2004 Hyundai GX350 Front brakes lock up without touching the brake pedal. Have new master, calipers,pads, front hoses. HELP!

disc brakes can only lock up because the fluid is not allowed to return to the master cylinder reservoir
That is normally because the compensating port in the master cylinder is blocked
You say that you have fitted 2 new master cylinders but did you check the brake pedal free play because if there is not enough free play then the piston cup covers the compensating hole and the problem that you have is the result

no
rear wheel brakes operate at different pressures because of the proportioning valve in the lines
you are correct that failing brake hoses can fail internally and lock the brakes on but I feel that that is not your problem
This is how disc brakes work
There is no check valve in the master cylinder as there is in a car with drum brakes so any fluid captured by pedal ( the exception here is if the car has read drum brakes in which case that section of the master cylinder will have a check valve )movement is pushed into the calipers and applies the brakes
Now on releasing the pedal , that fluid is allowed to return to the reservoir by a compensating port in the master cylinder
That port blocks up and the disc brakes remain on.
heat build up from the brake pads expands the fluid and so pushes the pads harder onto the disc( Lock on that you release by bleeding the calipers)
SO the problem will be in the master cylinder
having the brakes professionally flushed may relieve the problem for a period but a new master cylinder fitted professionally will fix the problem

First, try bleeding the longest brakeline first, then next longest, and so on.

You can try troubleshooting several ways. Here are some possibilities: Front rubber brake hoses collapsed internally.(There is 1 way pressure to calipers, Gravity return) Master cylinder, sticking Calipers, ABS valving.

A diligent mechanic would attach a pressure gauge on the Brakelines to isolate the problem. If the pressure did not return to Zero after the Brake pedal was released, the individual component can be identified and replaced.

The cost to run the tests is compared to Labor to replace the bad part. With testing you only buy the part you need. You pay a third party to replace the single bad part versus guesstimates and doing it yourself.

Front end rubber brake hoses, cheapest, then Master cylinder or single Caliper. But if replacing both Calipers, they combine to cost more than a Master cylinder. ABS unit $$$

Labor versus parts. Some of the parts may be needed with guesstimates so difference really is buying something you don't need YET. You can look at it a preventative maintenence.

Brake specialty shops. Can do testing. Some offer lifetime Warranties on parts. Quick You have already installed pads. Repair place may not guaranty without replacing your new pads.

I hope my solution is helpful, please rate.

Most often that condition is caused by the rubber hoses that connect the caliper to the "hard" brake lines at the chassis. The hose may appear good but internally it collapses and prevents pressure from releasing. After correcting the problem be sure to replace the front pads as likely they are burned. If the pedal pulsates replace the front rotors as well, or, if they are still in relatively good shape, have them re-surfaced.

Hi,
sound as if you really do have a problem!
From your description I presume that you are confident in doing your own work?
The quickest way to check your system is to use brake hose clamps to isolate each section of the braking system. I would suggest that you rent or buy a set of the clamps.

Put the truck on axle stands and make sure it's safely secured. ]
If possible have an assistant to sit in the cab and depress the brake pedal on your instructions.
Place a brake hose clamp on both front brake hoses and the rear axle hose.
Depress the brake pedal firmly.
The pedal should have minimum movement, and be rock-solid and you should not be able to depress it further.
If the pedal does go down when you apply it, the likely reason is that the Brake Master Cyl is by-passing internally, ie, only one section is active.

You stated that the M/C had been replaced. so we shall presume that the brake pedal is rock hard.
Go to the rear brake hose clamp and release it. Instruct your assistant to depress the brake pedal. If the brake pedal moves a significant distance, then (a) your rear brake shoes require adjustment (b) rhere is a leaking brake cyl, (check for brake fluid in the drum) or (c) you have a "lazy' or a piston (s) which have siezed during a brake actuation. That problem will require removing the rear brake drums for further inspection. Not the problem? Then adjust the rear brakes if required, then depress the brake pedal again. The downward travel should now be noticeably reduced.

