Brake Judder Part 2

 A couple of issues ago I gave the reasons why you can get brake judder during braking. This issue I shall tell you how I went about curing my problem.

With the car chocked, jacked up and resting on axle stands the front wheels were removed (it’s better if you loosen the nuts with the wheels still on the ground). I set up my Dial Test Indicator (D.T.I.) on the calliper. I have a magnetic stand, arm and DTI but you can get other types which are better at getting onto both sides of the brake disc and clamp on any solid component.

Rotating the hub and disc will give you the ‘run out’ or disc distortion. I have been told not to accept anything above two thousandths of an inch, mine were close to eight. If you cannot get hold of a DTI use the calliper as a fixed point and feel any in and out movement of the disc when it is rotated. Two thou’ is just about ‘feelable’ so anything above this is out of tolerance. 

The solution was either to re-grind the faces or re-new the discs. As my discs were probably the originals and worn I decided to re-new. Just as a side issue, if you have five thou’ distortion on all wheels you would have to move the pedal about an inch to push all cylinders and pads back onto the discs.

Back to changing the discs: - To do this the calliper will have to be removed but just to let you know how the pads are renewed I will also do that. At this stage the pistons can be pushed back into the cylinders (the new pads will be thicker than the old). There is a special tool to do this but because we will be throwing the old pads away we can lever the pads back, but in doing so will probably damage the face.

It is also desirable to undo the bleed nipple no more than a turn to let the fluid expel, otherwise you could damage a seal in the master cylinder and then tighten up again. Brake fluid is like paint stripper, so collect any expelled fluid into a jar via a tube. The bleed nipple always seems to be corroded in, so be very careful and use release fluid, a good spanner and slowly work back and forth until the screw can be undone (If you are unlucky and it breaks, it can be removed by ‘Classicar Automotive’ or others).

The pads are held in by retaining pins, which in turn are held in by ‘R’ pins (if these have been lost or broken, split pins will do). Remove these (light percussion may be needed) and pull out the pads. When renewing use the anti-squeal shims (the arrow is in the direction of rotation) or copper grease on the face that contacts the pistons (copper grease shouldn’t melt when the brakes get hot).

Put the retaining pins back in from the inside, I do it this way so if they do come out they will not damage the wheel, and then the lock pins. When removing the callipers you can reduce fluid loss by clamping the hose, I use a normal ‘Mole Grip’ but special tools are available. Then undo the nut that holds the rigid pipe to the flexible pipe, again be careful not to twist the pipe itself or it will need renewing.

Next undo the two large calliper bolts, the best tool is probably a socket, these will be locked either with lock washers, tabs, or as in my case, locking wire. The calliper can now be removed complete with the pipe. Keep and note the position of any shims, these keep the calliper, piston and pad combination parallel with the disc.

Next remove the hub and disc by prizing off the dust cap, remove the split pin and undo the large castellated nut. The hub and disc can now be pulled off with the outer bearing. The disc is bolted to the hub and the disc will have to be clamped somehow to loosen the five bolts, I used the protected jaws of a vice.

We can now clean and inspect the hub & bearings; part of the inner bearing will still be left on the stub axle. Use a proprietary cleaner or an appropriate fluid. Inspect all six bearing parts; the running faces should be smooth and not matt and pitted. If you do ‘run’ the bearings to see if they are smooth running, lubricate them first.

If you need to replace a bearing the surfaces will have to be knocked out of or off the axle or hub. Also now is the time to replace any damaged wheel studs. When joining the hub and new disc make sure the mating surfaces are clean, any defect here caused by a foreign body will be exaggerated at the face and you will be back were you started.

Tighten up the bolts, using new lock washers, evenly and in stages so as not to distort the new disc up to a torque of 50 lb ft. Now keeping everything clean, the bearings can be greased and the hub replaced on the axle, with the last bearing put in place the large washer on and the nut tightened.

When I say tighten, it really is not tight. First of all tighten the nut with the disc rotating to ‘seat’ everything in, and then undo it. Then retighten to just above hand tight (20 lb ins, that’s inches) then back off to the next position that you can get the split pin in, to lock the nut. You should just be able to feel movement when you gently ‘shake’ the disc.

If you tighten this too much, the bearing will overheat, expand and burst. Too loose and you will have a wobbly wheel. This is one adjustment you should keep checking with use. Now replace the dust cap (remember what happened to me on the Inca Trail, also the cap doesn’t have to be full of grease). 

The calliper is now put back, remembering to put the shim washers back where they came from, or you will have to set the calliper up to be parallel with the disc again. Tighten the bolts up to 55 lb ft for Girling or 85 for Dunlop callipers, and relock. If you are using wire, make sure it is proper lock-wire and it is in the right direction i.e. trying to tighten.

Replace the pads as above. Now tighten the brake pipe, remove the clamp if you used one and we are ready to bleed this side. There are many ways to bleed the brakes; one way is to suck the fluid and any air at the calliper using a vacuum pump, but the best way is to flush the whole system using pressure at the reservoir. Without special equipment this is hard to do, but we can pressurize from the master cylinder using the normal brake pedal.

One person can use a procedure to bleed the system but you have to guarantee an airtight seal at the bleed nipple, this is to attach a tube to the nipple and put the other end into a container. Pushing down on the pedal will push through the fluid & air and on the up stroke the master cylinder will be recharged with fresh fluid but it could also suck air through the nipple thread. So use a partner and on the down pedal movement, open the nipple and expel the fluid and air, when the pedal is down tighten the nipple so that on the up movement no air can be sucked back.

Continue this procedure until no more air bubbles are being expelled or all old fluid has been used if you are changing the brake fluid. Of course throughout this procedure the reservoir will have to be kept topped up. Any brake component changes should be done in pairs, so it’s now time to do the other side and then re-bleed both sides (air may have got as far back as the ‘T’ piece).

With the front high and the wheels off it’s the perfect time to get a set of spanners and check tighten all the nuts or bolts you can get at around the suspension area, also grease the ball joints at this time. The wheels can be put back on now and the car lowered. The wheel nuts should only have been tight enough to hold the wheel, as now we torque the wheel nuts to the correct tightness.

The ‘book’ figure for both the GKN type alloy wheels and the Rostyle steel wheels is 50 lb ft; I personally use a figure of 55 to take into account the twist in my extension bar and any paint etc, under the nut face. Even then I still check regularly and find they ‘back off’ with use.

Figures much higher than this will damage the nuts and studs.

Most Tyre fitting establishments do the nuts up using manual or air methods and then torque to your figure; but most are just showing you that they have been done up to at least your figure, they were already higher. They should be brought up to your figure in stages and evenly; do one nut then go clockwise missing a nut, continue until all five are correct.

Now all there is to do is test drive the car.

Brake Judder! The Saga continues.

Since my articles on brake judder, member Alexander Dorogoi has done some further investigation and talking to B G Developments (Brakes) has identified another reason for the judder, it is this: - When the Disc & Pad or Drum & Shoe combination get very hot, and they will, the pad material will infuse into the disc metal. This happens when after braking you hold the car ‘on the brakes’ thereby applying pressure between disc and pad. I am told that you can feel if this has happened by running a finger gently around the disc and feeling differences in the friction of the surface. Obviously the way to prevent this is to use the hand brake when stationary, this works on the back brake and is not so critical and the hand brake pad will not be hot. If the infusion has happened I am sure it will wear away with normal use or in bad cases the disc will have to be skimmed.