View Full Version : Braking system FAQ/diagnosis

01-31-2003, 12:03 AM
I'm bored and semi-knowledgable, so here's a small contribution to the site. Feel free to add your .02c and maybe this'll end up in FAQ section.

Braking basics:
The 240sx is equipped with a power assisted 4 wheel disc (vented front, solid rear) braking system using single piston sliding calipers. Some models came with the Antilock Braking System, which allowed for skidding wheels to lose some of their braking force to maintain optimal traction for braking. The standard fluid is DOT 3, pads semi-metallic, and rotors iron, of course.

Breakdown of the brakes:
When you push in the pedal, a rod attached to the end of your pedal is pushed into the brake booster, which, utilizing vacuum created by the engine, helps push pistons into the master cylinder, forcing fluid from the brake fluid reservoir through the lines and into the backs of the brake caliper pistons, forcing them against the backs of the pads (and using that force, also pulling the floating portion of the caliper toward the rotor, forcing the other pad to rub against the caliper). The brake system has 2 proportioning valves, contrary to most popular belief. One is in the master cylinder itself and is non-adjustable. The other is located on the underside of the car near the differential. Adjustment is not necessary, unless you're using a very heavily biased braking system, or your driving needs (mainly on a track) dictate that you do so.

Common problems and their possible (and probable) causes:
Squealing/howling, etc.-
Mostly at slow speeds in reverse, but depending upon problem severity, all the time and/or intermittent.
This is USUALLY caused by low pads. They have a metal wear indicator which, when rubbed against the rotor, makes a nice big squeal! Simple, eh? New pads usually have about 10mm of material before the backing. It can also be caused by items stuck between the baffle plate and the rotor, improperly cut rotors, warped rotors, glazed or heat cracked pads.
Easy fix: resurface or replace rotors and/or replace pads!

Mushy pedal-
Most of the time it's caused by air in the brake lines or excess moisture in the fluid. Air can get into the system by allowing the fluid in the reservoir to get to low, loose brake line connections, or overheating the fluid to it's boiling point. Moisture gets into the fluid via a poorly sealed master cylinder or reservoir. Brake fluid absorbs moisture from the air, so you should never use brake fluid from a non-sealed container.
Easy fix: This can be simply alleviated by bleeding the brake system with new fluid.
As far as differing DOT levels are concerned, I started typing out what I knew, and decided to link you to this page: It explains it in much more detail than I could, better than I could, and it saves me the effort.


edit:copied this article into most recent post in this thread for posterity

Pedal requires excessive effort to brake-
Usually it's a vacuum leak, preventing the brake booster from assisting you in forcing the fluid through the lines. Drive a car with no power brakes, and you'll understand better.
Easy fix: check for vacuum leak and functionality of brake booster, and fix it!

Pedal sinks to floor, NO BRAKES!!-
Say goodbye to your master cylinder (specifically the internal seals). No seals = no pressure = no brakes.
Easy fix: replace master cylinder with rebuilt/reman/new one, or rebuild it yourself.. Bench bleed it first (fill reservoir with good fluid, until no air bubbles come out. Make sure the outlets aren't sucking in air each time you pull out the piston.

Vibration and/or shuddering in brake pedal/steering wheel while braking.-
Warped rotors.
Resurface until true or replace.

On resurfacing (aka cutting or turning), ensure its done on a brake lathe. Many cheapo brake places (like brake depot, etc will swirl rotors to make for a quick resurfacing. It basically involves using a high speed abrasive wheel to make the rotors smooth. This is not what you want. A resurfaced rotor has a rough finish to it, but is perfectly even. For breakin of pads, you want the rough pad to come in contact with a rough rotor, otherwise you'll glaze them.
Glazing is when your pads (and the transferred material onto the rotor) get shiny and smooth. It's basically the pad material melting. Not a good thing for stopping, because it makes the two surfaces smooth.

