Weak brakes on the drive axle

My truck too, i cant stop for #### when bobtailing. i got used to it though and its not a big deal. stops awesome with a trailer but i hardly use brakes anyways.

 

I had an FLD that wouldn't want to stop when bobtailing, sometimes- scary feeling to put 200 lbs of pressure on the pedal and hope it will slow down a little.

You said the proportioning valve was replaced- if it was hooked up backwards in the first place it would cut braking power to the drives. It's supposed to cut braking power to the steers when bobtailing. So, if it was hooked up backwards before it was replaced it might still be hooked up backwards- most techs would unhook and switch one air line at a time from the old unit to the new unit- to make sure it was hooked up just like the old valve.

My best guess is that the lines are backwards, braking power should be reduced in the steers, not the drives if it's hooked up correctly.

 

this is how mine works, i push, push harder, then push even harder and all of a sudden, im peeling my face from the dash . its weird, its like its weak as hell and then all of a sudden at a certain pressure, it just freaking locks up, lol.

 

Faulty Foot valves can act that way, I've seen quite a few.

 

you all guys do all the wrong way!

first of all identify LCV, it may be Wabco, Knorr/Bendix, Haldex...

All they are different. So the quation is what is the suspention? air ore leafsprings?

Step #2 - some LSV ajust rear end brake pressure, some all the axels (MAN F2000 for example). According the law truck makers put stickers with LSV charaterisics on truck. use this info to ajust LSV

 

Well, what do you x-pect??? We're Mericans! We just bypass stuff and drive it like we stole it! Lol.....We don't know all about all that your a pee'n stuff. Give an ole boy a break! Lol.....

 

according the law you cant modify brakes, and it is not all, read carefully DOT and ECE directives and you see that LSV may not be required if your truck got ABS. not big rigs only got LSV, but pic up trucks and cars. In 90-s many car and truck makers removed rear LSV. But I wanna remind you that bobtailing w/o LSV and w/a ABS is dangerouse, you hit brake and rear axe sway redirect your truck in any direction with rear wheels locked and front keeping rotation. It is especially dangerouse on 4x2 trucktors.

for highway wehichles automatic LSV is approved only (manual LSV was legal till 80-s, but now it is used in agricultural application only).

LSV got two bolts to ajust loaded and non laden brake pressure.

Ok, some pics
LSV for boogie axe


LSV for air suspention

Ther are ports for supply, control and air bags pressure and output pressure port, line from output port runs to rear axel air relay.

BTW, in 90-s some EU truck makers used LSV with ABS. I'd say it is usefull thing for trucks used like empty-fully laden- empty))) no exessive tire wear. But now in EU all new trucks got EBS and all this old stuff is not used now

 

We've always used a form of ABS of sorts, it's the rapid on/off stabbing of the brake pedal with the right foot.

 

Yep, old school, I'd say it is one channel ABS. I sugest you can do all better the 4s/4m ABS if you have four brake pedals and four legs))))

OMG, trailer ABS/EBS is 2S/2M, so we need six pedals, and octopus driver))) LOL