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Brake problem - pedal soft, firm, soft; Trying to diagnose a braking problem - 91 Sprint
Topic Started: May 26 2012, 11:51 PM (2,934 Views)
hdsheena
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Still new, but learning...

So..
I was hoping a friend was going to help me with this, but he's unavailable, so I'm sort of on my own. Thus, I'm hoping some kind person 'round here will be willing to help me out.

A bit of background.
Car in question is a 91 Chevy Sprint, I've owned it for 3 years or so I think. ~300 000km.
My mom borrowed my car, as she does often does. She took it to the gas station, fueled up, everything was normal. Started it back up and braked to turn out of the station. No problems. Braked at the yield sign on the way out, and suddenly the brakes were .. strange.
When I put pressure on the pedal, it is quite soft (feels like lack of line pressure), then about half way down, it gets firm, then quickly goes soft again to the floor. There is some braking power, but not full, for sure.

I'm going to check it for leaks as best I can, starting under the hood (check fluid level and for any brake fluid). I can bleed the brakes, and probably will do that before investing too much effort into other stuff. But it doesn't feel spongy like air in the lines. This is different than anything I've dealt with before (hole in brake line, air in the lines, bad pads/rotors).

What I want to know is how best to diagnose the problem. In what order should I check/try things? What tests/information is important to figuring out what the problem is?

Thanks for any help at all! Anyone local want to come take a look in exchange for a beer? :)
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Jittney
Anchorage 92 XFi

Welcome, hdsheena :)
You're heading in the right direction.
Check the brake fluid level first.
Look for leaking brake lines underneath..
Brake booster check = does engine idle increase when you press the brakes about a 1/4 pedal?
Those are 3 free easy checks.
Bleeding brake lines is understandable if you've recently been into your brake fluid system.
Let us know what you find :news

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Car Nut
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Jittney is correct. Ck fluid level first. If low, check lines, wheel cylinders, etc for leaks. Is the brake pedal creeping to the floor when you apply the brakes? If so, & you determined that there are no leaks, you may have a bad master cylinder. To test, start car, pump brakes 3 times then hold steady pressure on the pedal. If it goes down toward the floor your master is probably on the way out.
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Bad Bent
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Facetious Educated Donkey

Also the brake booster hose #27 has been known to kink. It's a ~1/2" ID hose that may have a check valve in it.

If you have used Seafoam, now might be a good time to Seafoam the engine through the vacuum hose/brake booster connection. http://www.youtube.com/watch?v=vRAV5ln5cUc It will clean the hose out but it may have to be replaced eventually if it is truly deteriorated.

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hdsheena
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Still new, but learning...

So I headed out this afternoon to have a look...

>>>Check the brake fluid level first.
Not quite at the full line, but it hasn't been.. So no change.

>>>Look for leaking brake lines underneath..
No sign, but I didn't take the wheels off. Checked where I could see under the hood, too.

>>>Brake booster check = does engine idle increase when you press the brakes about a 1/4 pedal?
Sort of.. It changes, but I don't think it increases until after I let off the pedal?

>>>To test, start car, pump brakes 3 times then hold steady pressure on the pedal. If it goes down toward the floor your master is probably on the way out.
Definitely holds it's own, no movement toward the floor.

So, I did these checks, and the pedal felt normal. I decided to drive it up and down the driveway to see how it felt when moving... It won't move. It feels like all four tires are seized.

So any thoughts on brakes seized ON now?
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hdsheena
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Still new, but learning...

It's the rear passenger that's seized, so I'm sure it's a park brake issue. Just coincidence to have both at once?
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Potter
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Col. Potter

hose #26 rotted out on me... and i had no clue about it it would do the same as what your saying because when it built up enough vac it would work properly. i replaced all my hoses and its worked fine....... just something to keep an eye out for

And welcome to the site :)
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HelterSkelter
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#1 Pizza Driver

i would check for a collapsed, gashed brake booster vacuum line. the line may be ok when there is no use of the brakes and then as it starts to smoosh from braking the gash opens and the brakes feel hard because you are non power braking and then once the pressure reduces the brake booster hose will unsmoosh and the gash will close and the brakes will feel softer again.
Edited by HelterSkelter, May 28 2012, 07:09 PM.
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hdsheena
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Still new, but learning...

HelterSkelter
May 28 2012, 07:08 PM
i would check for a collapsed, gashed brake booster vacuum line.
Makes sense. Can you link me to, or briefly explain, the procedure for doing this?
Thanks!
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Thart10


New to Geos, but if your car has a Seperate Proportioning Valve between the Master Cylinder and the Calipers/Wheel Cylinders, it could be the problem. The early Chevy Sprint Turbos, Swifts & Firefly's had issues with them.
Edited by Thart10, Jun 19 2012, 04:42 AM.
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hdsheena
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Still new, but learning...

Just an update...
We swapped the booster, master and prop valve. Next is rotors and calipers on the front.. Hope that fixes it!
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