The braking system, without a doubt, contains your car's most important components. Nothing else matters without good brakes. Steering is important, but if the steering breaks, then the brakes will bring you to a stop. If the brakes go, all the steering will do is help you point your hot rod in the direction of the softest object you can find.
Last month we went over some of the basics of braking, explaining the different types of valves and master cylinders, and answering some of the more frequently asked questions. This time we once again turned to some of the braking experts and picked their brains on how to properly set up your system and help diagnose some of the simple problems that seem to pop up, preventing the braking system from performing at its best. They had the answers to all of our questions, and even some we didn't even know we needed to ask.
Brake Troubleshooting Guide Soft Spongy PedalA Defective Master CylinderSome fluid will bypass the seals under pressure if the bore in the master cylinder is pitted or the rubber seals have decreased in size, giving a lower pressure to the wheels and a softer, spongier brake pedal.
To test for this condition, remove the brake lines from the master cylinder and plug the outlets (obtain outlet plugs from a local auto parts store). When you apply the brake pedal, it should be high and firm. If it is spongy or slowly goes down, then either the bore is pitted or the rubber seals are bad.
No Residual Valves To Rear DrumsFor drum brakes to function correctly, there needs to be a 10lb residual pressure valve in the system. This residual valve will maintain pressure on the drum brakes to counter the spring pressure in the system. This keeps the shoes out close to the drum, giving a higher, firmer pedal. Every time you step on the pedal without the residual valve, all the fluid goes to moving the shoes out before drum contact.
If you remove the lines that go into the master cylinder and look at the brass seat in the outlet with a flashlight, you should be able to see the little black rubber check valve under the seat. You will need to install an inline 10lb valve if your master does not have one. Use a straightened paper clip to check for a residual valve. Insert the clip into the center of the brass seat; there is a residual valve present if it meets a restriction within 1/2-inch.
Air in the SystemThis is obvious, but sometimes all the air has not been removed after bleeding. One reason is the incorrect orientation of the bleeder screws in the wheel cylinders. A pocket of air will always remain if the screw is not at the highest point on the cylinder chamber. Check the screw orientation.
Master Cylinder Piston Diameter is Too SmallYou will experience a long pedal travel if the diameter of your master cylinder piston is smaller than required by the wheel system volume requirements. Determine what the original master cylinder bore diameter was and replace the master if it's too small. Remember, we're working with old cars and the master cylinder most likely has been replaced, and maybe incorrectly, by a previous owner.
Incorrect Booster Pin LengthThis is for power brakes only. You will get a spongy pedal if the pin that goes from the booster to the master is too short. The pin should be 1/64-inch from touching the master cylinder piston with the master cylinder mounted on the booster. Too long a travel before engaging the master cylinder gives a soft pedal feel.
Drum Brake Wheel Cylinders Too LargeIf the original drums have been upgraded to a larger drum system, then the fluid volume requirements have increased. Using the original master could lead to a long pedal travel with a spongy pedal feel.
Loose Front Wheel BearingsIf the wheel bearings are loose on disc brake systems, they will allow the rotor to wobble during use instead of running true. This will cause the pistons in the brake calipers to be pushed back farther into the caliper than they were meant to. The result is that the pedal must be pushed farther to the floor to take up this travel before the brakes engage.
Aerated Silicone Brake FluidSilicone brake fluid is known to aerate easily, giving a spongy pedal feel. Once aerated, it is very hard to remove the air.
Hard Line That Loops UpHard line that has been plumbed so it loops up above the level of the master cylinder and then back down can trap air and give a spongy pedal feel.
Restricted Vacuum Line or Restricted Booster Check ValveA restriction in the vacuum line to the booster or in the booster check valve will give a hard pedal since the booster will not be able to replenish the vacuum fast enough.
A Frozen Wheel CylinderA drum brake with a frozen wheel cylinder will give a very hard pedal feel since the brake fluid can't travel anywhere under pressure.
Binding Pedal LinkageLinkage that's too tight or linkage that's restrictive of pedal travel can give a very hard pedal feel.
