I'm getting ready to go out of town next week, and I can't get these brake's to feel right.

Quick history. Brake's have been getting worse. I just figure I needed new pad's. I put on all new rotor's, caliper's, and pad's, front and rear. Bled with my motive bleeder. Still a spongy pedal, and brake's not worth a hoot. Bled with someone pumping the pedal. Same deal. Every once in awhile,,I'll get a good pedal and have good brake's. But that only last's a short time. They feel like they are wet like I drove through standing water.

Today I changed the master cylinder. No better. Again though, once in awhile I'll get normal brake's. I can't see anything leaking anywhere. Once in awhile, my brakesmart will show I'm loosing pressure. But according to brakesmart, the sensor is either good or bad. They won't send me a new one unless I buy it for $65. I'm not pleased with them.

What else could it be. I'm about to pull my hair out. Could it be the hydro-boost. Is there anyway to test it.

I need to get this thing fixed before I go on another run next week. Thank's for any input.

 

If the pedal stays spongy if you pump it slowly five to ten times with the engine off (to bleed the hydro-boost) you can rule the hydro-boost out. I do not think it's the booster, the brake would feel spongy with the booster working but brake better.

Can you pump up the brake, and does it get better? If yes then there is still some air somewhere.
To get rid of it I usually use a vacuum brake bleeder (homebrew instructions will follow) and start from the rear right, then rear left, (if applicable the load proportioning valve), front right, front left.

The homebrew vac bleeder is made of 2 pieces of hose that wil just slip over the bleeder screws, and a glass jar. Punch 2 holes in the lid of the jar, and insert the hoses- one going to the bottom of the jar (will be connected to the brake bleeders) and one almost flush with the lid (will be connected to the vac source.)
I usually connect the vac hose to the vacuum pump on the truck and have a helper watch the jar and refill the reservoir while I do my round under the truck.

HTH

AlpineRAM

 

ABS BRAKE BLEEDING
ABS system bleeding requires conventional bleeding methods plus use of a scan tool. The procedure involves performing a base brake bleeding, followed by use of the scan tool to cycle and bleed the HCU pump and solenoids. A second base brake bleeding procedure is then required to remove any air remaining in the system.

Perform base brake bleeding,(Refer to 5 - BRAKES - STANDARD PROCEDURE) OR (Refer to 5 - BRAKES - STANDARD PROCEDURE).
Connect scan tool to the Data Link Connector.
Select ANTILOCK BRAKES, followed by MISCELLANEOUS, then ABS BRAKES. Follow the instructions displayed. When scan tool displays TEST COMPLETE, disconnect scan tool and proceed.
Perform base brake bleeding a second time,(Refer to 5 - BRAKES - STANDARD PROCEDURE) OR (Refer to 5 - BRAKES - STANDARD PROCEDURE).
Top off master cylinder fluid level and verify proper brake operation before moving vehicle.

 

Another way to get around the use of a scan tool is to go perform some abs actuated stops in some loose gravel then re bleed the brakes again. I have had more trouble bleeding brakes with all the vacuum mumbo jumbo. What works best for me is this

2 people
open bleeder at r/r of truck
have helper push brake pedal to floor
close bleeder
repeat until no air
continue at l/r wheel
then r/f wheel
then l/f wheel

 

Thank's for the replies.

It just so happens, they have my street tore up, and all I have is a gravel road. Now I know why. So I can activate my ABS. I'm going to try this method first. If that doesn't work, would a cheap scan tool from Advanced Auto,,,or similar, work for cycling the ABS. Or would this be something I need to take to the dealer. Thank's again.

 

Off the cuff have you looked at the lines coming from the hard metal lines too the calipers them selves? They could be wore out and expanding when the brakes are applied

 

I took it to the dealer and had the ABS bled. It seemed to be fixed at first. Then after about 10 mile's, I got an occasional pull to the left. That seem's to have went away. Then before I made it home, I got several spongy pedal's, but then it seemed ok. The next day when I got in it, it was right back to where I started.

What the heck could it be. Is there anyway to test the Hydro-boost.

 

Battery voltage is supplied to the ABM. The ABM performs a system initialization procedure at start up. A check of the ABS motor is performed at 15 miles per hour. Initialization consists of a static and dynamic self check of system electrical components.

The static and dynamic checks occurs at ignition start up. During the dynamic check, the ABM briefly cycles solenoids to verify operation. An audible noise may be heard during this self check. This noise should be considered normal. The ABS motor and pump are then checked at a speed of 15 mile per hour.

If an ABS component exhibits a fault during initialization, the ABM illuminates the amber warning light and registers a fault code in the microprocessor memory.

The ABM monitors wheel speed sensor inputs continuously while the vehicle is in motion. However, the ABM will not activate any ABS components as long as sensor inputs indicate normal braking.

During normal braking, the master cylinder, power booster and wheel brake units all function as they would in a vehicle without ABS. The HCU components are not activated.

The purpose of the antilock system is to prevent wheel lockup. Preventing lockup helps maintain vehicle braking action and steering control.

The antilock ABM activates the system whenever sensor signals indicate periods of wheel slip.

The antilock system prevents lockup during a wheel slip condition by modulating fluid apply pressure to the wheel brake units.

Brake fluid apply pressure is modulated according to wheel speed, degree of slip and rate of deceleration. Sensors at each front wheel convert wheel speed into electrical signals. These signals are transmitted to the ABM for processing and determination of wheel slip and deceleration rate.

The ABS system has three fluid pressure control channels. The front brakes are controlled separately and the rear brakes in tandem. A speed sensor input signal indicating a wheel slip condition activates the ABM antilock program.

There are Two solenoid valves (Isolation and Dump valve) which are used in each antilock control channel. The valves are all located within the HCU valve body and work in pairs to either increase, hold, or decrease apply pressure as needed in the individual control channels.

During an ABS stop the ISO valve is energized which acts to prevent further pressure build-up to the calipers. Then the Dump valve dumps off pressure until the wheel unlocks. This will continue until the wheels quit slipping altogether.

 

Nice write up. But what's it mean. Are you saying I have a bad ISO valve.

 

Could have dump valves sticking causing your intermitant spongy pedal. This condition will not give a code but brake pedal will fade then become hard when coming to stop. Since the ABM only checks the electrical components of the ABS it has no check for sticking valves.