Author Topic: Power brakes  (Read 5360 times)

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Offline ddahlgren

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Power brakes
« on: May 08, 2008, 03:29:18 PM »
Ok I am pretty good at a lot of things but and a big but this has me a bit stumped. I have an older pontiac withpower brakes.. an 87 v6 fiero that is fun to drive but have not in a long times as the brakes just plain suck. It has I think 9.5 or so unvented rotors and very typical looking GM calipers along with a power brake booster. It has had new pads(2 different sets) new lines turned rotors everything moves freely there is decent vaccum at the booster. When you step on the pedal they feel like power brakes in that they are a bit lower and very hard with the engine off and you can feel in the pedal the effect of the booster like you can in my newer Pontiac. The car just st5ops horribly and will not lock up a wheel short of maybe in an ice storm.. How in the world do you trouble shoot this thing? Oh there are no leaks(visible bhind carpet etc.) in the master cylinder and and the MC does not lose fluid and bleeding them at least shows there is flow to all the calipers... I haved owned one previous to this and it stopped just fine...
any ideas are welcome... :?
Dave

Offline Sumner

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Re: Power brakes
« Reply #1 on: May 08, 2008, 05:09:12 PM »
Ok I am pretty good at a lot of things but and a big but this has me a bit stumped. I have an older pontiac withpower brakes.. an 87 v6 fiero that is fun to drive but have not in a long times as the brakes just plain suck. It has I think 9.5 or so unvented rotors and very typical looking GM calipers along with a power brake booster. It has had new pads(2 different sets) new lines turned rotors everything moves freely there is decent vaccum at the booster. When you step on the pedal they feel like power brakes in that they are a bit lower and very hard with the engine off and you can feel in the pedal the effect of the booster like you can in my newer Pontiac. The car just st5ops horribly and will not lock up a wheel short of maybe in an ice storm.. How in the world do you trouble shoot this thing? Oh there are no leaks(visible bhind carpet etc.) in the master cylinder and and the MC does not lose fluid and bleeding them at least shows there is flow to all the calipers... I haved owned one previous to this and it stopped just fine...
any ideas are welcome... :?
Dave

I'm not crazy about just swapping parts, but if it was mine I'd put a new master cylinder on it and if that didn't do it I'd replace the booster even though it seems to be working.  The master cylinder is probably pretty cheap.  If you do that bench bled it before putting it on.

I'll bet a friend of mine on Rodders Round Table could probably figure this out.  I highly respect his diagnosis on things as he has a ton of knowledge and experience and I haven't found him to be wrong yet.  If you don't mind I'll e-mail your problem and see what he has to say.

c ya,

Sum

Offline Stainless1

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Re: Power brakes
« Reply #2 on: May 08, 2008, 05:34:22 PM »
Hey Dave, call Jon Amo, he does this stuff for a living and maybe he will trade some of that knowledge back that youv'e been expending on the scooter.
 :-)
Stainless
Red Hat 228.039, 2001, 65ci, Bockscar Lakester #1000 with a little N2O

1800guy

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Re: Power brakes
« Reply #3 on: May 08, 2008, 05:58:08 PM »
Newbie here, been lurking for a while so I guess it's time to jump in.  My suggestion would be in the area of carefully checking the brake hose for internal blockage, esp. on a 20 year old car.  More often bad hoses show up as a failure to release completely, but resticted flow can also prevent full piston travel.  And yes, like most people everything I know I learned the hard way.  Cheers, RJ

Offline Milwaukee Midget

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Re: Power brakes
« Reply #4 on: May 08, 2008, 07:16:10 PM »
Speculation, so take it just as that. 

As I recall, and this is from reading Road and Track many years ago, the V6 had bigger brakes than the Iron Duke.

At some time in the car's life, has the MC been replaced?  If so, perhaps the MC was replaced with the unit from the 4 cylinder, and might not be delivering the correct amount of fluid.  Again, speculation, but that might describe what's happening.

Just as a precaution, I'd also inspect the vac line to the booster - that's a long distance for vacuum to travel through a 21 year old piece of rubber.  Check you vacuum at the manifold, and then at the booster, they should read the same.
"Problems are almost always a sign of progress."  Harold Bettes
Well, I guess we're making a LOT of progress . . .  :roll:

Offline doug odom

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Re: Power brakes
« Reply #5 on: May 08, 2008, 10:48:00 PM »
Dave, What size is the master cylinder?  What size is the caliper bore? Is it the single piston floating type? What size is the booster diaphragm? Sounds like the vacuum assist is working. This is a case where size does matter.
Doug Odom in big ditch

How old would you be now if you didn't know how old you are?
If you can't race it or take it to bed - it ain't worth having.

