2000 mack rd688 brake help

[mechohaulic]

ALL bets going to the treadle (brake) valve is stuck/ sticking. hammered pressure  during quick stop !!  manually move the brake peddle by hand up/down ===back forth how ever you call it  == how much slack/ free play does it have???  floor mount  :: does the roller free spin or drag across the plunger causing it to jam. the dry plunger not traveling completely . rubber boot still there intact or have the usual rip allowing crud oxidation. 

[Joey Mack]

1 minute ago, mechohaulic said:

ALL bets going to the treadle (brake) valve is stuck/ sticking. hammered pressure  during quick stop !!  manually move the brake peddle by hand up/down ===back forth how ever you call it  == how much slack/ free play does it have???  floor mount  :: does the roller free spin or drag across the plunger causing it to jam. the dry plunger not traveling completely . rubber boot still there intact or have the usual rip allowing crud oxidation. 

rubber boot still there intact or have the usual rip allowing crud oxidation.    

[JoeH]

5 hours ago, Moparmike said:

brake have 2 different valves on them 

and air comes out both lines when off press 1 air the other has air when you remove it 

Are you saying you have air leaks? Can you clarify what you mean here? With truck fully assembled you need to run through a proper Air System PreTrip Inspection! This will find any air leaks you have that could be causing your issue! 

Maybe I'm not following the thread well enough, but I don't get the impression that air leaks were checked for "by the book."

Here's a YouTube video on how to run the air system pre trip inspection.

 

[JoeH]

23 hours ago, Moparmike said:

as I'm looking things over I let air build up and press brakes to get everything adjusted back out 

Reread this in original post and I'm a little confused, pressing brakes doesn't adjust them.

[JoeH]

23 hours ago, Moparmike said:

s cam is free not stuck when I press brake pedal it makes a noise can't remember if the noise was there before or after I slammed brakes

My R models all make a bit of a momentary farting noise in the pedal on initial brake application. I assume this is what you're referring to.

[JoeH]

23 hours ago, Moparmike said:

so I'm looking things over decided to replace all slack adjuster on back with new from mack 

New slack adjusters combined with indicating in one of the above quotes that you "pressed the brake to get everything adjusted back out"  is a red flag to me that you might not know how to adjust the brakes properly.  No offense meant, I'm just reading more thoroughly than I did my first time through.

[Moparmike]

17 minutes ago, JoeH said:

New slack adjusters combined with indicating in one of the above quotes that you "pressed the brake to get everything adjusted back out"  is a red flag to me that you might not know how to adjust the brakes properly.  No offense meant, I'm just reading more thoroughly than I did my first time through.

biggest reason for the change was a few didn't adjust (SELF) so I just replaced all 4

all the 1s I took off was on the truck when I purchased it about 7yrs ago

 

and most times I try catch fleet pride deals on drum and brake kits change most stuff all at once 

[Moparmike]

35 minutes ago, JoeH said:

Reread this in original post and I'm a little confused, pressing brakes doesn't adjust them.

let pressure build before I adjust the out 

[Moparmike]

43 minutes ago, JoeH said:

Are you saying you have air leaks? Can you clarify what you mean here? With truck fully assembled you need to run through a proper Air System PreTrip Inspection! This will find any air leaks you have that could be causing your issue! 

Maybe I'm not following the thread well enough, but I don't get the impression that air leaks were checked for "by the book."

Here's a YouTube video on how to run the air system pre trip inspection.

 

I took lines off whole truck was running to verify air pressure was there 

[Moparmike]

10 hours ago, mechohaulic said:

now this adds a new road to travel with the statement "pedal goes to floor and not stopping". if the plunger was oxidized and stuck the brakes should be partially applied; which apparently they are not. or plunger NOT going all the way down to activate brake valve, combine that with only the left rear front wheel acting up! three other wheels ( plus steer axle )should stop unit. I would look at the pedal; possibly remove it , hope alum bracket doesn't break ;clean all parts. hold the phone;;; old school thinking again== is this pedal floor mounted  as in olden days or hang from under dash ?? 

truck stops just not how it usually stops

 

takes longer even empty

[JoeH]

8 minutes ago, Moparmike said:

biggest reason for the change was a few didn't adjust (SELF) so I just replaced all 4

all the 1s I took off was on the truck when I purchased it about 7yrs ago

 

and most times I try catch fleet pride deals on drum and brake kits change most stuff all at once 

I hate self adjusters for exactly this reason. I put manual adjusters on my older trucks that can legally run them. A) it forces me/driver under the truck to look at things regularly,  b) when you adjust them all your brakes are synchronized and the truck stops straight with no pulling, C) you dont have to worry about the self adjuster failing.