Brake fluid getting low:
Could be caused by a leaky system (usually the beginnings of a MC failure), or just from the wear of the pads.
Fix: none. look for leaks & monitor the level of your pads.

Brake light on:
Low fluid, ebrake on (or handle relay touching), short at brake fluid level connector.
Figure it out.

Improving your braking:
The MOST important aspect of ANY braking system is the tires.
Go back and read that last line 2 more times.
You can have the trickest Brembo/Alcon/Baer brakes with 14" rotors, project Mu pads, stainless braided lines, etc, but if your tires are 13 year old cracking 165/80R12s (let's assume for the sake of demonstration that the tire/wheel is connected outside of the brakes), then you'll be outstopped by a Geo Metro.
The wider (to a point) and stickier the tire, the better you will stop. Ask anyone who's done a wheel upgrade while keeping their brakes the same(assuming brakes were in good shape).
To brake better, the following things are good to have for the following reasons:
Stainless braided brake lines- replacing stock rubber lines with these reduces the slop in the pedal and gives you a better feel for your brakes. The reason for this is that the stock rubber lines tend to flex when pressure is applied to them. Stainless lines can't flex.

Larger diameter, vented, crossdrilled/slotted, 2 piece, cryotreated or composite rotors.
Larger diameter means less effort needed to stop them. (example: stopping a spinning bike wheel by the tire versus the inner spokes).
Vented is something we have already (for the front), and it allows the rotor to cool more effectively.
Crossdrilling and slotting are popular these days as performance braking upgrades. They can have side effects such as premature pad wear, or rotor cracking, but the upsides are things like improved pad grip and pad ventilation (and cooling).
2 piece rotors are basically rotor outer pieces (which come in contact with pads) that are bolted to aluminum hats (where they slide onto the lugstuds). These are great because they reduce unsprung weight and rotational mass.
Cryotreating basically just makes rotors stronger and more heat & wear resistant.
Composite rotors (usually carbon fiber or some such) are much more exotic, but with price, comes performance.

Lightweight wheels: Also good in reducing rotational mass.

High performance brake pads:
Easily the best upgrade for an otherwide stock system, barring tires. Aftermarket pads range from cheap & useless to exotic and awesome. Racing pads can be too much for the street, as many need to be heated to a certain temperature to be most effective, and many throw off way too much dust to be user friendly for most drivers. There are many pads that make a good compromise between price, performance, and hassle.

the reason everyone likes switching to multi-piston fixed calipers is because they can provide more clamping force, spread more evenly onto the rotor, giving a better feel of the brakes, and more control during braking.
These tend to be the most expensive thing to upgrade, and require the most extra stuff to install (be it new rotors & pads, adapters or otherwise).

Master cylinder: useful for big brake upgrades, esp when using the entire braking system from another car on the S-chassis. (i.e. Z32 MC). Bigger MC bore diameter = more fluid pushed thru.

Caliper Upgrades:
Probably the most common upgrades done on S-chassis braking systems are 180sx/Altima brakes and Z32 fixed-caliper systems.
With the former, simply changing out the front caliper and rotor are sufficient to increase stopping power without compromising brake bias.
With the installation of the Z32 system, it is vital to install the complete system. Installing only the front calipers will significantly throw off the brake bias of the car and make it more dangerous to drive.
Spacer kits are available for Z32/BNR32 brakes for repositioning of the caliper to accommodate for larger diameter rotors.


this info is intended solely for the users of Zilvia, and is copyrighted 2003 by me, Kevin Pratt. Any use for the purposes of monetary gain and/or reproduction outside of Zilvia.net is prohibited by me without prior approval and/or compensation.

01-31-2003, 12:13 AM
ABS...Mind going into more detail on how that works?

Does it change the proportioning independently for all 4 wheels, reducing brake pressure to only the wheel or wheels that are locking up? So your rears don't lose pressure when your fronts start to lock, and gives you the perfect trail braking every time...this would be sweet. Or is it simply reducing braking to all 4 wheels whenever even just 1 starts to lock?

