Soft brake pedal on 1990 GMC Sierra. Replaced master cylinder, rear shoes are almost new, front pads are good.

first replace master cylinder the replace rear wheel brake cylinders.. Inspect front pads for non uniform wear, inspect front rotors replace if badly worn or scarred. You are dealing with 23-24 year old parts.

Once you have bled the air from the brake lines,clamp off the front brake hoses carefully.Does the pedal feel better now?
Then the pad /caliper fit is allowing too much play.Is the pedal the same?Then clamp the rear hose and try the pedal.Is it fine with the rear hose clamped?If so we now know the problem is at the rear brakes.One common low pedal rear brake cause is brake shoes that do not fit the drums.Remove the drums and look at the shoes,are they showing contact wear fully or just in the middle of the shoe?Remove a shoe and place it in the drum.Can you rock the shoe against the drum surface?Once drums have been resurfaced,the shoes will not fit fully against the drum allowing the shoe the flex when applying pressure to it and this can easily create a low soft brake pedal. Of course rear brake adjustment must be correct once brake shoe contact is correct.To correct brake shoe contact,have your shoes re arc-ed to fit resurfaced drums or install new drums.Don't overlook brake master cyl /brake pedal push rod adjustment too.

Pedal, not peddle. First make sure reservoir is full at the brake master cylinder. Then check your brake pads (for disc brakes) and/or brake shoes (for drum brakes). When the pads or shoes are very worn down, you will get a low pedal. If you have disc brakes on the front and drums on the rear wheels, first try just replacing the front brake pads. If you get a good pedal after the front is done, many people tend to let the rear brake shoes go unless they are making noise. These systems are designed to put most of the braking force on the front discs, so the rear shoes do not need changing as often.

in late model GM and Ford cars with quick take-up master cylinders, the quick take-up valve takes about 15 seconds to reseat after the brake pedal has been depressed, so you should wait at least 20 seconds before you press the pedal while bleeding. If the pedal is pumped too quickly while manually bleeding the system, you may never get the pedal to firm up. On a side note, did you check the level in the master cylinder after every 2 or 3 bleedin procedures? you could have been pumping air right back in the lines if the reservoir runs dry during the bleeding process.

Does the car have ABS ? And does it have an equalizer block for the 4 wheels ?
It sounds like the ABS has been affected, or there is an equalizer block that is stuck on the front wheels only.
Can you open the bleeder or the rear line on the master cyl and get fluid to come out by depressing the pedal ?

Did you bench bleed the MC before installing it? If not, that's the problem. MUST be bench bled or you'll always have air. If brakes lights are on, the brake switch is out of adjustment OR (since you replaced the MC), the push rod is out of adjustment

Check your lines going to the brakes the rubber hose on the 1993 chevy have a tendancy to crack and brake are you using an excess of brake fluid

try bleeding the master cylinder at the lines first as it indicates that there is still air in the pistons
loosen one line at a time and gently apply the brakes until any air bubbles come out tighten the line before releasing the brake pedal
do it for the other line and then bleed all the brakes starting at the longest line first and ending at the shortest line

Did you bench bleed your master cylinder before you installed it? If not you may not be able to purge the air from it.

George

Hi I'm litoshop if you bled the system and get no results then your brake pump needs to be replaced now before replacing make sure your pads are installed correctly because if there not this to can cause this problem good luck. if this information has been useful please let us know thank you.