RPM jump when brakes applied

[tobyw]

OK, some basic power brake system troubleshooting here... First, a little background info: I just finished the 8.8 install, along with new front and rear rubber lines. After thoroughly bleeding the brakes, I still have a VERY soft pedal and minimal, at best, brake function. They seem to pump and hold relatively decent without the motor running, but as soon as I start it, the pedal gets VERY soft again. I have yet to remove the o-ring from the combination valve.

1.) When I apply the brakes with the motor running, the RPM's jump ~200 or so. Is this indicative of a bad brake booster?
2.) Is the soft pedal indicative of trapped air in the system still? I bled them several times, but it's easy enough to do again...
3.) Will removing the o-ring improve pedal feel?

[zman]

1. - I don't know
2. - It could be.
3. - Removing the o-ring will not fix your problem (I still have mine in).

I suggest you get a couple of inverted male plugs and plug the lines going out of the porp valve. You can narrow down the problem if it is your front or rear brakes. Just plug the line (s) and see if your brakes get stiff. If they don't get stiff your problem may be the porp valve or booster.

If the problem occured after your 8.8 swap I would suspect something in your 8.8 brakes.

zman

[tobyw]

I don't remember the pedal being anywhere near this soft, and don't remember the RPM jump either... Wonder WTF could be causing those issues by simply swapping rear brake hardware and lines??? :steamin:

[Vancopbs]

Call me toby.

[MattPascoe]

Now theres the guy that would know the answer.

Be sure to post back here what the problem was so we can all learn from it.

[drexelsteve1]

just a thought, don't have the calipers on upside-down do ya? Must have read half a dozen threads just like this...remember, bleeder should be on top.

[tobyw]

Van is da' man!! He's giving me some tips and we are working through the problems. Once we get it all squared away, I'll post up...

[HEEPJEEP]

You check your vacuum line from the manifold? There still must be air in the lines too.

[Dan90YJ]

You check your vacuum line from the manifold? There still must be air in the lines too.


YUP check the vaccumm line to the booster. If it has a leak or the like it could cause this.

IT would also explain the lack of brakeing power (loose pedal)

[tobyw]

All is good now. A quick call to Van and he set me on the right track to PROPERLY bleeding out calipers, and within 5-minutes I had it licked. RPM jump was a function of the booster being over-extended via the pedal dropping to the floor, and since I'm running a tad rich at idle, the excess vacuum draw actually helped the motor run a little better.

[zman]

How were you bleeding them wrong? Glad it was an easy fix.

zman

[tobyw]

I didn't have the bleeder screw in the absolute 12 o-clock position while attempting to bleed the system, so there was still air in the caliper cavity that couldn't escape. Van reminded me that even though the bleeder screw is on the top side of the caliper, it still may not be at the highest point possible. So, I simply removed the caliper, shoved a block of wood between the pads to simulate the rotor, and held the caliper in my hands with the bleeder screw nice and high while my buddy ran the pedal. Worked like a charm :clapping:

[zman]

Thanks,

zman