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Weird pull on braking...

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dpimm View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dpimm Quote  Post ReplyReply Direct Link To This Post Topic: Weird pull on braking...
    Posted: May/03/2021 at 9:26am
My car has a pull on braking that I cannot seem to pin down. 

FYI- I am running 78 Matador front discs on a 68 AMX. I have been thru the front brakes 3 or 4 time and the backs 2X and cannot get rid of it. I have replaced all the calipers 2X, rebuilt the pressure differential valve, replaced every line and hose, LITERALLY the only things not new are the proportioning valve (which is rebuilt) and the rotors, which were turned. I know the rears are adjusted correctly. I have replaced the calipers 2X and even had stainless lines made and I cannot find anything leaking.

Is it possible that the pull is in the suspension/steering parts? Suspension is 100% new, including every bushing and new trunnion springs. Just wondering if maybe I have a shock that is failing causing one side to dive on braking... Is it possible that the sway bar is somehow not centers and that is causing it( I cannot see how that is possible, but...) 

I'm really stumped.Cry
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: May/03/2021 at 9:43am
Describe the pull in more detail please.
Is it the front or rear, left or right. Does it only happen on braking or is it happening in cornering also, at speed or low speed.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dpimm Quote  Post ReplyReply Direct Link To This Post Posted: May/04/2021 at 10:03am
Pull is only on braking, gets worse the harder you brake and is to the driver side front.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: May/04/2021 at 10:28am
Sounds like you have been through the brakes so look at the drivers side suspension. See if you can have someone at the side of the road observe the front driver side wheel while your braking.
Could be the lower ball joint, but I suspect the trunnion is cantering backwards, causing the wheel to tilt and pull the car. Basically steering you. The observer would see the front wheel gap between the tire and fender close up some. They may also see the wheel move inwards at the top.
The steering wheel will also fight you to go the new direction.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote troutwilly Quote  Post ReplyReply Direct Link To This Post Posted: May/04/2021 at 7:52pm
Another thought, though we still don't know which side its pulling towards. 

Maybe the other side is not really braking and the side its pulling toward is trying to do all the work.  If its the same as my 70, the driver side operates regardless of the distribution block, the passenger side operates through the distribution block.  You've bled the brakes several times....does one side bleed easier or more readily than the other?  I'm thinking distribution block.
Bill O.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: May/04/2021 at 8:56pm
The 68 AMX has a differential pressure switch that sees front and rear, proportioning valve on the rear, no side to side valving. Unless it's plumbed wrong!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ramblinrev Quote  Post ReplyReply Direct Link To This Post Posted: May/04/2021 at 9:44pm
By chance are you using silicone fluid? I have this crazy pull one way, then pull the other way going on on on my '74 Hornet (with all recent drum brakes/parts/master).  And I've been a big believer in silicone fluid. But I have come to figure out that the wheel cylinder cups have expanded a bit, and are causing trouble with one or the other wheel cylinders/pistons retracting properly.
I suppose the same thing could happen to a disc brake caliper seal...
74 Hornet Hatchback X twins
62 American Convertible (still worth the $50 I spent in 1973!) AMCRC #513, AMO #384
70 AMX 360 4-speed
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: May/05/2021 at 6:52am
After checking the front suspension for damage or looseness (as new parts break in they can loosen up), get the front end aligned. Could be that the new parts have just "seated in" and are throwing the front end alignment off. It's common for new rubber parts to loosen up and seat in after a few hundred miles.
Frank Swygert
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Post Options Post Options   Thanks (0) Thanks(0)   Quote troutwilly Quote  Post ReplyReply Direct Link To This Post Posted: May/05/2021 at 10:19pm
Originally posted by Trader Trader wrote:

The 68 AMX has a differential pressure switch that sees front and rear, proportioning valve on the rear, no side to side valving. Unless it's plumbed wrong!


Understand.

But I believe its the same as my 70.  To explain further, if you follow the front brake line from the MC, it goes to a "T" fitting on/near the firewall.  From there, one side of the "T" then goes to the passenger side front caliper through the pressure differential switch.  The other side of the "T" goes directly to the driver side front caliper.  Therefore, if there was a problem with the pressure differential switch it would only affect the passenger side front (and maybe the rears).

I referred to the pressure differential switch as the distribution block.  Sorry for the confusion.

Would still be nice to know which way its pulling.
Bill O.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mbwicz Quote  Post ReplyReply Direct Link To This Post Posted: May/06/2021 at 4:27am
Considering that everything is new, is there a chance that the left caliper is dragging? Brake pads get more efficient as there is more heat in them (up to a point). If the rotors are new and the pads are new, there likely isn't much clearance. If you really had to squeeze the new pads in place, especially on the driver side, maybe there isn't room for them to retract fully? I had this once and had to sand down the pads about .030 to get them to drop in easily. I used a caliper to try and keep the thickness even on the pad. If both front wheels rotate freely when the car is in the air, I would think that this is not the issue.

I'm just brainstorming, there is plenty of other ideas here.

My gut feel is that there is an alignment issue. As the weight shifts forward under braking, the car can 'self steer' if one side deflects more than another, or has a different alignment.

I'm not sure what you had for an alignment, but I'm used to getting a printout showing lots of information. I wonder if the front 'axle' is square to the car. In other words, if the wheelbase is 1/2" shorter on one side than the other, this could affect things. The strut rods could be adjusted so that you get proper caster on each wheel, but something may be off. Just another idea.
1970 AMX. Holy Crap its easy to spend money on this car!
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