toy with driving without sufficient hydraulic pressure, you may want to call on this system for some stopping assistance.