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.