All wheel drive GMC Yukon Denali brakes locking up...already changed out booster and master cylinder and checked all calipers

If it goes to the floor and no brake warning lites or anything comes up on the dash, check the brake booster--engine vacuum and vacuum to the booster. Just my opinion. The master cylinder usually bolts to the booster.

Check ur master cilinder , a way to know if your wheels are free, jack it up turn the wheel by hand if not realese the bleeder on top of the caliper and try again if it is free probably u need to replace master cil.

don`t just change parts because someone told you to without checking it out first.with the brake light on check the fluid level in the master cylinder if its down check the float in the cylinder as these will get stuck.also check for fluid leaks in the entire brake system and condition of the brakes themselves as when they wear especially the disc brakes the fluid will go down as the brake fluid takes up the worn brake material so the calipers don`t have to move as far to apply the brakes.the master cylinder is the component that holds the brake fluid under the hood that you would top up if low.the brakes booster is what the master is bolted to and gives the car power brakes as opposed to manual brakes.

If you have not checked the large vacuum hose that runs from the intake manifold to the brake booster, or the one way check-valve the hose connects into the booster with, please do so before rearranging your budget. If the hose or valve are dry rotted, cracked or not sealing tight at the ends, replace them and see what the booster does.
Also, the booster will usually make a continuous hissing sound if its bad.
It may be the master cylinder is bad.

As a rule, if the brake pedal goes to the floor with little or no effort, it's the master cylinder.
If you have to stand with both feet on the brake pedal, gripping the steering wheel to stop, its the brake booster 90% of the time. The other 10% is the master cylinder.

Hope this is clear as mud! and helps... let me know.

A few different things to look at. Either you have a bad master cylinder, master cylinder and booster, brake Caliper/calipers.

Look under the vehicle on inside of the wheels, if you notice any fluid on your tires then it is coming from the caliper piston leaking. They have rebuilt kits, but its way easier to just install new calipers.

Look in the engine compartment on the drivers side on the firewall, you will see the master cylinder with 4 lines running from it that it will look like this

Look at where it attaches in the back, if fluid is leaking in the back then you need a new booster. To be honest with you, since it has to do with brakes, if you are not mechanically inclined I would leave it to a professional. You can call a mobile mechanic to do the job usually about 50% cheaper on there labor rate, but just as experienced or you can tow it to a shop.

when locked try to relieve the brake line pressure by opening the bleeder on the wheel that is locked. if that unlocks the wheel then replace the hose for that wheel. if all wheels are locked then relieve the pressure at the master cylinder if that unlocked the wheels then you will need the flush out all the brake fluid with new clean Dot 4 fluid. and replace all the rubber in the brake system. the fluid was contaminated and caused the rubber parts the the brake system to swell. this would include the master cylinder, rubber hoses, wheel cylinder kits, calipers kits, ABS pump module.any part that has brake fluid in it. if you know that the wrong fluid has be put into the brake fluid .you may want to check with your car insurance company it may be covered .

The master cylinder seal is bad. (seal between master cylinder & booster) Common GM problem as they age.

you have a leaking caliper or master cylinder, depending on whether you have rear discs or drums. needs to be fixed ASAP. do the other side while you are at it and replace the pads/shoes.

Yes master cylinder and maybe ABS sending unit. Also is there a sound of an air leak it may also be your brake booster

1. What is the Brake Fluid Level?
2. Are you losing any Brake fluid at all? Are there any puddles or noticeable wetness on the ground or on the tires/wheels? With vehicle parked, the reservoir filled (overfilled), and engine running: pump the brakes and check on the ground and back sides (inside portions) of the wheels. Look for any brake fluid leaks.
3. Leaks from Calipers usually means a leak from the caliper piston. The rubber seals will be wet.
Leaks from Drums usually means a leak from the wheel cylinder.
4. In any case of leaks from the wheels - replace the calliper OR wheel cylinder.
For caliper leaks: change the brake pads, and clean the rotors with brake cleaner.
For drum leaks: clean the drum and brake hardware with brake cleaner, and replace the shoes.
5. If no leak is detected from the wheel area's: check under the Master Cylinder for wetness indicating a possible leak.
6. If no leak can be found: you MAY have a rear seal leak in the Master Cylinder which is pumping Brake Fluid into the Brake Booster (that large disk looking thing attached to the fire wall, attached to the Master Cylinder in front).
a. Use a large drip/catch pan under the Van in the area on the Master Cylinder.
b. Disconnect the brake lines from the Master Cylinder using a flair wrench.
c. Remove the two nuts (12 or 13mm?) attaching the Master Cylinder to the Brake Booster. If you have been using a lot of fluid, and have found no leaks, here is a good possibilty that all that brake fluid has collected in the Booster; so when you detach the Master Cylinder from the Brake Booster, all that fluid will come rushing out!
7. If there is Brake Fluid inside the Brake Booster: there is no cleaning that out. Brake Fluid is very caustic to the seals inside the Booster and should be replaced.
8. Of course the Master Cylinder will also have to be replaced.
9. After replacing the Master Cylinder/Brake Booster - make sure you get all that spilled brake fluid off the engine compartment area parts! I use a brake cleaner, then mild soap and then water to rinse. Clean any Brake Fluid off Paint imediatley as it will quickly dissolves finishes and paint.
10. If no leak is detected and the Booster is clear, there may be problems with the Booster One-Way Valve. This is attached to the outside of the Booster and has a hose from an "advance" connected to it. This maybe malfunctioning where you are losing power. Replace this first before deciding to replace the whole booster (in cases of NON-LEAKS ONLY). If that did not work, there may be inner seals inside the Booster that have failed. That means a new Booster.
11. In any of the above cases: Make sure you bleed the Master Cylinder correctly and bleed the brakes (at each affected wheel) correctly. Any air in the brake lines will decrease your braking proficiency or could result in brake failure.

Let me know if this helped or if you have any additional information or questions. Feel free to contact me at FixYa.com!