brake booster vs. hydraullic accummulator

[Ameer Antar]

Sounds like you may have 2 problems (maybe more!)... First, can you tell if the car has ABS? The vibrating could be the ABS kicking in due to a bad sensor or something falsely triggering the sensor, as someone already posted.  You ought to be able to test this out by turning the ABS switch off or pulling the fuse if you don't have a switch. If the brakes stop vibrating, then the problem is ABS related. Otherwise, it could be really bad rotors. You should really replace the rotors if the pads are finished... New rotors have a tempered surface when manufactured that's lost after many miles of use and even after turning/grinding them. You can get some OEM's reasonably priced at TPC.

The power assist issue can be more complicated. If you have no assist at all even during normal running, then it could be a problem with the servo (booster), hydraulic pump, or high-pressure hose to the accumulator (bomb). The accumulator is just like a reservoir that hold lots of pressure needed for emergency handling, or in case the engine shuts off and the hydraulic pump no longer is making pressure for the assist. 

The accumulator can be tested easily by running the engine for a while, then shutting off the engine. Pump the brakes about 20 times or until the pedal changes feel from power to manual. If the number is less than 10 than the accumulator is on its way out. 

If you have no change at all and it's always a manual brake feel, then I'd say you may have a problem with the servo, etc as mentioned above. The shop manual (including Haynes) has a short procedure to check the brake servo. (I think it was just to check the servo return line for any flow while the car is running. I think anything more than a few drops means a bad servo). Make sure you have the right Pentosin hydraulic fluid in there. You need to eliminate some possibilities before spending $$$ on hydraulic parts.
good luck.

-Ameer

----Original Message----
Date: Mon, 13 Dec 2004 15:34:01 -0600
From: Ahmad Helal <helal.ahmad at gmail.com>
Subject: Re: brake booster vs. hydraullic accummulator
To: quattro <quattro at audifans.com>
Message-ID: <78e7dc060412131334131c4de0 at mail.gmail.com>
Content-Type: text/plain; charset=UTF-8

1990 audi 100
Brake pads are fine, they were changed recently. i DO HAVE a hard
brake pedal and there is no assistane what so ever...feels like manual
brakes....ya that vibrating feels like something ABS would do...now I
am wondering if maybe something went bad causing something else to go
bad...not sure...