Possibly a stupid question about line lock test

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saxman - 31 Jan 2007 16:26 GMT

So, I've been doing some research, and I want to run a line-lock test
on my car to check for a bad master cylinder. I understand what to do,
but I have a simple, yet possibly stupid question: After you clamp off
the lines, do you turn the car on before stepping on the brakes? Or do
you do the test with the car off?

Thanks for the help.

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Tegger - 31 Jan 2007 16:34 GMT

"saxman" <erlloyd@gmail.com> wrote in news:1170260766.999763.166930
@q2g2000cwa.googlegroups.com:

> So, I've been doing some research, and I want to run a line-lock test
> on my car to check for a bad master cylinder. I understand what to do,
> but I have a simple, yet possibly stupid question: After you clamp off
> the lines, do you turn the car on before stepping on the brakes? Or do
> you do the test with the car off?

You don't need line locks to check for a bad master cylinder.

If your master cylinder is bad, the brake pedal will slowly go to the floor
under light, steady pressure. A hard stomp on the pedal will probably see
it remain off the floor. The fluid level will not change.

If you have a leak somewhere (piston, line, etc), the fluid level will go
down over time. The pedal will go to the floor at all times under any
steady pressure.

(I don't know if the newer high-pressure integrated-ABS systems can be
diagnosed this way.)

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Tegger

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saxman - 31 Jan 2007 16:56 GMT

> "saxman" <erll...@gmail.com> wrote in news:1170260766.999763.166930
> @q2g2000cwa.googlegroups.com:
[quoted text clipped - 20 lines]
> --
> Tegger

I'm having a hard time telling whether my brake is going all the way
to the floor consistantly. It seems that the pedal goes in further
than I remember to reach my normal holding position - but it doesn't
sink under normal circumstances. If I'm applying a lot of pressure
while stopped, the brake goes in quite far - but still, not really a
slow sinking.

This is why I'd like to do a line-lock test.

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Tegger - 31 Jan 2007 18:42 GMT

> I'm having a hard time telling whether my brake is going all the way
> to the floor consistantly.

Apply very light, steady pressure over a minute or so (as though you were
waiting in a drive-thru line somewhere). In the early stages, the pedal
will only occasionally go to the floor. Later on it will go to the floor
all the time.

> It seems that the pedal goes in further
> than I remember to reach my normal holding position - but it doesn't
[quoted text clipped - 3 lines]
>
> This is why I'd like to do a line-lock test.

If the pedal is just low and spongy, then you've got air in your lines.
Time to bleed the system. Line locks will tell you nothing.

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Tegger

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saxman - 31 Jan 2007 19:16 GMT

> > I'm having a hard time telling whether my brake is going all the way
> > to the floor consistantly.
[quoted text clipped - 17 lines]
> --
> Tegger

I just got the system flushed today - so I'm sure there is no air in
the lines. Hence - why I want to do a line-lock test.

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HLS@nospam.nix - 31 Jan 2007 23:09 GMT

> I just got the system flushed today - so I'm sure there is no air in
> the lines. Hence - why I want to do a line-lock test.

A simple test will give you a go or no go answer. Much like Tegger told
you.\

With the car off, pump up the brakes and then hold a constant pressure on
the pedal. If it holds for a while, and then heads to the floor, look at
the
master cylinder.

If you get no results from this, then go further.

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Tegger - 01 Feb 2007 00:13 GMT

<HLS@nospam.nix> wrote in news:kk9wh.15042$ji1.12324
@newssvr12.news.prodigy.net:

>> I just got the system flushed today - so I'm sure there is no air in
>> the lines. Hence - why I want to do a line-lock test.
[quoted text clipped - 8 lines]
>
> If you get no results from this, then go further.

I just thought of something: rear calipers that incorporate a parking brake
are fairly complex inside, with lots of nooks and crannies. It is
distressingly easy for air bubbles to get trapped inside.

If the rear calipers have been changed (or if air has been introduced into
the system by running the MC dry) the only sure way to remove all the air
bubbles is to bleed the calipers OFF their mount brackets, turning and
tapping with a nylon mallet as you bleed. The turning and tapping shocks
the bubbles free.

Don't know how VW front calipers are constructed, but the above paragrpah
may apply to them as well.

Finally, don't overlook ABS (if so equipped). I'm guessing here, but there
may be air trapped in the ABS mechanism in the master cylinder.

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Tegger

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Nate Nagel - 01 Feb 2007 01:08 GMT

>>>I'm having a hard time telling whether my brake is going all the way
>>>to the floor consistantly.
[quoted text clipped - 20 lines]
> I just got the system flushed today - so I'm sure there is no air in
> the lines. Hence - why I want to do a line-lock test.

ISTR that you are working on a VW? Does it have rear drum brakes? I've
found with VW's that the rear drum adjusters don't have enough range to
compensate for worn drums; this will cause a low pedal. Replacing the
rear drums will fix it right up.

good luck,

nate

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saxman - 01 Feb 2007 13:30 GMT

> >>>I'm having a hard time telling whether my brake is going all the way
> >>>to the floor consistantly.
[quoted text clipped - 34 lines]
>
> - Show quoted text -

The VW Jetta has Disc brakes all around.

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Calvin - 31 Jan 2007 17:33 GMT

> So, I've been doing some research, and I want to run a line-lock test
> on my car to check for a bad master cylinder. I understand what to do,
[quoted text clipped - 3 lines]
>
> Thanks for the help.

I would test it with the car OFF. The amount of vacuum available will
never be constant with the engine running. The more vacuum, the more
assist and the farther down the pedal will go with the same force from
your foot. It is possible to detect a falling brake pedal if you
start with a low vacuum and then the vacuum rises since you have taken
your foot off the gas and the engine is pumping harder against the
closed throttle. Shut the car off and pump the brake pedal until its
stiffness becomes constant and then STAND on the pedal to see if it is
falling under constant pressure. With the engine running you won't
ever be able to tell whether it is the cylinder's resistance or the
pedal's force that is changing.

Just a shade tree fixer here, so take this for the free advice it is.

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