Archived Brake pedal fade

[RColston]

I just completed a full boat brake upgrade on my 96 2500 RWABS truck and after bleeding my brakes, my brake pedal will not hold under pressure. Just slowly bleeds down!! No rebuilt parts here, every thing new, full EGR severe brake package, including new master cylinder. Did a search on brake bleeding but no similar problems. Any ideas???



Rick C.

 

[B.G. Smith]

Brake

Look at the master cyl. and see if you are losing any fluid, if not then the master cyl. is leaking by inside. bg

 

[RColston]

This problem did not exist before the brake job. Appeared after completeing the job, prior to the new master cylinder. That is precicely why I installed the new master cylinder. Just finished re-adjusting the rear brakes, adjusted them tight, then backed them off till they just barely had any drag. Master cyl. sis a brand new wagner unit, not a rebuild.



Rick C. :{

 

[illflem]

Rick, I would have figured the master cylinder also. It has to be leaking somewhere. Inside the ABS unit? Give Sam Peterson a call 8-5 pacific 360/357-4958

 

[RThompson]

Chances are you still have air in the system. Bleed right rear first and then left rear. Proceed with right front and then left front. Pump the pedal 'slow' to build up and make sure you shut off bleeder 'before' the pedal bottoms. Did you bench bleed the master clyinder before installing? Very important to do so.

Good luck.

 

[RColston]

Did all of the above. Suspect it is something to do with the ABS?? Maybe. Hey Bill, how's your tractor.

 

[MidwestMotorman]

shoe shape

Could it be that your rear shoes have not set in to the drums? You would then brake on only a small surface of the drums. This would rapidly wear part of the pad. Recheck the rear brake drag (are you doing this with the rear axles pulled out to eliminate limited slip drag?).



Did you turn the rear drums? Is it possible they were overturned and you are maxing out the rear brake cylinders, possibly making them seep out some fluid?



If everything else fails to work, I would pull the rear drums and look at the rear pads to check the wear pattern. Also check for a possibly leaking rear cylinder.

 

[RThompson]

Some veteran mechanics have been down this same road many times. With all the hydralic controls, (clutches, throttles and servos) mechanics have lost a lot of time and money sorting out how to correct the problem of spongy hydralics. Before you spend more money on parts go back and look at each component. If you are using pressure bleeder, try bleeding manually with someone who has a lot of patience and who will advise you to pedal position as it is depressed. Lots of patience and fluid... I have had to slip . 004 feeler gauge into rear clyinders to confirm air that is trapped there. (This post brings back a lot of bad memories. )



Keep us posted:

Good luck!!

 

[RColston]

Well, here is the latest. Took truck to reputable dealer and had them flush and bleed the system, washing out all of that wonderful Motul 600 brake fluid. Picked up truck, dealer said it was all better. Seemed ok at first then pedal began to fade same as before. I may go get another new master cylinder in case this one is defective ( believe this, new 1 3/16 rear wheel cyl from EGR. Brake job all done bleeding brakes fluid running out from left rear drum. Disassembled left rear examined rear wheel cylinder missing one rubber wiper cup !) Accolades to Glenn from EGR in Corona CA, stand up guy, gave me new part, fluid etc. great service. Is there anything in the RWABS system that would allow fluid to bypass similar to an internal leak in a master cyl?

 

[MidwestMotorman]

ABS sensor

Your ABS system could be bleeding it back. What if a rear sensor was faulty and read the rear as locking up and thus releasing the pressure? Could it set a code if it was that way? Maybe you can unplug the rear sensor to eliminate that possibility. Could be that the ABS valve is bad and leaking the pressure away.



Keep a searchin'

 

[B.G. Smith]

Brake fade

I do not want to be argumentative but the fluid can not return from the system to the master cylinder while the pedal is being depressed. The ABS system pulses or modulates the pressure when activated but the fluid is not bypassed. bg

 

[MidwestMotorman]

It's not argumentative when an education is taking place So it is my understanding that the ABS system must just have a modulating chamber that allows the brake pressure to be varied? Dodge truck systems do not have a pump, just the modulation chamber correct? I know GM uses an actual pump in the ABS system. That makes it very handy to bleed air as one just has to turn on the ignition and push the brake. Then the pump just pressurizes the system for you.



Sorry for taking the post a little off topic.

 

[dan_gilson]

The DC shop manual has a specific procedure for bleeding the ABS. I am not at home but will try to remember to look it up when I get there. You should only have to use it WHEN you replace something upstream from teh ABS unit like the master cylinder.

 

[Missouri Mule]

What I'm reading you bled the brakes.



Did you bleed the master cylinder after you installed it?

 

[dan_gilson]

Replacing everything complicates things a little.

The shop manual says to bench bleed the master before mounting it on the truck.



I remember reading some "better" instructions about bleeding ABS systems but it is not in the shop manual.



The order that the shop manual says is:

1. Bench bleed the master

Then bleed in the following order.

2. combination valve

3. right rear wheel

4. left rear wheel

5. right front wheel

6. left front wheel



If ABS then:



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 CONPLETED, disconnect the scan tool and proceed.



Bleed the brakes for a second time.



I think what the scan tool test does is set the ABS pump to full open so that any air can be released.



One other note: Make sure that you are bleeding into a bottle. It really helps when bleeding sigle handed BUT also give you less chance of sucking air back into the syetem. The shop manula has a picture. Here is also another writeup: https://www.turbodieselregister.com...id=24058&highlight=Brake+AND+Bleed+AND+Bottle

 

[Pit Bull]

It's air you just have to find it. Good luck.

 

[RColston]

Well, this is the deal. After taking truck to the dealer to have brakes bled, to no resolution of pedal fading problem, I talked to a Friend of mine who has a fleet of (8 trucks that definitely all gross 10k lbs. or more and eat brakes regularly) work trucks. Told him the problem. He has had the same problem on trucks that major brake jobs are done. Only cure was to replace both, I repeat both, the master cylinder and the brake vacuum booster:{ . He has experimented replacing just one or the other, fade remained. Same exact problem as mine:{ . Well $234. 04 later :{ I am the proud owner of a new vacuum booster. After this is installed I will have 3 parts in the system that are not brand new, 1, rear brake drums ( freshly turned ) 2, brake pedal and 3, the firewall that the whole mess bolts to. I'll let ya know how it is after the booster is installed . ( And to this end- NO HO TRUCKS IN CALIFORNIA IN 2003#@$%! #@$%!