If all is well at the rear brakes. refit the brake hose clamp. Go to the passengers side front brake and have your assistant apply the footbrake. Pedal rockhard/minimun movement? Release the brake hose clamp whilst your assist has pressure on the pedal. spin the front wheel by hand, and note if (a) the brake pedal has excessive downward travel. (b) the brake pads are contacting the brake disc, (the wheel will cease rotating and you will hear the pads contact the disc.)

Pedal displays limited downward travel and pads contact disc? Refit the brake hose clamp and go to drivers side brake and follow the same procedure. If the pedal has excessive downward travel then you have found your inital problem.

If releasing both front brake hose clamps results in excessive brake pedal travel, then the problem will be easier to address if you deal with one side first, complete the resolution, test by using the brake hose clamps, then start / complete the other faulty brake.

Whichever side you start to work on, be methodical, boring as it sounds.
remove the road wheel, but before doing so, place a hand on the top and bottom of the tyre and rock the wheel away from you and check the bearing play. If memory serves me correctly, that year Dodge has the discs in one piece with the hub.

You have removed the wheel. Now, have your assistant turn the steering onto full right lock. Before continuing, I would like to remind you that the vehicle is up on axle stands and you have secured it safely, in order to conform with accepted safety parameters, correct?

The steering is now on full right lock and you can see both disc pads. Now, very carefully check the position of the caliper in relation to the disc pads. Is there and equal spacing on each side? Now, have your assistant release the brake pedal and very carefully observe the travel of the disc pad pistons. They should retract and the hub should turn freely by hand. A very light drag is allowed between disc pad and disc, but it should NOT be discernible when you rotate the hub by hand.

With no pressure on the brake pedal, and using an appropriate tool, attempt to have the caliper pistons retract into their cylinders / bores. Completed? Use caution as it is very easy to break / damage a disc by using undue force when retracting the pistons.

There is now an obvious gap between disc pads and disc (or rotor..sorry) Now, carefully check that the pad guide pins are not deformed and that the pads ride easily on them. if a pad jams when the brakes are applied, then, when the piston retracts, when force is removed from the brake pedal. There is an appreciable gap to close, upon the next application of the brakes!

Some types of disc pad retaining /guide pins are a tight fit, and it is very easy to tilt a pad when fitting the pins, causing the disc pad to fail to retract fully, and again, displayed by a brake pedal with excessive travel.

When the brake pedal is applied, the brake fluid has to fill the caliper piston bores, then exert pressure on the piston to force it against the disc pads, and they in turn are forced into contact with the disc. If the piston has to move an appreciable distance before contacting the disc pad, that takes more brake fluid to fill the bore of the piston, and the master cylinder piston has to travel further,resulting in a brake pedal that displays excessive travel .

I notice that you did not mention the type of effort or number of applications of the brake pedal which resulted in a firm pedal.

If you fitted replacement calipers, can I presume that you fitted new guide pins to the calipers?
Last but not least, (a) are they the correct calipers for the vehicle as regards piston bore size? The brake Master cyl will not be able to fill the bores of the calipers with enough fluid to drive the pistons out to apply the disc pads, if the bores are oversized. The pedal will also display excessive travel.
(b) If the brake master cyl is overfilled, when the brakes are applied, the master cyl will force fluid to the calipers, expand the caliper pistons, but will be unable to release the application to the pistons due to the fluid being unable to return to the master cyl as the allotted reservoir space has been filled with static fluid. When the brake pedal is depressed again, the Master cyl cannot service the caliper pistons on the first stroke as the pistons are locked at full stroke / travel in the bores, resulting in the brake pedal going to the floor, or giving that impression.
HOWEVER, that condition, if the vehicle is driven any distance, will result in the obvious odor of overheated disc brake pads, and the vehicle struggling to display any state of acceleration.
(c) Are they in fact the correct disc brake pads? It would be wise to remove a guide / retainer pin and check for free movement of the pad on the remaining pin. All ok, then refit the pin which you removed and test again. The pads have to be free to move on the pins, and thus align themselves with the face of the disc / rotor when the brakes are applied. Some people coat the pins with never-seeze or hi-temp grease when fitting them, others prefer them to be dry.