Edited December 7, 2023 by JoeH

[Joey Mack]

1 minute ago, JoeH said:

I hate self adjusters for exactly this reason. I put manual adjusters on my older trucks that can legally run them. A) it forces me/driver under the truck to look at things regularly, when you adjust them all your brakes are synchronized and the truck stops straight with no pulling, C) you dont have to worry about the self adjuster failing.

EXACTLY !!!!

[JoeH]

7 minutes ago, Moparmike said:

I took lines off whole truck was running to verify air pressure was there 

Air moving doesn't = full air pressure.

[JoeH]

3 minutes ago, Moparmike said:

truck stops just not how it usually stops

 

takes longer even empty

Does the truck pass the air pre trip? The rubber diaphragms in the service chambers do rupture, and a bad one can dump enough air pressure to cause problems.

The short of the pre trip is build air pressure, shut off truck. Listen for leaks. Release parking brake, listen for leaks.  Apply service brake pedal fully, listen for leaks.

The whole while you are watching your air gauges for pressure loss.

[JoeH]

If that checks out then note back to us the following:

With parking brake set, what angle are the push rods to the slack adjusters? Maximum braking force is at 90 degrees when fully applied.

I'd also like to know how much travel each air chamber pushrod has. To get these measurements, release brake, put a zip tie on each pushrod just barely against the face of the air chamber. Then set the parking brake, and measure how far off the chamber face the zip tie is. Measure each of the 4 drive positions and let us know what they are. You should have them set to 1 inch.  1-3/4 inch is usually maximum legal travel. Much more than that and your chamber maxes out it's travel before it has the shoe fully applied to the drum.

[JoeH]

I have those zip ties on all my trucks with self adjusters, and in each truck I carry a 3 inch scrap of flat bar with quarter inch gradations on it under the seat. Gives a quick reference to know where your brake adjustments are.

[mechohaulic]

want to get really technical=checking brake system should be a three check process; Ready  1-distance from can base to clevis pin center when brakes released== 2- same place of measure can to pin center BUT with brake pedal/trolly down (service brake applied) 3- same place of measure WITH  parking brake applied. 

reason for three times ; can tell brake throw vs released ALSO  measure with parking brake applied will indicate a bad or weak brake can which having a broken internal spring usually punctures a diaphragm . newer chamber have a red throw rod which indicates it's traveling tooo far. the  zip ties are  the best way to go. manual adjusters got my vote. any checking of brake pedal yet ??

[mechohaulic]

have seen "mechanics" do a brake adjustment having released  1 1/4 distance , set parking brake shows  1 1/2 throw. conclusion " brakes ok only throw 1/4 in. apply service brake throw 2in. bad brake chamber not applying full pressure with parking brake . chalk on floor to write measurement or on unit tooo many numbers for me to remember back then

[mechohaulic]

Moparmike don't give up on BMT . diagnostic solutions are challenging when not physically there for hands on checking.

[mrsmackpaul]

Do you have a air gauge you can hook to lines to check pressure ?

 

Paul 

[JoeH]

On 12/6/2023 at 7:50 PM, Moparmike said:

biggest reason for the change was a few didn't adjust (SELF) so I just replaced all 4

all the 1s I took off was on the truck when I purchased it about 7yrs ago

 

and most times I try catch fleet pride deals on drum and brake kits change most stuff all at once 

On the intermediate axle of Mack Camelbacks, the slack adjusters are mounted "backwards" I believe, so is it possible you have the new slack adjusters installed wrong? The rearmost axle adjusts normally, but the intermediate axle requires a reverse action slack adjuster. Your parts dealer should have given you two and two different adjusters,  not four of the same.

When we bought my 1995 RD688S it took us a while to find out why the one wheel was always walking out of adjustment. The above is why. Previous owner installed the wrong style self adjusters.  This truck now has all manual adjusters.

After a certain year, I think around 2000,2001 you're not allowed to use manual adjusters, but I think if a cop pulled you over and you explained a) brakes are all adjusted and b) synchronized I don't think he could argue with you other than if he's a complete ass and c) you know exactly where your brake lining life is at.

Edited December 9, 2023 by JoeH

[JoeH]

Its also possible if you don't know about the reverse slacks to install them on the wrong axles even if they gave you two and two.

Edited December 9, 2023 by JoeH

[JoeH]

Which is another reason why I'd love to know what your adjustments are set to.  If your adjustments are set properly and maintain adjustment well then I'd be wondering about the treadle valve not regulating out enough brake pressure.

[Moparmike]

Hey guys I appreciate the help 

it's in the shop 

I been in trucking for about 18 yrs and last yr I decided to jump in to excavation and demolition and dirt Works 

so I really don't have the time to figure it out especially when it's the truck i pull my e

equipment with .mostly

I'll keep you posted and I appreciate all the help and response

 

[MACKS]

I don’t think u can have the slacks adjusted for one inch of travel they will self adjust to just under two inches of travel,that’s the way mine always worked anyway..