Great writeup!

01-31-2003, 12:27 AM
ABS (damnit, DSC)

ABS is a simple system. It involves the monitoring of 3 or 4 sensors (front left wheel, front right wheel, and differential (or left and right rear axles). It's a binary system which uses a magnetic sensor (which can get confused by iron shavings) that detects the ridges and valleys on the hub of the front wheels or rearend/axles.
When the computer is receiving a steady stream of 10101010101, it applies braking force normally (ie, doesn't get involved)
When it receives 1111111111 or 000000000000 or whatever, it realizes that the wheel has stopped spinning (and is therefore skidding), and has to reduce the braking force to that wheel enough for it to stop skidding.
It seems like a complex system, but it's just an analysis of 3 or 4 inputs, done in a few thousandths of a seconds to make 3 or 4 sets of minute adjustments.
ABS is a VERY good thing.

01-31-2003, 07:19 AM
Thanks, you posted this right when I needed it. I give it 5 stars.

02-02-2003, 08:39 AM
How to bleed brakes
For the 240sx, you must bleed in a specific order to eliminate air from the lines. It's really easy to remember what the order is, too.. Simply start at the point furthest from the master cylinder and work your way in. In most cars, this means starting at the side furthest from the driver.
In USDM S-chassis, since the brake hardlines run along the passenger side framerail that makes the driverside rear brake the furthest from the master cylinder.

That means:

There are a few ways to do this:
1.) using a source of vacuum on the bleeder and sucking the fluid through.
2.) having a partner pump the brakes for you. Basically, your partner builds pressure in the system, and when you release that pressure, you force out old fluid and air. Make sure your partner holds the pedal down when you're releasing, and lets you know when the pedal has sunk.
3.) submerging a tube going from your bleeder into a container of old brake fluid. Just keep pumping the brakes. The new fluid will be sucked into the master cylinder more easily than the old fluid can be sucked back in.

In all of these methods, you must ensure that your brake master cylinder does not run out of brake fluid. Otherwise, you've negated everything you've done by allowing air into the lines.

Once you're finished bleeding, pump the brakes until they're stiff before turning the car on. This insures proper pedal feel.

Good luck!

02-03-2003, 12:09 AM

mrmephistopheles stated how the ECU determines when the ABS comes on. Wheel speed sensors(the magnetic sensor) are at ALL wheels(from what I've seen. what good would it be if 3 of your wheels were good and one locked up on ice and to started to pulse?). They detect when to make the ABS operate.

Normal braking- When the brakes are applied, fluid is forced from the brake master cylinder outlet ports to the normally open inlet ports of the Hydraulic Control Unit(HCU). The prake fluid travels through the HCY, ininterrupted, andout the outlet ports of hte HCY. Fluid then travels to the braking units just like in a normal system.

Anti-Lock Mode- If the driver gets into a situation where the brakes are applied hard enough to cause a wheel to lock u, the ECU will send signals to the HCU. The HCU will use these signals to prevent wheel lockup from occurring. Although the exact method the HCU will use to do this will vary between the different ABS systems. the following will occur:

- The ECU will determine what wheel is in danger of lockup. This decision is based on the wheel rotation speed and the deceleration rate of the other wheel(s).

-The ECU will then send an electrical signal to the HCU. This activates the solenoid that controls the isolation/inlet valve for the hydraulic circuit for that wheel. This action cuts off the fluid flow, preventing any more master cylinder hydraulic pressure from reaching the wheel.

-The ECU then looks at the sensor signal from the affected wheel again.

-If the affected wheel is still decelerating too quickly, the ECU will send a signal to the HCU that will open the normally closed dump/outlet valve for that circuit. This dumps any applied pressure from the brake back to the reservoir.

