2000 mack rd688 brake help

[Moparmike]

ok had a driver slam brakes in front of me so I had to slam slide duck everything

 

so now my brakes don't feel the same  takes longer to stop if I can when loaded

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

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

driver side rear front axle does nothing

brake chamber doest move s cam not moving so I figure chamber 

put new chamber Samething not moving 

there's air on both lines 

where or what should I look at its 2 different valves in line I guess 1 is abs with a plug the other is big valve 

 

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

any help is appreciated

thank you 

[Joey Mack]

air the lines switched? the chambers are not isolated on the service side.  one valve should be sending service air to the service side of all of the brake chambers..  

[mechohaulic]

did you check brake lining???  heavy slam on brakes with worn lining there is a possibility the cam jumped over . or no movement due to cam standing straight up on shoes won't return . back the slack adjuster off far as possible ; might take a little effort. hit or miss parts changing gets expensive 

[mechohaulic]

"driver side  rear front " stated  would indicate tandem axle , might have a service valve per axle based on being a 2000?? can't recall excessive brake pressure blowing out a service valve.  have seen excessive pressure locking up/slowing down brake application when a cam stands up or jumps the shoe rollers and jams.

[mechohaulic]

does a 2000 still have removable backing plates OR any at all?? block the unit by tires NOT only in gear; jack up the problem wheel . brakes off /backing plate removed , check wheel operation manually spin wheel; set brakes /check wheel then release, you got the idea ... remove clevis pin in slack work slack manual  while someone else spins the wheel ; what happens . 

[mechohaulic]

reread original;; new chamber installed, if the push rod on chamber had to be cut ;;;; where measurements take with BOTH CANS in same mode==fully extended or compressed. have seen chambers cut when one is measured released (still on truck) new one measured (no air) extended. could be the new noise. and explain no movement on new can. shaft tooo long . i still go with jumped cam. 

[mechohaulic]

28 minutes ago, Joey Mack said:

air the lines switched? the chambers are not isolated on the service side.  one valve should be sending service air to the service side of all of the brake chambers..  

don't know Jojo seems to be in NC ;; you up for a road call ???🤣😂😱.past my bed time , I can't go LMAO

[JoeH]

I'm wondering about S cams going over-center.  If any brakes were close to being out of adjustment then a real hard brake application can push them far enough that the tips of the S go passed the rollers. 

On all my trucks I release my brakes, put a zip tie on the pushrod right against the air chamber, then apply the parking/trolley brakes.  Measure the travel of the zip tie from the reference face of the air chamber, more than 1-3/4" travel is a red flag, and that wheel needs to be adjusted. That's my redneck check my brakes myself method when I don't have a second person to work the pedal.

It is also possible to bend the bracket the brake chamber bolts to.  Its a replaceable plate, you have to pull S Cam and then it's 4 bolts around the tube the S Cam goes through.

Edited December 6, 2023 by JoeH

[JoeH]

You could also have broken friction material off the shoes, or cracked a brake drum.  When the brake drums get worn out and thinner they're easier to blow apart. But I'd expect that carnage would be obvious.

[Moparmike]

3 hours ago, Joey Mack said:

air the lines switched? the chambers are not isolated on the service side.  one valve should be sending service air to the service side of all of the brake chambers..  

yes sir lines are hooked up correctly

[Moparmike]

1 hour ago, JoeH said:

You could also have broken friction material off the shoes, or cracked a brake drum.  When the brake drums get worn out and thinner they're easier to blow apart. But I'd expect that carnage would be obvious.

yes took drum off to verify brake are more  then half of new and I adjusted slack all the way around to confirm its not stuck or binding 

[Moparmike]

3 hours ago, mechohaulic said:

"driver side  rear front " stated  would indicate tandem axle , might have a service valve per axle based on being a 2000?? can't recall excessive brake pressure blowing out a service valve.  have seen excessive pressure locking up/slowing down brake application when a cam stands up or jumps the shoe rollers and jams.

brake pedal don't seem to have the pressure it use to have 

it goes almost to floor and not stopping

I was wondering about a pressure valve 

[mrsmackpaul]

If the brake pedal is going to the floor and your not stopping properly it sounds like your brakes are out of adjustment 

Now for old school junk like I have the slack adjuster should  be set at right angles to the chamber rod

Then adjust the brakes up

The rod shouldn't moce past 90° on full pressure

It sounds like (as others have said) you have jumped a S cam

Pull the slack adjusters off and soak in diesel and bung them in vice and put a rattle gun and work them round and then soak a bit longer

You will be surprised at just how dry old grease will be 

Slack adjusters rarely need changing unless the locking part is stripped

Hook shop air up and give full allocations and watch and listen 

Wedge a bit of wood between the seat or steering wheel so you can hold the full application and walk about and listen 

Some ideas

 

Paul

[JoeH]

Do you have access to a helper? You need to have someone in the cab working the brake while you are under the truck.  With wheels chocked of course!

[JoeH]

Some truck manufacturers install a pressure gauge to tell you how much brake pressure you're applying. You can put a gauge on your system so you can see if you're getting good pressure through the foot valve. I suppose it's possible you hurt the foot valve, but I doubt it. I assume the truck has self adjusters? I think you need to be under the truck while someone else works the pedal to see if all the slack adjusters are functioning properly.  Maybe you ripped one of them up internally.

Does the truck pull to one side or the other when stopping? The brakes would be working better on the side it pulls towards.

Have you run through the pre trip air system check to listen for air leaks? Maybe you ruptured an air chamber.

[mechohaulic]

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 ?? 

[mechohaulic]

bad diaphram was a thought but earlier post states brake chamber replaced on LFR; not saying another can isn't leaking .possibly the (R)  valve for that axle gummed up hard brake application fouled it up; but that would screw up both wheels.

[Joey Mack]

How about a knot in the hose created during the repair?? 

[Moparmike]

23 minutes 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 ?? 

floor mount

[Moparmike]

5 minutes ago, Joey Mack said:

How about a knot in the hose created during the repair?? 

everything is straight on hoses

 

you ready for me bring it to you mr Joey  👀

[JoeH]

If the brakes aren't working I'd bring joey to the truck lol!

[Joey Mack]

I would if I still ran my service truck.. I have a cushy city job now...  I am currently waiting an hour and a half for the garage to come jump start our crane, because I am not allowed to put jumper cables on it..  I'm going nuts,  I used to hustle and get trucks back in the road in short time..

[Joey Mack]

'R' cabs have the treadal valve in the floor, right where crap from your boots can get in there and jam things up..  it's odd how only 1 brake is not working..  is the clevis pin rusted solid? Did you have it off the slack adjuster, or did you just screw the camber on to the clevis while it was still attached..?  Very strange....

[Moparmike]

2 hours ago, Joey Mack said:

'R' cabs have the treadal valve in the floor, right where crap from your boots can get in there and jam things up..  it's odd how only 1 brake is not working..  is the clevis pin rusted solid? Did you have it off the slack adjuster, or did you just screw the camber on to the clevis while it was still attached..?  Very strange....

I remove clevis pins 

everything is free

I believe all brakes are not getting the pressure that they should but I could be wrong

the foot valve under the floor makes a noise when you press the brakes not sure if it's been there are not

 

I'll get a picture of it later today 

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 

but when empty the truck stops still not very good 

but at 70000 lbs I'm afraid that someone else will slam brakes 

[Joey Mack]

How about removing the service line on the bad chamber and pressing the brake pedal to see if a strong rush if air comes out..