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Old January 14th, 2007, 12:19   #8
Join Date: Jan 2002
Location: Alberta
TDI(s): 2002 Jetta

Thanks people. It sounds like this is an almost-normal VW brake pedal behaviour but why??

Here's my thinking based on good knowledge of manual hydraulic brake systems and scant knowledge of ABS:
  • There are no external leaks or brake fluid consumption
  • Engine-off testing would seem to prove that the master cylinder is good. I can stand on the pedal and it will not continue to go down - the master cylinder is not bypassing internally or leaking externally.
  • Booster problems would either make it more difficult to depress the pedal (e.g. vacuum leak - less assist) or maybe hold the pedal down, but not all the way down to the floor(?)
  • Pedal has a direct mechanical linkage to the master cylinder with booster as an assist only.
  • If the pedal ever goes to the floor, that means by definition that the pistons in the master cylinder have moved further into the master cylinder displacing brake fluid.
  • The master cylinder has only two output lines, likely to diagonally opposite wheel pairs (RF&LR, LF&RR) via the ABS module.
So, if there are no external leaks and the master cylinder isn't bypassing, having the pedal go to the floor, ever, means that brake fluid is getting pushed out into the system (ABS unit, lines, calipers). Where can this fluid be going??!!

Does the ABS pump behave like a hydraulic accumulator somehow?

I'm not desperate to "fix" this. I just want to understand it.
BugBoyOne is offline   Reply With Quote
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