Unusual brake problems - RoadBoss

Hi again all, wishing to call upon your vast experience for a problem that has existed for years now...

My '79 Road Boss is a well maintained vehicle, always greased and brakes adjusted each 4 weeks (about 3500km), bla, bla...

When the vehicle is cold, the brakes are fine - well balanced, and work perfectly.

As the vehicle warms up OR I do more loads, I find that the front brakes provide more braking effort than the rears - you can "feel" that the front brakes are working harder. It is like they are doing too much work.

When stopping with a load or even empty when it is hot (meaning the truck has been working all day) it does not pull to one side or the other, and both front brakes are of the same temperature (yes, you can put your hands on the spider, but not the drum). There is no lock up, but obviously when empty and in an emergency stop the backs will lock.

I feel that possibly there may be a fault in the brake treddle valve that only occurs when hot, but that really doesn't make sense although the brake treddle valve is only a few inches (8 or so) from the turbo outlet. Could heat be causing a problem here????

Also, as the front brakes are air pressure activated, whereas the rears are air pressure drop/release activated, I wonder if there is a problem with too much air pressure being applied to the front. My air pressure is most always at 120lb (according to the dash gauge) and the compressor cuts in at 105lb to build it back up to 120lb again.

As a result of this heavier braking effort on the front, my front tyres become "square" or scalloped, unbalanced, and bloody uncomfortable to drive at 100km/h; and I am lucky to get 12 months out of a front set.

I am up for a new front pair again (last pair went on Last November, have done 34,000km since then) but before I put another pair on I feel I need to get to the bottom of this problem.

Have you ever seen/heard of this type of thing?
cheers again
Bretto

It might just be to do with the material of the shoes. Years ago I had a Mitsubishi truck, 5 tonner with air over hydraulic brakes that worked well. Then I got the back brakes relined with new "non asbestos" stuff. These would drag when hot as the shoes were expanding more than the drum. You had to set the adjustment well off. Front brakes (probably asbestos) stayed in adjustment either hot or cold. The truck stopped alright but I suspect the front ones were doing most of it.

TRB Man
GuddayM8
U might have to refresh me memory
How many HP?
Jakes Y/N
towing Trailer Y/N

the main aim is to keep the brakes as cold as possable
Heavy braking will generate heat n the drums expand n th@'z when U get brake fade

When approaching a Green traffic light U kno wot the next color iz gunna B
So Back off early unless U R 123% sure the lights R in your favor
Approach intersection @ a speed in a gear th@ enables U to accelerate or poke it down a gear to stop when brakeing
Guessin approach @ 1800rpm or 1500 RPM do skip shift down n make full use of Jakes (I hope U got)
LOL
the operating pressure of the compressor n tanks = 120psi
normall cut in 90psi
the brakes only use about 60psi under max brakeing
the reason being to leave sum in reserve
this is to keep a minium controlled brakeing pressure
U may have to drive a bit slower when loaded n take in account of the terrain down hill takes a lot more brakeing than up hill brakeing
But U already kno th@
the lower the gear the more retardation from the jakes

Rule of thumb
Never go down a hill faster than wot U'd pull the same load going the other way (UpHill)
LOL
Cya
[ch9787]

Bretto, early Road Boss's, like yours and mine have single curcuit brakes, later ones had dual curcuit with the pendulum pedal. There isn't much to replace or go wrong. If you go to Air Brake Services on Beatty rd at Archerfield you can get a new treadle valve for about $80, and a new relay valve for the same dollars. If the treadle valve is cactus you will have an air leak at the exhaust which you'll probobly hear with the motor off. If air leaks from the exhaust on the relay valve with the park brake on it is cactus, if it leaks from the exhaust with the park brake off it is the seals in betweenthe 2 chambers on the maxi brakes. The hotter brakes get, the less effective they are. Best bet is not to use them.

Did you fix up the king pins and shackle bush's. Rocklea truck parts have chinese steer tyres for about $250 each, I think Samios is similar.

ways to alter pressure acting on shoes 1) change size of air booster or 2) If there is a choice of holes in S cam lever move the push rod up or down a hole

Hi guys, in order of response:
Atkipete: You know, this problem actually became apparent after I had replaced the front shoes two years ago. They are the non-asbestos ones in the black box with red writing. Maybe that is as simple as it is?...
Swishy : Yep, copy that, no trailer, jakes, works all good cold but as the day goes on they get worse - even one pound of pressure on the brake pedal and the thing feels like it is falling over itself stopping.
Cam: I don't seem to have any air leaks from that area at all. I must admit that I am thinking of taking a drive from here (Helensvale) to Air Brake Services on Wednesday because, as you say, $80 for a new treddle valve is cheap if that fixes it. Did the king pins, but the shackle bushes are still on the to-do list. Yep, Samios will get the nod for some chinese steers. Interesting that on the drive, TR688 Triangles are bald at 32,000km but the recaps in front of them are almost gone but they have 73,000km on them. Hmm, Triangle = chinese for "Soff lubber".
Mammoth: this already has small boosters, and the pins are in the top holes (least amount of movement).

Suggested prognosis from what you guys have suggested: I think that I might change my brake adjusting techniques from tightening and then back off 1/2 a turn to tighten and back off 1 turn - therefore leaving plenty of movement. Try that, then maybe a drive to Archerfield for a treddle valve, fit it, then try that.

Whilst under the old chook today, I find that the left lead bogie brake material is almost worn out but the right lead bogie is like new; me thinks somefink not right there either but one thing at a time I say.
cheers guys - keep ya posted
Bretto

TRB Man

2nd Rule of Thumb

Donot switch Jake Brakes on till the engine has reached operating tempreture

If U got jakes or simular

Also
(U gotta have an also)

the lower the gear = the more reatardation from Jakes

LOL
Cya
[ch9787]

Bretto. Sounds like you are almost onto it -airlines to rear might be restricting air and certainly there is a lazy booster. Another thought is that those fronts have symptoms of being leading shoe type(i didn't think they used them on trucks), which have a self-servo effect in forward direction but near useless in opposite direction. Problem is if there is no taper or wrong material on the lining they can grab.

Easy to find out if you have the same problem as mine. Jack the wheel up when the brakes are hot and see if they are dragging.

Guys, I reckon that I have cured the front problem by doing my adjustments differently (do up tight, and back off 3/4 turn).

But there is still a grabbing brake somewhere else...

On inspection, I find that the left front bogie brakes are near worn out yet the right front bogie is like new, and so is the rear bogie like new. So, I suspect that much of the braking effort is being done by possibly the left front bogie. Interestingly, that one had a new booster on it a few months back and the shoes were like new back then. I have checked the airlines today to make sure the right hose goes to the right port on the booster, and they do. Funny though, the tyres on that bogie are not worn any more than any others - maybe the braking effort is on those brakes, which is then transferred through the drive shafts to all the other tyres and therefore that is why the tyres are OK.

What would make one bogie wheel do all the braking???? Hmmm, any ideas??
Bretto