Brake pedal goes all the way to the floor, will slow the car a little, but wont fully brake. master cylinder is fine, calipers and pads are new, brake lines look okay, it's holding brake fluid fine, lines have been bled multiple times, and we can't find a leak anywhere. Any ideas why my brakes still wont work? I'm pulling my hair out trying to figure this out! Thanks!

Question edited for clarity. Wrong type of pedal/peddle, wrong 'brake/break.

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It is all guesswork due to the absence of stating what your vehicle is, but you mentions ABS, so...

You need a professional grade bi-directional scan tool computer to electronically open the ABS Valves, and you need a pressurized brake bleeding unit which pumps brake fluid upwards under light pressure from the caliper to the master cylinder reservoir. It also has the benefit that air bubbles like to rise.

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little low fluid w/no sign of leak is fluid going into calipers from pad wear ---causes of brakes to floor --need brakes replaced--failing master--air in lines--leak--flex line failure--caliper failure

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.

Low brake pedal almost always indicates a compromised hydraulic system. Check you brake fluid level. If it is ok and you haven't had anything replaced in the brake hydraulics system lately, such as a wheel cylinder or brake caliper, then the primary suspect is the master cylinder.

Hello Patricia,

Have you checked the master brake cylinder reservoir for brake fluid loss?

I don't suspect the master cylinder as you have replaced it... but am more suspect of a leaking brake line or fitting which should have
been discovered when you had the system bled.

The only time I ever had a rear brake problem with a 2500 series Chevrolet was due to seized rear brake caliper guide pins...as your calipers are also new that is not your problem.

If your brake pedal feels spongy and travels to the floor board you have air in the system, a brake fluid leak or a failure in the power steering system, such as a broken hose, broken power steering pump drive belt, or failed pump, would result in a loss of pressure to both the hydro-boost and steering.

Find more information here:

. Hydro Boost Power Assist Systems Operation Diagnosis and Repair

Time to redo the bleeding of the system again...starting with the master cylinder...you do not have to remove the master to re-bleed it. disconnect both brake lines from the master, and start again by pushing pedal to floor, and rebleed each line at the master...this is critical to work properly...you mentioned new master cylinder, if remanufactured I would return it. As noted above, after confirming the master is fully bled...rebleed each caliper beginning with the rear, farthest away from the master...you must have "air" in the brake lines. Hope this helps.

well, check the brake lines for leakage after adding brake fluid to the full mark. If the brake pedal goes all the way to the floor, it's likely there is a leak somewhere after the master cylinder. If the pedal gets hard after adding fluid, and pumping it several times with the car off, then the master cylinder is still ok.

If air has gotten into the master cylinder, it may have to be bled out on a bench, or using a scanner if that's the way your vehicle is.

Bleeding the Brake System (ABS) Auto Bleed Procedure NOTE: Perform a manual bleeding procedure. If the brake pedal height and firmness results are not achieved, perform the auto bleed procedure below.
NOTE: Perform this procedure when replacing the brake pressure modulator valve or electro-hydraulic control unit.

1. Perform the manual brake bleeding procedure. Refer to Bleeding the Brake System (non-ABS).
2. Check the master cylinder fluid level often to prevent running the system dry.
3. Press the brake pedal firmly and run the Scan Tool Automated Bleed Procedure (for Gas engines) once, or the Function Test (for Diesel engines) four times. Make sure to release the brake pedal between each test.
4. Perform the manual bleed procedure again. This should remove the remaining air from the brake system.
5. Bleed the system as many times as needed to obtain the appropriate feel of the brake pedal.

no they are inverted flare fittings no seals check all lines and calipers/wheel cylinders if all are dry remove the vacuum hose from the booster if there is brake fluid in the hose the master cylinder is bad

pump the pedal about 10 times and see if it comes back up, u need to pump the caliper pistons out after a brake job.

bleed the brake again. if the pedal goes to the floor with little resistance, then the master cylinder is your problem.
the shim is to depress the noise when you applied the brake. you can skip that, if the brake works without it.