In closing, I would recommend that you check the full travel AND RETRACTION of the caliper pistons in their respective bores. It is not unknown for re-built / new parts to be defective.

It would be interesting to hear if any of the above proved to be beneficial in resolving your problem.

Disc Brake Pads for Suburban 1500 Removal & Installation Front To Remove:
Front caliper mounting bracket

1. Remove the cover on the master cylinder and siphon out 2/3 of the fluid. This step prevents spilling fluid when the piston is pushed back into the caliper bore.
2. Raise and support the vehicle safely.
3. Remove the wheel and tire assembly.
4. Compress the brake piston back into its bore using a C-clamp.
5. Remove the 2 caliper mounting bolts. Lift the caliper off the rotor.
6. Remove the inboard and outboard pad.
7. Remove and discard the anti-rattle clips.

To Install:
Caliper bracket sleeves

1. Thoroughly inspect, clean and lubricate all caliper slide points, bolts and hardware.
2. Install new anti-rattle clips.
3. Position the inboard and outboard pads into the support bracket
4. With the 2 pads in position, place the caliper over the brake rotor and align the holes in the caliper with those of the mounting bracket.
5. Install the caliper mounting bolts through the sleeves in the inboard caliper ears and into the mounting bracket.
6. Tighten the mounting bolts to: 80 lb. ft, (110 Nm).
7. Install the tire and wheel assembly. Lower the vehicle.
8. Add fluid to the master cylinder reservoirs so they are 1/4 in. (6.35mm) from the top.
9. Test the brake pedal by pumping it to obtain a hard pedal. Check the fluid level and add fluid as necessary.

Rear To Remove:

1. Remove 2/3 of the brake fluid from the master cylinder.
2. Raise and safely support the vehicle.
3. Remove the wheel and tire assembly.
4. Remove the brake caliper mounting bolts. Suspend the caliper from the frame with mechanic's wire. Do not allow the caliper to hang from the brake hose.
5. Remove the brake pads from the caliper mounting bracket.
6. Remove and discard the anti-rattle clips.

Rear brake caliper

To Install:

1. Install new anti-rattle clips.
2. Position the brake pads to the caliper mounting bracket.
3. With the 2 pads in position, place the caliper over the brake rotor and align the holes in the caliper with those of the mounting bracket.
4. Install NEW caliper mounting bolts. Tighten to: 30 lb. ft, (40 Nm).
5. Install the wheel and tire assembly.
6. Lower the vehicle.
7. Refill the master cylinder to the proper level with fresh brake fluid. Pump the brake pedal slowly and firmly to seat the brake pads. Burnish the brakes as needed.