-Once the affected wheel comes back to a speed whick is acceptable to the ECU, the ECU will return the inlet/isolation and dump/outlet valves back to their normal positions. This allows brake fluid to travel to the brake units once again.

-These actions will repeat until the brakes can be applied without risk of lockup. Different traction situations will require more or less cycles of the valves.

02-03-2003, 12:29 AM
Pull your wheel off and insure your pads have shims and that they're lubricated. Having shims on pads is rather important.


Originally posted by /etc/shadow
Seems like the front left of the car.

02-03-2003, 12:34 AM
For lubrication you can use very little dielectric compound. The stuff they sell for specific brake use is exactly the same. Just make sure not to contaminate anything with it(pads, rotors, etc.)

02-05-2003, 07:10 AM
The lubrication Nissan recommends is called PBC (Poly Buytl somethin) grease. It's a mixture of lead, copper and some other ****. It comes in shim kits and tends to be rather expensive (a 12oz jug is about $40) However, it's really great stuff. (Kinda has the same properties as anti-seize.)

07-20-2003, 05:07 PM
here's the VTR Brake Fluid info, copied to here for posterity.
Brake Fluids
by Kenneth Streeter,
with input from Mike Burdick, Shane Ingate, Chris Kantarjiev, a "Skinned Knuckles" article, and various other sources


The discussion of whether to use DOT3, DOT4, DOT5, or the new DOT5.1 brake fluids in Triumphs is a common topic. The information provided herein should help you to decide which of these brake fluids are best for you and your car. I would point out that I am not an "expert" on the topic, but have collected the experiences of many other Triumph enthusiasts, as well as opinions of professional auto restorers. I have tempered my findings with my own experiences and opinions.

I would also take this opportunity to point out that the type of brake fluid used in your car is far less important, from a safety standpoint, than a properly functioning braking system. If you are working on your own brakes, be extremely careful, don't skimp on poor components, and bleed the brake system very carefully and thoroughly.

DOT3 brake fluid is the "conventional" brake fluid used in most vehicles. One of the most familiar brands is "Prestone."

DOT3 fluid is inexpensive, and available at most gas stations, department stores, and any auto parts store.
DOT3 will damage natural rubber brake seals and should not be used in any car suspected of having natural rubber seals (most Triumphs prior to 1968).
DOT3 fluid eats paint!
DOT3 fluid absorbs water very readily. (This is often referred to as being hydroscopic.) As such, once a container of DOT3 has been opened, it should not be stored for periods much longer than a week before use.
Since DOT3 fluid absorbs water, any moisture absorbed by the fluid can encourage corrosion in the brake lines and cylinders.
DOT4 brake fluid is the brake fluid suggested for use in late model Triumphs. The most familiar brand is "Castrol GT-LMA"

DOT4 fluid is available at most auto parts stores, and at some (but not all) gas stations or department stores.
DOT4 fluid does not absorb water as readily as DOT3 fluid.
DOT4 fluid has a higher boiling point than DOT3 fluid, making it more suitable for high performance applications where the brake systems are expected to get hot.
DOT4 fluid eats paint! Small leaks around the master cylinder will eventually dissolve away the paint on your bodywork in the general vicinity of the leak, and then give rust a chance to attack the body of your car!
DOT4 fluid is generally about 50% more expensive than DOT3 fluid.
Since DOT4 fluid still absorbs some water, any moisture absorbed by the fluid can encourage corrosion in the brake lines and cylinders.
DOT5 brake fluid is also known as "silicone" brake fluid.

