Aztek 2001 shuddering brakes

Aztek 2001 FWD, front rotors, rear drums.
As an opening note, this is a problem that has been with this car since the delivery day. Three GM dealers and one independent garage had their
hands on this, but no one could ever fix it.
Ok... Here we go...
The problem is a kind of vibration or shuddering when I apply the brakes. It is mostly noticeable when I drive downhill and/or when I drive at 100 km/h (did I mention I am Canadian!?!?) or more and press the brake pedal. The shuddering will start when the speed decreases below, let say, 70 km/h and until the car stops. It is not always easy to tell, but at times, the shuddering is clearly happening mostly at the rear right wheel. Driving in the city, it is rarely noticeable unless I apply the brakes strongly. My brake pedal has always been spongy and I have to push harder than any other car I ever owned or rented to bring the car to a stop. If I just apply the parking brake pedal on the highway and go slowly from, say, 110km/h to 60km/h, the shuddering is even stronger, to the point that the rear of the car starts seriously bouncing. If I drive below 60 km/h and apply the parking brake, then the shuddering is not so clear. Of course, this is an average description since the behaviour will vary depending on how hot the brakes are. Also, I noticed that it seems worse on a humid or rainy day. I must mention too that I engage my parking brake every time I leave the car as an old habit I acquired when I had a house with a sloping driveway (GM mentioned at some point that this might be causing the problem?!?!?). Lastly, I am not sure if this helps, but the brake pedal does not pulsate much (or not at all) when the shuddering happens.
The different mechanics that worked on the car have been content with playing with the rotors/pads/drums/linings till we are all blue in the face but this never solved the problem. They have turned and replaced all these parts quite a few times through the years. Typically, when I start fresh with a new garage, the mechanics diagnoses the shuddering and, right away, he wants to play with the front rotors/pads. When I complain that it really seems to originate from the back, he then wants to play with the drums/linings. The only constant fact here is that no one, I mean no one has ever given a thought about looking for other causes. I am running out of options and my extended warranty is ending soon. I want GM to acknowledge the problem and most importantly... Fix it! I wasted so much money and time on this issue, it is just driving me insane.
Here are some questions:
- From the description, can one conclude that the problem is at the rear and not at the front? I am basing my assumption on the parking brake test and the fact that when regular braking is very bad, the shuddering is clearly coming from the rear wheels (mainly at the right). - Since the parking brake causes the shuddering on its own, doesn't that eliminate the hydraulic system from the equation? - My present dealer says that the problem has to be with the brakes since it happens when I apply the brakes. I think this is simplifying the issue. Can someone point me to a list of possible parts that could be the culprit since I am no mechanics myself (how about the hardware, backing plate or others...). - Does changing the linings always mean changing the shoes too? I thought the linings were glued to the shoes by the manufacturer?
Feeling inspired?
Thanks
Simon
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Simon Till wrote:

If you had this clearly on the front it could be caused by things other than the brakes themselves. Suspension and shocks could do it in strange cases. So I believe the same could happen in the rear. By this time I suppose everyone has checked the rear struts and suspension parts for looseness, alignment or damage.
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If it were me I would get a dial indicator on the rear hub, suspect r/r, maybe, but anyway and check for runout of these hubs, if they are over the limit this can cause the drum not to run true on hub, causing the condition you describe. If you pinch of one brake hose at a time in the rear you may be able to pin point which wheel ass'y is suspect.

-
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also just to be sure, have you tried different tires/wheels?

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Good stuff for my dealer to hear about. Thanks. Excuse my ignorance, but when you say pinch one rear line at a time, do you mean you can road test that or you mean for some other tests on the lift. I am scared to think the former is even safe!
Simon

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wrote:

Have you ever had a ACCURATE 4 wheel alignment done on the vehicle? Set right TO specs, not just within range? Check ALL suspension bushings, and if that has all been done borrow a set of wheels and tires from a different vehicle that does not have the problem.
My Trans Sport recently exhibited the exact symptoms, and I ended up replacing both right rie rod ends (inner is inside the rack) and right lower ball joint.

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Hi there,
Thanks for the help.
Actually, the fact is that GM refuses to look at anything else than the brakes. They also have the weird obsession that it HAS to be the front end since they tried 3 sets of drums and changed the linings at the back in the last few weeks. But I refuse to let them work on the front rotors and pads because I have been there before and it doesn't solve the problem. From all the replies received, I guess GM should have been looking at other stuff just like I've been begging them to do for a while now.
O.K., now I need to take a crash course in suspensions so I can sound like I almost know what I'm talking about when I visit the dealer again tomorrow.
Simon
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