In my opinion, the number one issue with g-body brakes stems from the step bore master cylinders. Most 1982 and up GM cars and trucks use these step bore master cylinders with quick take up calipers to help with fuel economy. Do and internet search for quick take up calipers, low drag calipers, and step bore master cylinders.
Step bore master cylinders are harder to bleed and they have a bypass valve built into them that can malfunction. If not all the air is out of the master cylinder, you will get a spongy pedal with a lot of pedal travel. If the bypass valve malfunctions, you will get a spongy pedal with a lot of pedal travel. The bypass valve regulates the volume of brake fluid in the master cylinder to the front quick take up calipers. Because a quick take up caliper requires more volume, the step bore master cylinder supplies more volume through the larger step bore within the master cylinder. When the volume the caliper is reached, the pressure goes up and the bypass starts to function to allow the primary, smaller bore in the master cylinder to increase the pressure. If everything is functioning correctly, you will get a good firm pedal. If you have air in the master cylinder or a faulty bypass valve you will get a lot of pedal travel and spongy brakes.
When manufactures rebuild these master cylinders, they may not replace the bypass valve, which may still be faulty and still give you greater pedal travel and spongy brakes.
If you do not have leaking front calipers I believe your problem lies in the master cylinder. Research on the internet on step bore master cylinders and how to bleed them. If you think you need to purchase a master cylinder pay for a new one. It will more than likely be cast iron, but it will have all new components.
If you decide to purchase calipers, buy new non low drag calipers. Where do you get these? US Brakes sells stock "metric" g-body calipers. Does your auto parts stores sell new non low drag calipers? I don't know and visually you cannot tell. With non low drag calipers, you can run your stock step bore master cylinder or you can run a stock 1978 to 1980 "g-body" 24mm strait bore master cylinder. The advantages of a strait bore master cylinder is its easier to bleed and has less parts to malfunction.
Step bore master cylinders are harder to bleed and they have a bypass valve built into them that can malfunction. If not all the air is out of the master cylinder, you will get a spongy pedal with a lot of pedal travel. If the bypass valve malfunctions, you will get a spongy pedal with a lot of pedal travel. The bypass valve regulates the volume of brake fluid in the master cylinder to the front quick take up calipers. Because a quick take up caliper requires more volume, the step bore master cylinder supplies more volume through the larger step bore within the master cylinder. When the volume the caliper is reached, the pressure goes up and the bypass starts to function to allow the primary, smaller bore in the master cylinder to increase the pressure. If everything is functioning correctly, you will get a good firm pedal. If you have air in the master cylinder or a faulty bypass valve you will get a lot of pedal travel and spongy brakes.
When manufactures rebuild these master cylinders, they may not replace the bypass valve, which may still be faulty and still give you greater pedal travel and spongy brakes.
If you do not have leaking front calipers I believe your problem lies in the master cylinder. Research on the internet on step bore master cylinders and how to bleed them. If you think you need to purchase a master cylinder pay for a new one. It will more than likely be cast iron, but it will have all new components.
If you decide to purchase calipers, buy new non low drag calipers. Where do you get these? US Brakes sells stock "metric" g-body calipers. Does your auto parts stores sell new non low drag calipers? I don't know and visually you cannot tell. With non low drag calipers, you can run your stock step bore master cylinder or you can run a stock 1978 to 1980 "g-body" 24mm strait bore master cylinder. The advantages of a strait bore master cylinder is its easier to bleed and has less parts to malfunction.
