More Brake Problems

[MEL1523@ACS.TAMU.EDU]

;But the damn brake light NOW stays on 'most all the time!!!  Oh, it 
;might go out for a few minutes, but it's at random times as far as I 
;can discern...there's no pattern.
;
;So - I *WAS* ready to be convinced the only problem was a faulty 
;pressure warning light, because there was no other problem appearing.  
;The switch screws into the brake servo and is easy to replace.
;
;But TODAY, my wife said the brakes felt spongy twice when she started 
;it after it had been sitting for 3+ hours.
;
;I would SURE welcome opinions and suggestions on how to diagnose and 
;solve this damn problem.  I am not willing to go to the dealer and 
;let them start hanging $300 to $1000 hydraulic parts on the car 
;until they find what's wrong (seems to be their only approach 
;to most problems...) but this is a potential safety hazard, and I 
;GOTTA find a solution.

OK, here's my take on the Audi hydraulic system.  Actually it's a pretty 
straightforward design as far as industrial hydraulic systems go.  There is 
a central positive displacement pump.  Positive displacement means that 
the pump pushes a constant volume of fluid through itself regardless of how 
much pressure it takes.  The Audi pump is actually two pumps in one.  They 
share a common source line to the reservior.  One is for the brakes the 
other is for the steering.  I don't think the systems are connected at all 
once past the pumps.  Therefore I don't understand the connection between 
steering problems and brake assist problems.

So,how do you control the pressure of a system with a pump that is capable 
of destroying itself?  You put a pressure relief valve in the discharge 
line.  This is like a safety valve.  Only it always is discharging.  When 
the pressure gets to a certain point (About 2000 psi for the brakes) this 
valve (held against it's seat by a spring) opens and lets the fluid back to 
the reservior.  When the system is off, you don't want the hydraulic 
pressure stored by the accumulator (basically a hydraulic "capacitor") to 
act backwards on the pump, hence the check valve.  All the servo (the 
"customer") sees is a nearly constant 2000 psi.  It can take some if it 
wants and the relief valve will close a bit to make the pump flow constant.  
If we take more than the pump is flowing, the accumulator delivers the 
rest, and the relief valve will close off for a second.  If the accumumator 
is shot, and we take too much, (like if we stab the pedal fast), the pump 
pressure drops to zilch fast (almost instantly), and we get the brake 
warning light for low pressure.  For what it's worth, that's basically how 
every hydraulic system in the world works (like forklifts, backhoes and 
such) only most don't have an accumulator.

In the Audi installation, the pump is just that, a pump, actually two.  A 
common line for new fluid, and two discharge lines, one from each pump.  The 
accumulator is actually the accumulator, check valve, and relief valve all 
in one device.  The pump pumps to it, and it stores it, and relieves it back 
to the reservoir, or sends it to the servo.  

One of the ways the pump can go bad is for the vanes (or gears) to be sloppy 
from wear, and leak back on themselves under pressure.  The manual sez, to 
check the delivery rate after putting a relief valve in the discharge of the 
pump.  This checks if the pump can deliver it's supposedly constant flow 
while under pressure.  For us, it's 0.3 L/min at 2030 psi.  What if you 
don't have VW1354 to do this?  You do have a relief valve.  It's the 
accumulator.  Take the line from the acc. back to the res. (it's the one 
held on by a hose clamp) and put it into a thing in which you can measure 
volume.  AFTER the fluid is flowing at a constant rate (it may appear to 
speed up flow as the accumulator fills, or start with a full accumulator), 
take a timed measurement. Don't empty the reservior.  It's not the best way 
to do the test, but it'll tell you if you have most of the 300 mL, or more like 
100 mL.  If your servo leaks a bit...enough to affect the data, you'll have 
to capture that flow the same way and add the two together to get the total 
pump flow.

If you only get to 1800 psi, it may be the pump not able to deliver that 
pressure/flow combination.  The light trips at 120 bar (1740 psi)...pretty 
close to 1800..well within toleracnes anyway, so it may be trying to tell 
you the truth...that your pressure is low.  If the pump delivery is low 
under pressure, it's the pump.  Especially considering that 1800 is less than 
the 2030 it's supposed to deliver.  If the pump is ok, it may be the relief 
valve opening too soon (i.e. at too low a pressure), and dumping all your 
fluid/pressure...in which case (grit your teeth here) you need another 
accumulator...this time for the relief valve.  

Only thing is..you said they feel spongy?  That would indicate air in the 
brake lines.  Low pressure would make the pedal feel harder due to less 
assist.  Discharge the accumulator, and feel the pedal.  It should be pretty 
hard without the assist.  Any sponginess without assist will feel like Jello 
with the servo helping.

Good luck,

 - Mitch Loescher
   Chemical Engineering
   Texas A&M University