Undersized BoosterUsing a booster that is too small for a master cylinder with a large bore, such as 1 1/8-inch, will give a hard pedal. Your booster size/car weight/cylinder bore size must be designed correctly.
A Tripped Combination ValveIf you lose half your braking system and the combination shuttle valve cuts off half the system, this can give you a hard pedal. This type of combination valve can be tripped during the bleeding process. To center the valve, open a bleeder screw on the half of the system that still has pressure. Depress the brake pedal and tighten the bleeder screw; this should center the valve.
The Wrong Master CylinderYou need to use a master cylinder with a 1-inch bore diameter or smaller if you have manual brakes. You will have a very hard pedal if you're using a bore larger than 1 inch. The rule of thumb when plumbing the master cylinder is that the larger fluid reservoir volume will feed the front disc brakes.
No PedalIncorrect Booster PinIf you have power brakes, the pin from the booster to the master may be far too small or not present at all; in effect, the booster pushes on nothing!
Brake DragIncorrect Booster Pin LengthIf the pin from the booster that pushes on the master is too long, the master will not return to the correct rest position when you release the brake pedal, retaining a residual pressure in the system. Try shimming the master off the booster about 1/32-inch and see if the drag disappears. If it does, remove the master and adjust the booster pin inward.
Frozen Parking Brake CableIt's often overlooked, but a parking brake cable that's frozen in the applied position will cause brake drag.
A Frozen Wheel Cylinder PistonIf a wheel cylinder piston freezes it could cause the shoes to drag on the drum.
Pre-Load Pressure on the BoosterIf your pedal linkage binds, it may retain a pre-load pressure on the booster that causes the booster to apply pressure to the master.
Incorrectly Adjusted DrumsHaving the drums too tight will cause drag.
Too Much Fluid in the Master CylinderThe brakes will drag as they are heated up if the level of brake fluid is right to the top of the master cylinder. Leave a half to 1 inch of air above the fluid between the rubber cap diaphragm and fluid top.
Very Sensitive PedalWrong Pedal RatioYour pedal ratio may be too high. Power boosters will require a 4:1 to 5:1 ratio. You're getting too much mechanical advantage if your ratio is around 6:1, making the brakes extremely sensitive.
Now that you know how to set up a system correctly, here's how to find out what may be wrong with your system if it isn't performing to your expectations.
The Most Common Causes of Poor Brake Pedal Feel1.The bleeder screws on the caliper are not facing up.2. The master cylinder was not bench-bled or was not bled completely.3. A defective rebuilt master cylinder with pitted cylinder bore or defective pressure seals.4. The master cylinder bore size is too small for system requirements.5. Using disc/drum master on a disc/disc system.6. The master cylinder is lower than the calipers or wheel cylinders and not using correct residual valves.7. Lines or components are near a heat source.8. Lines that lope up higher than the master cylinder and then come back down will trap air.9. Not using a residual valve for rear drum brakes.10. Drum brake wheel cylinders are too large.11. Silicon brake fluid has a tendency to trap air and causes seals to swell.12. Rear caliper parking brake/pistons are not set properly with rear disc brake system.13. Rear calipers are not being bled properly (most brake problems with four-wheel disc systems come from rear brakes).14. Improper pedal adjustment with too much free play.15. Old or poor-quality brake hoses.
How To Diagnose a Problem 1. Disconnect the brake lines from the master cylinder while leaving the cylinder on the vehicle.2. Obtain solid plugs for the master cylinder outlets with the correct threads.3. Plug both master cylinder outlets and try the pedal. The master cylinder is bad if the pedal is very spongy or goes to the floor. The master cylinder is fine if you have a good, firm pedal.4. If the master is fine, connect the line to the front portion of the system and try the pedal again. The front part of the system is fine if the pedal remains firm. The problem is with the front half of the system if the pedal goes to the floor.5. Connect the rear portion if the pedal is fine with the front part of the system connected. The problem is in the rear if the pedal goes to the floor.6. It will be easier to fix once you know which portion of the system has the problem.