Possibly a stupid question about line lock test

"saxman" wrote in news:1170260766.999763.166930 @q2g2000cwa.googlegroups.com:

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.)

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.

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.

"saxman" wrote in news: snipped-for-privacy@h3g2000cwc.googlegroups.com:

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.

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.

innews: snipped-for-privacy@h3g2000cwc.googlegroups.com:

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.

wrote in news:kk9wh.15042$ji1.12324 @newssvr12.news.prodigy.net:

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.

innews: snipped-for-privacy@h3g2000cwc.googlegroups.com:

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

innews: snipped-for-privacy@h3g2000cwc.googlegroups.com:

The VW Jetta has Disc brakes all around.