[200q20v] Re: Brake Light / accumulator

C1J1Miller at aol.com C1J1Miller at aol.com
Mon Dec 11 14:17:20 EST 2000

In a message dated Mon, 11 Dec 2000  2:02:48 PM Eastern Standard Time, George Sidman <sidman at montereynet.net> writes:

<< The Brake light on my 91' 200TQW came on some time ago, and
a during a visit to the mechanic, I mentioned it. When I
picked up the car the light was out. This weekend I noticed
that the master cylinder sensor was disconnected. I
reconnected it and the brake light came on. Blau tells me
that the sensor detects low hydraulic pressure, but there
appears to be nothing wrong with the brakes or steering. The
accumulator might need replacing? The sensor could be bad?
The Chris Miller and SJM sites don't have anything on this
subject. Any wisdom/advice.......??
George Sidman

Hi George,
Probably the bomb...
If you search for "brake light" on my site, the first result is the braking repair page.  After that, you need to read a bit, as I don't have the "brake light is on" as a link.  I should, though.

Page down to "Bomb or brake booster":
The standard tests: 

* If the brake light (the warning light on the dash that also comes on when the parking brake is up) stays
on for several seconds after starting the car, suspect the bomb. 

* After driving, shut down engine and pump brake pedal until it loses power assist (gets stiff). If it takes
less than 10 presses of the pedal, you could have a bad bomb. 

* Press brake pedal as hard as possible while engine is running and car is stationary or moving slowly. If
brake light comes on and the pedal initially feels like it has no assist, the bomb may not be holding

A thorough test: 

1. Engine running at idle, steering to right or left full lock 20 seconds,      at the same time pump brakes
hard 10 to 15 consecutive times. Does the brake light come on? Yes = bad belt, pump, or bomb (check
valve).  Pressure bleed before exchange! 

2. If the light doesn't come on: stop engine, pump brake 10 to 15 times (short      quick strokes). Brake
light on? Yes = bad bomb (pressure accumulator). 

3. If light doesn't come on: Drive the car in a very safe place at 65 mph      then apply full stop pressure
to brakes (emergency stop pressure!).      Does ABS actuate several times? Yes = good bomb! Brake
light on? Yes =      probably bad ABS sensor. Above assumes booster cylinder has been tested for leaks
! They don't seem to hold the N2 charge for more than 6-7 years (diffusion of the gas through the

Just to eliminate the possibility that it's the booster itself, unscrew the return hose from the booster (the
thin one, that goes to the reservoir). Do this with the engine OFF naturally! If the booster continuously
drips the snake fluid out of that hole then its internal check valve is no more (12 droplets is OK though). 

Replacement: Make sure you get 2 copper washers for banjo bolt of the high pressure line from the
hydraulic pump to the bomb. I recommend you use this opportunity to replace the hydraulic fluid
(Pentosin) for the brake booster and the steering rack. During the bomb replacement, you will have to
disconnect the hose leading from the reservoir to the pressure accumulator (bomb). At this time you can
drain the Pentosin, assuming it has not been replaced recently. 

1. Decide first whether you will reuse the "pentosin" (hydraulic fluid) or replace. If you want to reuse,
have available a very clean 1 quart container, or one not as clean if you intend to replace. 

2. After car is in position, pump the brake pedal (with engine off) 30 to 40 times to relieve pressure from
the "bomb".  Don't neglect this step; the Bomb will act just like one if it is disconnected without first
removing the residual pressure.  No joke, mechanics have lost fingers doing this!  It should be perfectly
safe once the pressure is removed. 

3. Disconnect the hose from the hydraulic reservoir to the "bomb" (hose clamp), remove the hose by
rotating (twisting by hand) and quickly insert it into your recovery reservoir. Allow the system to drain

4. Loosen the 17mm banjo bolt from the high pressure hose (from the hydraulic pump) and remove by
hand retrieving the two copper or soft material washers      ( both need to be replaced) located above
and under the hose connection.  There should be minimal fluid presence at this time. 

5. Loosen the 11mm connector of line from the "bomb" to the booster cylinder at the "bomb" side. 

6. Loosen the nut at the front side of the "bomb" and carefully lift the bomb up and diagonally towards the
passenger side. This will allow the front screw to move free from the short slot in the retaining bracket. 

7. Allow the bomb to drop a little and twist it (rotate)in both directions while pulling towards the front of
the vehicle, until the back rubber bushing becomes free from the back hanging bracket. The bomb is now

8. Install the new "bomb", tighten the forward retaining nut, attach the line to the booster cylinder and high
pressure hose ( 2 new copper washers ), reattach the hose to the hydraulic fluid container, and refill. It
will take some time for the pump to refill the bomb to proper pressure, usually a good 5 minutes or so. 

Hydraulic Servo 
There is a relatively easy test for hydraulic servo (booster amplifier ???) leakage: After running the engine
at about 2000 RPM's for a few seconds, stop the engine. Carefully disconnect the metallic return line
from the servo to the Pentosin reservoir (have rag handy) and observe for a couple of minutes. Only a
few drops of Pentosin should come out from the servo. If a more definite flow is present, the servo is
leaking. If the servo leaks during this test, there is a high probability that the pressure accumulator is O.K.

  hth, chris

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