Sounds like your front brake dics are warped,It is a common problem with the 9r,especially those fitted with the six pot tockico calipers.Also check your brake calipers,the six pots tend to sieze pistons and need regular cleaning .
Try chaining fork oil.
check your forks for play.
check your pads for uneven wear.
also make sure the stearing head bearings are ok.
hope somethig helps.
could also be caused by the unevenly worn front tyre.
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does this happen when you brake gently or when you have to brake hard? cars with ABS - anti-lock braking system have a feature which help prevent the car from skidding when braking. If wheels lock, then brakes will automatically release for a moment and then lock again. The experience feels very much like what you are describing. check this wiki out Anti lock braking system
holden commodores are know for vibration when braking know as brake shudder.
To correct this replace brake pads and machine disc rotors .
Clean hubs before refitting disc rotors .
Check and make sure you can achieve the run-out within holden specifications .
To achieve the correct run-out Front disc need to be under 0.5
When doing run-out place front disc back on hub once all surface areas have been cleaned .
secure disc on hub with wheel nut just even so its all flush.
Then use a dial gauge and measure run if you can achieve on 1st stud move around all studs if u still cant get the front within 0.5 replace hub.
follow same procedure for rear but remember rear is0.8..
If you did nor replace the attaching hardware when replacing the rear shoes and drums then there is a good chance that the rear brakes are not performing the self-adjustment necessary for them to work properly. This results in the rear brakes not braking effectively and placing extra stress of the front brakes. The increased stress on the front brakes causes excessive heat on the rotors and they will warp and this causes the shuddering you feel. Inspect the rear brakes to ensure that they are working as designed.
Section 06-03: Brakes, Front Disc
1999 Taurus/Sable Workshop Manual
REMOVAL AND INSTALLATION
Procedure revision date:
Raise vehicle on hoist.
Remove wheel and tire assembly. Be careful to avoid damage or interference
with caliper bleeder screw fitting and front disc brake rotor shield (2K004).
front disc brake rotor (1125) and disc brake caliper (2B120) assembly in such a
way as to prevent deformation of front disc brake rotor , and nicking,
scratching or contamination of brake linings/rotor surfaces. NOTE: To prevent damage, do not allow disc brake caliper to hang by the
front brake hose (2078) .
Remove caliper anchor plate bolts and discard. Position disc brake caliper
out of the way. Prevent separation of caliper housing from anchor bracket
because contamination of guide pin journals can occur.
NOTE: If excessive force must be used during front disc brake rotor
removal, the front disc brake rotor should be checked for runout prior to
If additional force is required to remove front disc brake rotor , apply Rust
Penetrant and Inhibitor F2AZ-19A501-A or equivalent meeting Ford specification
ESR-M99C56-A on front and rear rotor/hub mating surfaces. First, strike front
disc brake rotor between lug bolts (1107) with a plastic hammer. If this does
not work, attach 2-3 Jaw Puller 205-D027 (D80L-1013-A) or equivalent and remove
front disc brake rotor . Remove front disc brake rotor from wheel hub (1104)
assembly by pulling it off the lug bolts.
If original front disc brake rotor is being installed, make sure rotor
braking and mounting surfaces are clean.
to clean rust and foreign material from rotor and hub mounting faces when
installing new or old front disc brake rotors will result in high rotor runout,
which will speed up the development of brake roughness shudder
Apply a small amount of Silicone Brake Caliper Grease and Dielectric Compound
D7AZ-19A331-A or equivalent meeting Ford specification ESE-M1C171-A to pilot
diameter of front disc brake rotor.
Install front disc brake rotor on wheel hub assembly.
Install disc brake caliper and new caliper anchor bracket bolts on front disc
brake rotor . Tighten caliper anchor bracket bolts to 88-118 Nm (65-87 lb-ft).
to tighten lug nuts (1012) in a star pattern may result in high rotor runout,
which will speed up the development of brake roughness, shudder and
Install wheel and tire assembly on vehicle. Tighten lug nuts to 115-142 Nm
(85-104 lb-ft) using Rotunda Accutorq® Lug Nut Socket 164-R0303 or equivalent on
a 1/2 inch air impact wrench, or use a torque wrench.
Pump brake pedal prior to moving vehicle to position brake linings.