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A soft spongy pedal with manual disc brakes front and 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. 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. Wrong master cylinder
When you add disc brakes to the rear you must use a true 4 wheel disc master cylinder. The pressure and volume requirements to the rear are much greater than that required by drums, disc brakes require more pressure and volume. A four wheel disc master will have a longer stroke and will provide more pressure to the rear disc brakes. Air in rear calipers
Rear calipers are very hard to bleed properly. Sometimes they can be bled only when removed from the car.Try taking them off, place a block of wood between the pads and bleed while tapping and orienting the bleeder screw up. Rear caliper problem
If you are using Cadillac ElDorado rear calipers there are some important things you should know. One of the biggest advantages of a disc brake system is the fool proof self adjuster. Not so with this rear GM system. The rear calipers adjust off the parking brake. The parking brake is incorporated into the caliper. You must set the parking brake every time you park the car.The rear caliper pitons utilize a one way clutch inside the caliper piston. When the parking brake is applied the clutch senses when there is .030" or more clearence between the friction material and the rotor on the inboard side. When there is more than .030" the clutch turns inside the piston adjusting it out keeping the rear brakes adjusted. If you do not set your parking brake every time you will start to lose brake pedal (low and spongy) and the adjuster mechanism will not work any longer. Also: never use rebuilt calipers on the rear because the rebuilders use the old pistons and the pistons were the reason the caliper failed in the first place.
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