YOU NEED REMOVE A LITTLE BRAKE FLUID FROM MASTER CYLINDER.THEN YOU RAISE VECHICLE SAFELY.SUPPORT IT WITH JACK STAND.MAKE SURE VECHICLE IS ON LEVEL SOLID GROUND.REMOVE TIRE OR WHEEL.THEN YOU NEED A LARGE C - CLAMP.PLACE C - CLAMP OVER BRAKE CALIPER AND BRAKE SHOE BACKING.TIGHTEN C - CLAMP JUST ENOUGH PUSH CALIPER PISTON BACK IN CYLINDER BORE TO REMOVE CALIPERS FROM ROTORS.TO REMOVE BRAKE CALIPER YOU NEED 2 OPEN AND BOX END WRENCHES ONE TO HOLD CALIPER SLIDE PINS AND THE OTHER WRENCH FOR LOOSEN CALIPER BOLTS.THEN YOU REMOVE CALIPER AND REPLACE FRONT BRAKE SHOES.IF YOU NEED TO REMOVE BRAKE ROTORS JUST REMOVE THE BRAKE CALIPER CARRIER SUPPORT BOLTS THE REMOVE THE BRAKE CALIPER CARRIER SUPPORT THEN REMOVE ROTOR.YOU NEED TO REPLACE BOLTS WITH NEWS AND USE LOCK TIGHT.THEN YOU PUT IN NEW BRAKE PADS IN CALIPER PUT CALIPER BACK ON SUPPORT CARRIER.REPLACE WITH NEW BOLTS USE LOCK TIGHT THEN TORQUE BOLTS TO 26 FT LBS.MAKE SURE MASTER CYLINDER COVER IN PLACE.WHEN YOU FINISH REPLACING BOTH FRONT BRAKE PADS PUMP BRAKE PEDAL TO SEAT BRAKE SHOES AGAINST BRAKE ROTORS SO YOU WILL HAVE FIRM BRAKES WHEN YOU APPLY THEM. THEN CHECK YOUR BRAKE FLUID LEVEL IN MASTER CYLINDER MAKE SURE ITS NOT LOW. MAKE SURE YOU HAVE FIRM BRAKES BEFORE DRIVING.IF YOU HAVE ANY PROBLEMS CHANGING FRONT BRAKE PADS YOU CAN GO TO ANY AUTO PARTS STORE AND BUY A HAYNES REPAIR MANUAL.

Remove brake fluid from the master cylinder brake fluid reservoir until the reservoir is approximately 1/2 full. Discard the removed fluid.

1. Raise and safely support the front of the vehicle. Remove the front wheels.
   

1. Remove or disconnect the following:
2. Front brake caliper guide pin bolts
   

• Brake caliper by slowly sliding it up and off the adapter and brake rotor. Support the caliper out of the way with a strong piece of wire. Do not let the caliper hang by the brake hose or damage to the brake hose will result.
  

1. If necessary, compress the caliper piston into the bore using a C-clamp. Insert a suitable piece of wood between the C-clamp and caliper piston to protect the piston.
2. Outboard disc brake pad from the caliper by prying the brake pad retaining clip over the raised area on the caliper. Slide the brake pad down and off the caliper.
   

• Inboard disc brake pad from the caliper by pulling the brake pad away from the caliper piston until the retaining clip on the pad is free from the caliper piston cavity
  

To install:

1. Be sure the caliper piston has been completely retracted into the piston bore of the caliper assembly. This is required when installing the brake caliper equipped with new brake pads.
   
2. If equipped, remove the protective paper from the noise suppression gaskets on the new disc brake pads.
   
3. Install or connect the following:
   
   • New inboard disc brake pad into the caliper piston by pressing the pad firmly into the cavity of the caliper piston. Be sure the new inboard brake pad is seated squarely against the face of the brake caliper piston.
     
   • Outboard disc brake pad by sliding it onto the caliper assembly
     
   • Brake caliper assembly over the brake rotor and onto the steering knuckle adapter
     
   • Caliper guide pin bolts and torque to: 35 ft. lbs. (47 Nm) on 2002–04 models or 26 ft. lbs. (35 Nm) on 2005 models
     
   • Front. Apply the brake pedal several times until a firm pedal is obtained.
     
4. Check the fluid level in the master cylinder and add fluid as necessary. Road-test the vehicle.
   

For the rotors:

1. Remove brake fluid from the master cylinder brake fluid reservoir until the reservoir is approximately 1/2 full. Discard the removed fluid.
   
2. Raise and safely support the front of the vehicle. Remove the front wheels.
   

1. Remove or disconnect the following:
2. Front brake caliper guide pin bolts
   

• Brake caliper by slowly sliding it up and off the adapter and brake rotor. Support the caliper out of the way with a strong piece of wire. Do not let the caliper hang by the brake hose or damage to the brake hose will result.
  

1. If necessary, compress the caliper piston into the bore using a C-clamp. Insert a suitable piece of wood between the C-clamp and caliper piston to protect the piston.
2. Outboard disc brake pad from the caliper by prying the brake pad retaining clip over the raised area on the caliper. Slide the brake pad down and off the caliper.
   

