There are three main factors which decide how well brakes will perform.
1) The pressure applied to the brakes
Unless the car is under 2 years old the chances are that the braking efficiency has deteriorated to some degree. Most brake fluids are hygroscopic (absorb water) and systems should be recharged with fresh fluid at least every 2 years. Always use a good quality brand name fluid (DOT4 or 5.1) or, if you are racing, the special high temperature fluids available. The standard rubber hoses do deteriorate over time so a set of braided steel hoses are always a good investment because they prevent pipe expansion and eliminate spongy pedal syndrome. Lastly, the wheel cylinders or caliper pistons should be operating efficiently. The piston should move freely within the bore and there should be no visible wear. Piston seizure is quite common and can sometimes go unnoticed, especially on rear discs.
2) The ability to dissipate heat
Even with a super efficient hydraulic system, if the pad's maximum working temperature is exceeded, the car will give up stopping no matter how hard the pedal is pressed. High performance pads will provide braking at higher temperatures, typically 450°C for road spec., and these items should be high on your shopping list. Ventilated front discs (also known as rotors) are a standard fitment on most modern cars, but replacing these with grooved or cross-drilled items will aid braking efficiency for two reasons. Firstly, pad dust and road dirt are removed more effectively by the action of the grooves or holes sweeping across the pad surface and secondly, they will reduce the build up of hot gases forming between the pad and disc surfaces (NB. This condition increases as pad sizes get larger). One last but perhaps pretty obvious point hot brakes require airflow to dissipate heat (unless you have some very special water-cooled calipers as fitted to some racing formulas, etc.). So any improvement to the airflow around the brakes will pay dividends e.g. cool air ducting.
3) The brake Surface area
To improve braking effectiveness by a considerable factor the disc size and pad area has to be increased. Unfortunately this is not just a case of fitting a pair of larger discs and a bigger set of pads. Bigger pads need bigger calipers and larger discs invariably require larger inside wheel sizes! Most modern car braking systems use a single piston floating or sliding caliper arrangement that is economic to produce but not 100% effective. Two pot fixed calipers are an improvement but with only one piston each side, the pad size is very restricted. 4 and 6 pot calipers allow for much larger pad areas to be employed with greater control over pad distortion (a major problem with the floating caliper system).
Finally, a few answers to some popular questions asked about brakes:
Q: Are grooved and cross-drilled rotors noisier than plain discs?
A: Yes and no! There is more than one factor to consider here - the number of grooves or holes, the pad material and the type and size of the disc. Generally speaking they will be a bit noisier, however, a few manufacturers claim that for some applications they are quieter.
Q: I have just fitted a new set of pads - how do you advise bedding them in?
A: Again, not a simple straightforward answer. For many motorists bedding in is the process of knocking off any high spots on the pad faces until full contact is made with the disc. But with high performance pads, the correct stabilisation of the friction material can be paramount in their ultimate performance. Some pads are supplied pre-stabilised but many others will require this process to be carried out on the car. This usually involves bringing the pads up to their operating temperature and allowing them to cool naturally but may involve more complex procedures for race applications.
Q: Most upgrades concern the front end, but what about the rear?
A: Some rear disc conversions are available and high performance pads are available for most cars. However, for normal road use, the rear brakes on rear wheel drive cars supply only 25% of the total braking effort and on front wheel drive cars it is only 10%.
Q: Will high performance pads will wear my discs out?
A: To improve the high temperature efficiency of brake pads the organic material commonly used has a metallic content added to it. These metallic particles will be slightly more abrasive but the major factor in disc and pad wear will be how hard and how often the brakes are applied.
And REMEMBER: Always replace pads and discs on both sides together!