[12VEG]

My brakes have never really felt that solid, I've adjusted the rears, replaced the front pads, checked the shoes/drums for excessive wear, found none, bled the whole system, replaced the wheel cylinders w/HD ones, and the thing still can't stop that well. I'm wondering if it's a problem with the RWABS?

Yesterday I had to brake hard, and felt the symptom more than I ever had before: If I have to slam the pedal down it will grab hard for a second, lock the rears for barely a chirp, and then the pressure I'm applying to the pedal has to be significantly increased to gain any more braking power and even then standing on it I can barely make 'er stop. Not good if your about to rear-end somebody.

Hope that was clear enough. Any ideas?

 

[Motorboat]

Hydroboost diaphram?

 

[95dually3500]

Needs a new booster and or master cylander

 

[jnicewan]

I second the hydroboost. If you have to stand on the pedal it may not be giving you enough assist. Have you looked into that. Could possibly be the P/S pump getting weak too.

 

[12VEG]

well, the p/s works just fine, and I assume if it was getting too weak to power the hydroboost I would also feel it in the steering, right?

What about it being a problem with the RWABS? Are there any components of the RWABS that could be causing this? normally it brakes OKAY, but when I need to slam 'em on - like I said earlier - it grips hard at first, then just looses juice.

I'm not familiar with hydroboost systems though, so if this sounds like the problem to those of you with experience then I'm all ears...

 

[dozer12216]

When you increase the diameter of the tires it affects the mechancial advantage of the brakes. There is a porportional decrease in braking ability. Couple that with some brake pads that are not of OE quality. It can be a disaster.

 

[12VEG]

Hell, all I have are 265's...

Until now I've made the truck go faster, I suppose it's time to focus on making it stop better too...

 

[RPegram]

If the hydraboost was bad, then the pedal would be hard the first mash and all subsequent ones. When you jab the pedal and get a chirp out of the rear that is the RABS doing what it is supposed to do. Now, after that initial chirp you should feel the pedal pulsate like they do when rotors warp. This is the RABS allowing fluid to go to rears under hard braking, but not enough to cause rear wheel lockup. You can temporarily bypass the ABS unit (or simply unplug it) and see what happens then. Be careful since now you can totally lock up the rears.

Another thought would be the proportioning valve. It may be you aren't getting enough fluid to the front. When you bleed the brakes make sure you get a good stream of fluid out of each bleeder. Remember, shut the bleeder closed before the pedal bottoms out. Something else that could cause low pressure to the front would be the master cylinder. If there was a little nick or tear in a cup inside of it, pressure would be reduced, but most times you can diagnose this by just holding steady pressure on brake pedal and it will start slowly going down towards floor.

Does pumping the pedal do anything with how high it comes up? Try it with truck just idling and sitting still. Quickly mash, let up, mash again several times very quickly and see if point where pedal stops rises. If it does ride, then two things come to mind: rear adjustment or air in lines. If rears are adjusted up, then that leaves air. Air can sometimes be a bear to get all of it out.

 

[Shibbyutman]

Rear wheel ABS will NEVER make the pedal pulsate because it doesn't have a pump motor to evacuate the low pressure accumulator within the ABS valve.

What ever type of assist you have on your truck is what I'd be looking at. The steering can still feel fine because the pump is working like it is supposed to but the hydraboost valve or vacuum diaphragm can both fail causing much of what you are feeling. The assist will be good on the initial hit if the damage to the unit is after some displacement of the pedal.

 

[12VEG]

Found this:

DIAGNOSIS AND TESTING - HYDRAULIC BOOSTER
The hydraulic booster uses hydraulic pressure from the power steering pump. Before diagnosing a booster problem, first verify the power steering pump is operating properly. Perform the following checks.


•Check the power steering fluid level.
•Check the brake fluid level.
•Check all power steering hoses and lines for leaks and restrictions.
•Check power steering pump pressure.
NOISES
The hydraulic booster unit will produce certain characteristic booster noises. The noises may occur when the brake pedal is used in a manner not associated with normal braking or driving habits.

HISSING
A hissing noise may be noticed when above normal brake pedal pressure is applied, 40 lbs. or above. The noise will be more noticeable if the vehicle is not moving. The noise will increase with the brake pedal pressure and an increase of system operating temperature.

CLUNK-CHATTER-CLICKING
A clunk-chatter-clicking may be noticed when the brake pedal is released quickly, after above normal brake pedal pressure is applied 50 - 100 lbs. .

BOOSTER FUNCTION TEST
With the engine off depress the brake pedal several times to discharge the accumulator. Then depress the brake pedal using 40 lbs. of force and start the engine. The brake pedal should fall and then push back against your foot. This indicates the booster is operating properly.

ACCUMULATOR LEAKDOWN

1.Start the engine, apply the brakes and turn the steering wheel from lock to lock. This will ensure the accumulator is charged. Turn off the engine and let the vehicle sit for one hour. After one hour there should be at least two power assisted brake application with the engine off. If the system does not retain a charge the booster must be replaced.
2.With the engine off depress the brake pedal several times to discharge the accumulator. Grasp the accumulator and see if it wobbles or turns. If it does the accumulator has lost a gas charge and the booster must be replaced.

________________________________________________


I'm gonna try the above right now and see if I have any of the symptoms.

...Quickly mash, let up, mash again several times very quickly and see if point where pedal stops rises. If it does ride, then two things come to mind: ...

This confuses me a little bit. Pump the pedal down a few times and see after how many times it stops returning???

I'll try unplugging the ABS too, if it clears up the problem does that mean the ABS unit is bad?

I just remembered that I have the manual too, so I'll try some troubleshooting from there as well.

Thank you for the replies, keep 'em comin, and I'll keep y'all updated...

 

[JHORN53]

Hissing unless brake pedal pressured

Dodgebabyjane here! My 92 12v 2wd has hissing that ceases when brake pedal is applied! And YES, I applied brakes yesterday and nearly drove into the pasture PAST the road. Any knowledge you can share would help! :thanks:

 

[Shibbyutman]

sounds like you have a torn vacuum booster diaphragm or another vacuum leak somewhere.

 

[JHORN53]

Thanks Shibbyutman

Shibbyutman,

I so appreciate your reply! 8 degrees here, a good day to be under the dash and hood!

So do I replace the booster AND the master cylinder? Know SOME, not a mechanic, frequently do read and study and do my own with help of sons. How does one locate LEAKS in the system?

Any visual materials/diagrams would be helpful!

 

[Shibbyutman]

The booster is the big round thing that is bolted to the firewall. Unless you're leaking brake fluid somewhere (check once you get the booster off, some times a bad seal in the master cylinder is what causes the booster diaphragm to fail) you really don't need to replace the master cylinder.