• Inboard disc brake pad from the caliper by pulling the brake pad away from the caliper piston until the retaining clip on the pad is free from the caliper piston cavity
  

To install:

1. Be sure the caliper piston has been completely retracted into the piston bore of the caliper assembly. This is required when installing the brake caliper equipped with new brake pads.
   
2. If equipped, remove the protective paper from the noise suppression gaskets on the new disc brake pads.
   
3. Install or connect the following:
   
   • New inboard disc brake pad into the caliper piston by pressing the pad firmly into the cavity of the caliper piston. Be sure the new inboard brake pad is seated squarely against the face of the brake caliper piston.
     
   • Outboard disc brake pad by sliding it onto the caliper assembly
     
   • Brake caliper assembly over the brake rotor and onto the steering knuckle adapter
     
   • Caliper guide pin bolts and torque to: 35 ft. lbs. (47 Nm) on 2002–04 models or 26 ft. lbs. (35 Nm) on 2005 models
     
   • Front. Apply the brake pedal several times until a firm pedal is obtained.
     
4. Check the fluid level in the master cylinder and add fluid as necessary. Road-test the vehicle.
   

For front brake pads replacement you need only usually wrench set, inclusive 7 mm allen key also. But for rear brake pads replacement you need obligatory a special caliper piston pressing tool, in order to press back rear caliper piston with parking brake automatic adjustment!!! For front brake pads. First you must verify yours front brake disc diameter: 280 mm or 288 mm. (On my car y have 288 mm). After that you can buy the brake pads (with wear sensor). For change front brake pads you must raise vehicle, remove wheels, extract the retaining spring of the caliper, and remove the caliper as follow: 1. Do not disconnect the brake hose from the caliper, and do not allow the caliper to hang by the brake hose! 2. Remove top and bottom caps (on back side of the caliper) for access to guide pins, then unbolt and remove them from the brake carrier. Remove the caliper. 3. Now you must thoroughly clean the brake calipers (free of grease). 4. Remove outer brake pad from brake carrier. 5. Pull inner brake pad out of brake caliper piston. 6. Check up the brake fluid level on the reservor, and emptying if neccessary! 7. Push piston back into brake caliper housing. 8. Install inner brake pad (with expanding spring) in brake caliper piston. (Arrow marked on pad - if exist, must point in direction of brake disc rotation when vehicle is moving forward). 9. Install outer brake pad into brake carrier. 10. Bolt brake caliper housing to brake carrier using two guide pins. Tightening torque is 25 Nm. 11. Install both caps. 12. Insert retaining spring into brake caliper housing. Important: Depress the brake pedal firmly several times while the car is stationary so that the brake pads adjust to their normal operating positions!!! Check brake fluid level and top up if neccessary!!!

Ok Have you recently replaced the pads? Do they act normal after they cool down? Are the pads in good condition, pads that are so worn that the caliper pistons come out beond their normal limit may stick there due to excessively worn pads. If so make sure they are at least factory grade quality Good are bendix, the only thing that can cause your symptoms is a bad master cylinder because it affects BOTH front calipers equally if it was a caliper it is likely to affect only ONE. Let me ask you this happens after driving in heavy traffic or after along drive?
If So there have been countless cases of boiling brake fluid, Old Brake Fluid gathers moisture and its boiling point is lowered, Fluch out ALL the old brake fluid and have new put it.

also is there any chance even the slightest chance that something other than brake fluid even a drop or two is enough to do damage. has been added to the master cylinder at any time? If so you need to replace EVERY PART IN THE BRAKE SYSTEM INCLUDING THE ABS CONTROLLER THAT HAS ANY RUBBER PIECES IN IT CALIPERS HOSES WHEEL CYLINDERS,MASTER CYLINDER just for your information.
I believe you have a bad master cylinder but do not buy a rebuilt one only bran new and preferably from the dealer.
Please contact me if you need more help and would you give me a good rating if you feel my information is helpful.
Thank you ,Randy