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A soft spongy pedal with power disc brakes front and drums rear could be caused by one or more of the following conditions: A defective master cylinder.
If the bore in the master cylinder is pitted or the rubber seals have decreased in size then some fluid will bypass the seals under pressure 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 of the master cylinder is pitted or the rubber seals are bad. Hard line that loops up.
Hard line that's been plumbed so that it loops up above the level of the master cylinder and then back down can trap air and give a spongy pedal feel. No residual valves to rear drums
For drum brakes to function correctly there needs to be a 10 lb residual pressure valve in the system. This residual valve will maintain a 10 lb pressure on the drum brakes to counter the spring presure in the system. This keeps the shoes out close to the drum giving a higher firmer pedal. Without the residual valve in the drum system, every time you step on the pedal all the fluid goes to moving out the shoes before drum contact. If you remove the lines that go into the master cylinder from the rear line 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.If your master does not have one you will need to install an inline 10 lb valve. Incorrect booster pin length.
If the pin that goes from the booster to the master is too short then you will get a spongy pedal. With the master cylinder mounted on the booster the pin should be 1/64" from touching the master cylinder piston. Too long a travel before engaging the master cylinder gives the soft pedal feel. Air in the system.
This 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. If the screw is not at the highest point on the cylinder chamber then a pocket of air will always remain. Check the screw orientation. Master cylinder piston diameter too small.
If the diameter of your master cylinder piston is smaller than required by wheel system volume requirements then you will experience a long pedal travel. Determine what the original master cylunder bore diameter was and replace the master if too small. Remember with an old vehicle the master could have been incorrectly replaced by a previous owner. Drums too large.
If the original drums on the rear 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. Incorrectly arched shoes.
If the rear drums have been turned too much the shoes may not match correctly giving minimul shoe to drum contact and spongy brakes. Replace drums and shoes with new.   If you can't find the answer to your question here, email our
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