DOT5 doesn't eat paint.
DOT5 does not absorb water and may be useful where water absorption is a problem.
DOT5 is compatible with all rubber formulations. (See more on this under disadvantages, below.)
DOT5 does NOT mix with DOT3 or DOT4. Most reported problems with DOT5 are probably due to some degree of mixing with other fluid types. The best way to convert to DOT5 is to totally rebuild the hydraulic system.
Reports of DOT5 causing premature failure of rubber brake parts were more common with early DOT5 formulations. This is thought to be due to improper addition of swelling agents and has been fixed in recent formulations.
Since DOT5 does not absorb water, any moisture in the hydraulic system will "puddle" in one place. This can cause localized corrosion in the hydraulics.
Careful bleeding is required to get all of the air out of the system. Small bubbles can form in the fluid that will form large bubbles over time. It may be necessary to do a series of bleeds.
DOT5 is slightly compressible (giving a very slightly soft pedal), and has a lower boiling point than DOT4.
DOT5 is about twice as expensive as DOT4 fluid. It is also difficult to find, generally only available at selected auto parts stores.
DOT5.1 is a relatively new brake fluid that is causing no end of confusion amongst mechanics. The DOT could avoid a lot of confusion by giving this new fluid a different designation. The 5.1 designation could lead one to believe that it's a modification of silicone-based DOT 5 brake fluid. Calling it 4.1 or 6 might have been more appropriate since it's a glycol-based fluid like the DOT 3 and 4 types, not silicone-based like DOT 5 fluid. (In fact, Spectro is marketing a similar new fluid which they are calling Supreme DOT 4, which seems less confusing.)
As far as the basic behavior of 5.1 fluids, they are much like "high performance" DOT4 fluids, rather than traditional DOT5 brake fluids.


DOT5.1 provides superior performance over the other brake fluids discussed here. It has a higher boiling point, either dry or wet, than DOT 3 or 4. In fact, its dry boiling point (about 275 degrees C) is almost as high as racing fluid (about 300 degrees C) and 5.1's wet boiling point (about 175 to 200 degrees C) is naturally much higher than racing's (about 145 C).
DOT5.1 is said to be compatible with all rubber formulations.
DOT5.1 fluids (and Spectro's Supreme DOT4) are non-silicone fluids and will absorb water.
DOT5.1 fluids, like DOT3 & DOT4 will eat paint.
DOT 5.1 fluids are difficult to find for sale, typically at very few auto parts stores, mostly limited to "speed shops."
DOT 5.1 will be more expensive than DOT3 or DOT4, and more difficult to find.
General Recommendations:
If you have a brake system that doesn't leak or show any other signs of failure, but has old seals in it, don't change fluid types as a result of reading this article. If it isn't broken, don't "fix" it -- you may simply break it instead!
Flushing of the brake system every couple years to remove any absorbed or collected water is probably a good idea to prevent corrosion, regardless of the type of brake fluid used.
DOT3 is dangerous to use in Triumphs with natural rubber seals, and thus should not be used in such cars, except as a temporary "quick fix to get me home" solution. (If this is used as a "get-me-home" solution, bleed the system as soon as possible, and be prepared to replace all your seals.)
DOT3 is an adequate brake fluid for use in later Triumphs, although it is rarely preferred. My recommendation would be to simply not use it.
DOT4 fluid, for a slight increase in cost, will give significantly increased resistance to moisture absorption, thus decreasing the likelihood of corrosion compared to DOT3.
DOT4 fluid has a higher boiling point than DOT3, making it preferable for high performance uses such as racing, autocross, or excessive use of the brakes in mountainous areas. For even greater braking performance, consider going to DOT5.1 or a high-performance version of DOT4 fluid.
DOT5 is a good choice for the weekend driver/show car. It doesn't absorb water and it doesn't eat paint. One caveat is that because it doesn't absorb water, water that gets in the system will tend to collect at low points. In this scenario, it would actually be promoting corrosion!
DOT5 is probably not the thing to use in your race car although it is rated to stand up to the heat generated during racing conditions. The reason for this recommendation is the difficult bleeding mentioned above.
When changing from one fluid type to another, as a minimum, bleed all of the old fluid out of the system completely. For best results, all the seals in the system should be replaced.
As always, your experiences may vary.