Offline ddahlgren

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Re: Power brakes
« Reply #6 on: May 09, 2008, 07:54:35 AM »
Hi thanks for all the suggestions. I dove this car as a daily driver for about 6 years and just sort of dealt with it. It has ad the fluid flushed,new pads rebuild the caliper, turned rotors, a couple of sets of pads, new flex lines basically everything but a new proportioning valve master cylinder and booster. I have owned the car since it had 30k miles on it and it only has 66k now... I never realized how bad they were until I bought a Grand Prix for a midsize car that thing stops. Both have 4 wheel disc brakes with the Grand Prix ones being a bit larger though not by much but it is a heavier car. Anyone want to buy a low miles Fiero that needs brakes?? LOL Quite serious as there is a limited amout of time and energy I am willing to put into a car I don't drive.
Dave

Offline Stainless1

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Re: Power brakes
« Reply #7 on: May 09, 2008, 08:59:45 AM »
That was the info we needed Dave, it is an old, low use car.  Likely the proportioning valve is stuck, not putting enougn juice to the front.  Of course, I am not a trained professional, at least not for brakes.....  :roll:
Stainless
Red Hat 228.039, 2001, 65ci, Bockscar Lakester #1000 with a little N2O

Offline ddahlgren

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Re: Power brakes
« Reply #8 on: May 09, 2008, 03:07:23 PM »
Jon Amo had the same suggestion as to the proportioning valve.. Though saying that I can not lock up any wheel front or rear..

Dave

Offline enjenjo

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Re: Power brakes
« Reply #9 on: May 10, 2008, 01:02:13 AM »
I believe in 87 the Fiero still had Chevette brakes. Not very good to begin with. The later ones had better brakes, but they don't retrofit easily. I would  look at using a more agressive brake pad, it may eat rotors faster, but worth it if it stops better.

There are other things you can do, but a bit of engineering goes into it. Brake pads are easy.

Offline ddahlgren

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Re: Power brakes
« Reply #10 on: May 11, 2008, 09:39:00 PM »
High performance Chevette pads are a little rare LOL.. Sort of like looking for a turbo kit for a Fiat 600.... I may play with it time frees up but been having a lot of projects kick my butt as of late so time is scarce and lots of race dates coming very soon.. Dave

Offline RidgeRunner

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Re: Power brakes
« Reply #11 on: May 11, 2008, 11:24:18 PM »
Dave,

       PM sent.

            Ed

landracing

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Re: Power brakes
« Reply #12 on: May 12, 2008, 01:12:58 AM »
One of the basic things to consider here is you are trying to compare the braking of a new car versus a car that is over 20 years old. technology has changed and only thru advances in Traction control and and four wheel abs has the braking been improved. Along with weight reduction in new cars. (Cadillac lost 300 + Lbs of wiring when they when to full CAN enabled BUSS systems).

Although we have a problem here. I have talked with dave on this and he states he can get a full stream of fluid out off all wheels at caliper, I just dont remember if this had a load sensing proportioning valve or not, or a master that is bypassing could be the culprit not giving full range of master cylinder stroke or even a mis adjusted master cylinder push rod.

I suspect a proportioning valve problem still. Raise the whole vehicle and step on the brakes and see if you get pad actuation on brake apply. And really try to move the wheel... I think you will find the problem

Jon

landracing

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Re: Power brakes
« Reply #13 on: May 12, 2008, 01:29:11 AM »
WAIT,

I think I got it, the rear calipers on that system have a adjusting screw in them for ebrake operation. When replacing pads or rotors you need to reset the adjuster on the rear brakes. this is done with a 14mm wrench. take the rear ebrake cables off the caliper, then rotate that screw until very slight drag in felt on the rotor. reinstall the cable and make sure you still have same amount of drag, If too tight then back off that screw.
This works because of the screw type system within the caliper bore. If its too far out and not adjusted properly then you only get so much travel within the stroke of the master and fronts are doing all the work and the combination valve only sends so much fluid pressure to rear if the valve is working correctly. Lots of front brake operation and tons of pedal effort to stop that steel can.
Based on our discussions this sounds like the fix. Please report back.

Jon