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Pedal to the metal...brake, that is!


gatyo

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I am seeking some sage advice from the collective wisdom of this Forum. I have

an '97 STS, w/ about 125Kmi on it, and the brakes have always felt less than

stellar. I have had this car for about five years and I don't know its previous

history. The brakes always felt spongy w/ lotsa pedal travel. When idling and

my foot just resting on it, the pedal is about half-way down, but if I press a

little harder, it slowly sinks to the floor. Also, in semi-emergency situations

I have to push it really hard, and the ABS kicks in only when the pedal is

almost on the floor.

I know there are many possibilities, but first let me say what *I think* are

NOT the causes. The hydraulic system was flushed with new fluid last summer,

and there is no loss of brake fluid in the MC's reservoir. This was done by

the dealer so I *assume* it was done properly and no air was left in the system

(when air is in the system, can not we still achieve a firm pedal by pumping

it?).

My first question: What's the maximum volume of fluid the MC can displace?

If this fluid flows to places other than back to the MC reservoir, for instance

into a ballooning brake hose or two, or back into the Brake Pressure Modulator

Valve's accumulator due to a stuck open valve in it, shouldn't I *see* a drop

in the fluid level in the MC while flooring the pedal? I do not see that.

So, I am inclined to suspect an internal leak in the MC, and here's my second,

main question: How to diagnose the darn thing? Is there any way other than by

swapping parts and thus by way of elimination? Parts aren't too cheap for this

automobile either...

Thanks: -Bela

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Gatyo, if its going to the floor I'd say dead MC but other than that I couldn't tell you how to diagnose it. Ballooning brake line maybe but if the pedal goes to the floor I'd be skeptical but you could have both. Cost wise order the the parts from Rockauto or Gmpartsdirect. Mine are less than stellar also but my pedal won't go to the floor I'm planning on changing the brake lines when it warms up a bit. How much fluid does the MC displace? Can't help you there :(

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Thanks, Joe and Larry.

The only way I can imagine troubleshooting the MC w/o swapping in a new one, is to unhook the brake lines, plug the holes and push the pedal. If the MC is good, simply bleed it at the joints. If it's bad, go get a new one, bench-bleed it followed by a bleeding at the joints.

Gmpartsdirect and/or Rockauto are great and I have dealt with them before; however, in this case I prefer a local parts store - because of quicker turnarounds. Advance Auto (Partsamerica) has a NEW Cardone (Select?) MC for $99.99 and I might be able to rebuild the old one and have a spare.

Your comments, horror stories, etc. are welcome.

P.S.: Yeah, I'd rather get an AC Delco, but it's almost twice as expensive as Cardone's, even at Gmpartsdirect.

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OEM parts only hurt once... :lol:

Disconnecting and plugging the ports on the MC and then testing the brake pedal is a good idea to isolate the problem. If it is the MC, you'll need to bench bleed it and then bleed all four calipers after installation.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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True, Kevin, it won't hurt bleeding at the calipers, but according to the FSM, that might not be necessary:

[First, it describes how to purge air at the MC ports after installation, then...]

"F. If it is known that the calipers and wheel cylinders do not contain any air, then it may not be necessary to bleed them. Check for firm pedal feel and proper brake pedal travel."

This boils down to whether "it is known"...

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