Sorry for the long description but i think most people just dont have a clue, no offense But if your really lazy just skip to the last paragraph.

For those of you with rear disk brake conversions, and perhaps even with front/disk rear/drum combinations, i found some useful info i'd like to share. Check it out, taking all the math out and giving you simple percentages, let's talk about brake bias. When you apply the brakes, hydraulic pressure runs through your brake lines, which eventually end up applying pressure on the pads to the rotors to slow you down. Of course when a car slows down it's weight gets thrown towards the front suspension while the rear suspension actually has less weight on it. Taking this into consideration we see why up front, in any vehicle, the brakes are bigger and better than the rear, thus the common front/disk rear/drum combination.

So to break it down, we have four brake calipers, two for the front, two for the rear. If nissan didn't alter anything when we apply 100% pressure on the brake pedal that inturn would end up being distributed evenly to all brake calipers (25%each). In this scenario the two rear wheels would lock up before the front wheels. if we keep applying pressure we'd just keep locking up the rear wheels. If however we take the 100% brake pressure coming from the brake pedal and distribute it say 60% for the front calipers (30%each), and 40%for the rear (20%each), and apply the brakes again we would see, that now the front and rear brakes lock up at pretty much the same time, thus we can come to a halt sooner.

The dual valve proportioning valve, is the part that divides up how much each brake caliper gets. In our case, being dual it controls how much the rear gets and how much the front gets. So if nissan designed the proportioning valve to give 100% brake pressure towards the front and 0% to the back we would only have pressure in the front brakes and thus the rear brakes become useless, if we slammed on the brakes, the front wheels would lock up and the rear not at all. If the dual proportioning valve we desinged to go with a brake bias of 90% to the front and 10% to the rear the front would still lock up. So now we are seeing how we can adjust this proportioning valve so that upon heavy braking, both front and rear wheels lock up at the same time, this would mean an excellent braking improvement in normal driving as well as an excellent 'feel' when slowing down.

Ok so on to the point my 96 alti locked up the fronts and then the rears(i'm not proud of it, wish i had ABS) when under heavy braking. But after swapping out the drums for disk, when under heavy braking, the front would lock up way before the rear, to the point were i thought something was wrong. If during hard braking i pulled onthe e-brake really hard the front would still lock up before the rear. So i went to see what the hell and i found there are alti's with ABS-disk, Non-ABS Disk, and Non-ABS Drum, each actually has a different proportioning valve, each one has it's own brake bios, for example:

ALL 93-97 SE altimas (with or without abs or whatever the case)have a brake bias of 65% to the front and 35% to the rear.

All 93-97 Models with ABS have 65% front and 35% rear brake bias

All 93-97 Models Except SE Without ABS have 68.7% front and 31.3% rear brake bias

For 98-01 it gets simpler:

All 98-01 SE models have 62.5% brake bios up front, 37.5% to the rear

All 98-01 alti's except SE models have 65%front, 35%rear brake bias

phew!

So point is, as for me i have the dual proportioning valve which gives me 68.7% brakes force up front and the rest, 31.3% to the rears which means that my front brakes are getting more than twice as much pressure than the rear brakes, no wonder they lock up so easily! What i would like to do is change the brake bias but it's not like it has a lever which let's you regulate how much brake bias you want to the front and to the rear, it's internaly set and can't be changed.

The good news is theres another way we can change brake bios, sort of, we simply replace the part, with one of the valves listed above, they are all interchangeable so, it's all to your liking whether you want 65/35 brake bias 62.5/37.5 brake bias, or 68.7/31.3 bias it's all up to you, of course i'd recomment the one that makes the rear brakes work the most, (62.5/37.5) why do you think all the SE models had them? They are the best option.


So if you still don't understand anything that i've said, i'll make it simple, if you want the best brake distribution available and thus the best brake performance and longetivity, Find ANY SE 93-01 altima model, OR ANY 93-01 Model with ABS, get it's dual proportioning valve, swap it in with your existing one and wallah, guaranteed the best bang for the buck for better, smoother, braking.

Great piece of writeup. You mentioned ALL 93-97 SE Altimas are a good choice. But the 98-01 Alti's, except SE models, has the same bias as well. I'm confused. From your observations, I would say than that the 98-01 SE models is probably the best choice instead.

Better tires with more grip can reduce how much lock up you are seeing.

True but that would be like saying if you put crappy tires up front and sport tires in the rear, that would also solve the problem, the thing is we want to stop harder while locking up at the same time. So with this mod you can get sport tires on all four wheels and all lock up at the same time.

You are correct All 98-01 SE altimas give the most brake bias to the rearso they are the best choice

So it goes like this
Best choice 98-01 SE
2nd best All 93-97 SE, OR 93-97 With ABS
3rd and worst possible choice and you are really screwed if you have this and more than likely you do,
are the 93-97 altimas who don't have ABS

I like your line of thinking, mostly. It's the best option for using all of your tires' grip when braking and coming to a stop the quickest.

There are aftermarket proportioning valves out there, I'm not sure how you'd route them in a stock Altima, that will allow you to change proportioning by the switch of a lever, in case anyone wants that option.

I for one prefer the rears lock up before the fronts, that way in case of a panic stop if I need to I can still maneuver the car via the front tires, but then again I didn't have ABS on my Altima. If someone were going for perfect braking then you'd want slightly more up front to use as much grip of your tires as you possibly can (as you mentioned).

So what, one would have to replace their Master Cylinder to change their proportions?

Exactly i'd much rather the rears lock up a bit before the fronts, when the fronts lock up your guaranteed to get an annoying flat spot since all the weight is on them but if you lock up the rears, theres not much weight back there so it wont be nearly as bad. And yeah the more brake bias you can send to the back the more predictable steering becomes. Its like riding a bike if you want to slow down while your in a turn, you don't even touch the front brake cables you just tag on the rear brake cables. And yeah in an emergency if your rear tires lock up you can totally steer where you want to go vs if your fronts lock up, good luck...