All you've ever wanted to know about brakes

[Glance91LX]

I've noticed alot of post recently on brakes and this is my area of knowledge so I figured I'd give you everything you've ever wanted to know. Any Mod feel free to make this sticky because it will answer and solve alot of problems. That and to show the probe people that I still know my stuff

Dual Master Cylinders:
A dual master cylinder consists of 2 single master cylinders operated by a common brake pedal. A dual master cylinder has separate outlet ports to send fluid to the brakes. This splits the system into two halves, with each having its own separate lines, pistons, reservoir, cylinders, etc. Each hydraulic system is called a circuit or channel. These circuits are called primary and secondary circuits. Since each circuit is separate, one circuit could fail and the other one would still work. Helping prevent total system failure makes this type of brake system safer.

The two basic ways a hydraulic brake system can be split are vertical and diagonal...the probes use a diagonal system so I will only cover it.

Diagonal Split system

In a diagonal split system the left front and right rear brakes are the primary circuit. The right front and left rear brakes are the secondary circuit. In this type of system each circuit does an equal amount of work. This type of system is used mostly in fwd cars. In a fwd vehicle the front brakes provied approximately 80% or more of the braking power. If a fwd vehicle had a vertical split system and the primary circuit failed the secondary circuit (15%) would not be powerful enough to stop it.

[Glance91LX]

As long as both master cylinder pistons and their respective hydraulic systems are full of brake fluid and operating normally, hydraulic pressure from the primary piston is the force that moves the secondary piston. If the primary braking system suffers a failure such as a ruptured wheel cylinder cup or a broken brake line, the system will lose fluid including the fluid in the primary section of the master cylinder.

Diagnosis and Repair:

When diagnosing a master cylinder keep in mind that it is a hydraulic pump. Look for fluid leakage at the rear seal low or no hydraulic pressure and a brake pedal that slowly sinks to the floor. Never check just one part of a brake system. The problem could be elsewhere and more than one thing could be causing the problem. Always check the entire brake system. If it's determinded that the master cylinder is faulty, it must be rebuilt or replaced. REPLACING THE MASTER CYLINDER IS BEST!

Bleeding the master cylinder.

First Place the master cylinder in a vise and attach the bleeder tubes from the lines back into the cylinder. Use a dowel rod or similar tool to slowly work the master cylinder piston back and forth. Use long full strokes. Pumping too fast causes the fluid to foam and tiny air bubbles will be trapped in the cylinder. Air in the clyinder makes pedal response feel spongy. Continue bench bleeding until no air bubbles can be seen in the reservoir. After installing the master cylinder, bleed the brake system in the proper sequence. Start by bleeding the lines at the master cylinder.

[Glance91LX]

Bleeding foundation brakes:

Sequence:

Bleed in an X pattern. The order is RR, LF, LR, RF.

There are 5 basic methods of bleeding brakes but due to alot of people being limited on tools we'll go with your basic pedal bleeding.

Pedal bleeding requires 2 people. One person opens and closes the bleeder screws and observes the emission of air bubbles. The 2nd person pumps the brake pedal slowly to move the fluid through the system and expel any trapped air. Slip a piece of hose over the bleeder screw. Insert the free end of the hose into a jar partially filled with brake fluid. The person can determine accurately when all air has een purged from the system by observing when bubble-free fluid flows from the hose. Air in the system is visible as bubbles emerge from the end of the hose inserted in the fluid as the pedal is pressed. Keeping the end of the hose inserted in the fluid prevents accidental admission of air into the system when the bleeder screw is open. The pedal must be pumped slowly to prevent any undue pressure buildup and increase in turbulence. Manual bleeding requires checking the fluid level in the master cylinder and replenishing it periodically. If the level drops below the vent or replenishing ports, air will enter the system. The bleeder screw should be open only when the pedal is depressed and closed before the pedal is released. Place a spacer under the pedal to limit master cylinder travel, because on some vehicles corrosion bulds up at the end of the cylinder. If the piston moves through this corrosion the cups in the master cylinder could be damaged.

[Glance91LX]

Front dics brakes:

Caliper: a caliper is a hydraulic cylinder that applies the pads.

Probes have a Floating caliper aka sliding caliper. The floating caliper has the pistons on one side. A floating caliper is not rigidly attached to the vehicle. Instead, it floats on slides on various pieces of mounting hardware, such as a guide pin. When the brakes are applied the piston pushes the inboard pad out against the rotor. This causes the caliper to slide inward, pulling the outoard pad against the rotor, now squeezing the rotor between the pads. The 3 basic parts inside the caliper cylinders are the quad seal, dust boot, and piston.

Quad seal: Its three functions are to seal, to retract, and to self adjust the piston.

Dust boot: This mounts between the outer edge of the piston and the cylinder bore. It helps keep out moisture and other contaminants.

Piston: If you're rebuilding your caliper if the piston is pitted scored distorted or otherwise damaged replace it.


I hoped this helped out somewhat. If there is enough demand for it I can go into power assists, ABS and traction control systems. I'm here to help with any brake problems via pm or email.

~G~

[mitchgt]

Thread to my MAJOR caliper problem.. anyone with knowledge please help.

[AMD IS THE BEST]

I have a quick question about my breaks. What happens is when I am driving around and I go to stop, when I put the break petal it drops to the floor. I then release the petal and the apply pressure again and everything is fine. WEIRD

[FLAPOC-Pres]

Master cylinder has a bad seal.
My opinion.

[Glance91LX]

yep I belive that too, the seal lets the piston travel so far and with a bad seal it's traveling to far. Replace the seal

~G~

[nboller]

Couldn't it just be air in the lines?

[our89gt]

Brake problems

The history of my break problem is a bit involved; a local mechanic wouldn’t touch this problem after I gave him this story. I purchased my 89 GT from a wrecking yard with the knowledge of a brake problem. What happens is when I initially hit the brakes they go almost to the floor, but there is still enough power for them to stop the vehicle within a good distance. But when I pump the brakes and then hold them they work as they should without the break pedal going down any further and the car slowing down at the correct rate.

I have replaced the master cylinder, the front rotors, and all four brake pads. There are no visable fluid leaks and I do not have to add more brake fluid. The car has ABS and the Anti-lock light is on, I intend on having the code diagnosed soon but wanted to know if you guys had any ideas because I am LOST?

[FLAPOC-Pres]

The abs pump deal is probably shot.

[351stang]

im having the same problem!! someone help me!! my pedal goes all the way to the floor when i first press it and then i can pump it and it builds resonable pressure, it all started the other day when my cars abs went to $hit and my brakes just about locked up at 80 mph and then i got out and there was brake fluid dripping from the master cylinder as if it backed up into it, i check it daily and theres no leaks anywhere, just a little moisten near the rear of my master cylinder and i cant get the cap off the abs reservoir now, damn car.

[Csmicfool]

What are my options for a leaking brake line on the secondary cylinder? It's a slow leak, but a leak that is starting to weaken my stopping power noticeably, and the leak is comming from an obvious place... the lines appear rusted through or on the verge thereof.

I've not been able to find OEM or aftermarket brake lines specifically for the model. I have 3.0 LX model, anyone with my model have to replace brake lines that might point me in a good direction?

[our89gt]

351 mustang--> Well at least you have a noticeable leak to start from, our mysteries brake problem is giving up no visual pointers what so ever.

Flaproc-pres--> Do you know someone that had this problem in the past? Or are you floating ideas?

[FLAPOC-Pres]

I have worked on cars that have had this problem before, brakes are generally the same.