I have recently had new discs and pads fitted and had brake fluid changed. However, my brakes are still rubbish, the car shakes when braking and under very hard braking the pedal goes to the floor. Do I need yet more discs and maybe change master cylinder or somthing?

 

Cheers, Dan

Hi Dan,

 

Ok this is not actually as rare as you would think after discs and pads have been changed, there are a few reasons but more over its often mistakes made during the fitting and incorrect poor brake diagnosis.

 

So I am guessing you have had the front discs and pads done, when the old pads are removed and the calipers removed there is a tendancy by many to squeeze the caliper pistons back without wondering where the fluid is going and at what rate.

 

The fluid of course is been forced back through the system pipework/ abs unit and then against the natural flow of the master cylinder rubbers seals, this is by far the most common cause of master cylinder failure. Some may disagree but our workshop experience bears this out by how many cylinders we have changed over the years when owners or other garages have forced the fluid into the master cylinder in this way.

 

Of course the easiest and correct way is to lock off the flexible caliper line and open the bleed nipple on the caliper BEFORE easing the caliper pistons back, a suitable bleed pipe will need to be on the nipple to prevent fluid getting everywhere, air is not such an issue at this point as you should always bleed the calipers anyway in part to just get rid of the brake fluid in the caliper and line as this is the section of fluid that has been getting most hot.

 

So that done correctly now onto the fitting of the new discs, so after the old one are removed the hub area needs a real good clean, ensuring no bits at all are left apply a very small smear of silver grease not copper grease ( see note below ) for the disc to slip over the hub easily.

 

Temporarily fit at least three wheel nut and hand tighten, now rotate the disc and check for runnout, if there is any then remove the disc and move the disc by one hole and refit accordingly, this can be due to small variations on the hub face and the cause of brake shake.

 

Leave the wheel nuts in place whilst you do the rest of the build up to prevent the disc moving away from the hub where some bits could fall in and undo all your good work finally bleed that caliper before moving to the next one.

 

One other point that repeatedly gets overlooked is the rear brakes, poor or low effecient rear brakes add more work load to the front, this can cause overheat and fade and general poor brake effeciency, check the condition of the discs on the rear, not just the outer face but get the car up and check the inner faces, this will often tell you more about the condition than the front, rear pads also have the habit of getting stuck in the caliper due to the anti rattle plates ( not the shims) fitted at the ends position of the pads, we have found removing these has no detrimental effect (no noise) but ensures a free pad operation through out the pad life.

 

Finally pads, these can make a huge difference, paying expensive does not always guarantee you get the brake feel you want, matching them to the type of driving you actually do is more effective.

 

So if you drive occasional quick but in the main like to take the cruise route rather than the track then there is absolutely no reason to go buy £100 + pads that are designed for totally different use, in fact they will under-perform due to been out of their design heat range.

 

On a final note, pads of all types have a back plate that has been struck from a larger section of metal, the edges of which often have a flashing lip, this is then painted or coated in some way. ALWAYS remove this lip edge, if you do not the pad will be tight in the caliper and can lead to jamming of the pad which leads to poor brakes, and wheel shake due to unblanced caliper / pad application.

 

Remember breaking in new pads takes at least 100 miles on new discs when changing pads alone the mileage is maybe more than double this, on badly scored discs the pads may never properly bed in due to glazing.

 

Note: Copper grease melting point is lower than than the average brake disc heat on a 300zx , hence it liquifies and runs out, we always use and recommend silver grease which is more than double the heat range of copper grease.

 

Jeff TT

Edited March 6, 201114 yr by JeffTT