New member intro / front brakes may not be working...

[Longview]

Hello!

I am not new to the site, but I am a new member. I've not posted anything yet because I've typically found success simply searching for answers already spoken to regarding questions I have. I've fixed several fuel problems, replaced the radiator and another coolant leak, and a few other issues.

So, I do have a problem though. I have a 1986 M923A2 that I use for our tree company as a chip truck. I believe the front brakes are not helping to stop the truck much at all. I'll share my process of elimination and any advice you have would be greatly appreciated.

Problem identified:
1. In an emergency stop situation the rear brakes seem to want to lock up, but not the front.
2. When towing and with a load on the back, all 4 rear drums are very hot to the touch, but both front drums are lukewarm and I can hold my hand on them.

Process of elimination:
1. Brake pressure test completed- no drop in primary or secondary air pressure with engine off and holding brake pedal down over the course of 60 seconds.
2. Wheels removed, drums removed, adjusted shoes to make sure they are making contact with drums, and cleaned drums and pads.

No improvement in braking and the front drums are still lukewarm with the rears still very hot.

What should I check for next? I've read the TM and am on the step where I start testing the limiting valve, but I have not started that yet. Is there anything else that would cause the front brakes to not get enough pressure? Overall the braking feels pretty weak.

 

[Jbulach]

There was an MWO done that neutered the front brakes on these trucks in an “attempt” to make them “safer” to drive. I believe it was part of the ABS MWO if I remember correctly. I have plans at looking into changing mine back closer to a normal bias one of these days…

 

[Jbulach]

Also, welcome aboard! Just one thing, we need pictures of this truck, especially shots of her at work!

 

[Longview]

Thank you for the info, and if I had ABS the MWO states the Limiting valve (LQ2) would make the front brakes reduced to 67%….

But the plot thickens! (Suspenseful music)

I don’t believe my truck had the ABS MWO installed. No abs warning light, and no noises before startup to indicate an ABS system (like others describe for their trucks), no electrical wiring of any sort to any of my brakes…

So what should I check next for little to no braking force to the front wheels?

 

[Longview]

Ok, so did some testing and found the issue. The Q2 valve that normally gives people problems with not dumping the air after releasing the brake pedal (and thereby keeping the front brakes engaged) is also what is limiting the air supply TO my front brakes by a significant amount. This limiting factor is now gauge tested and applies in two ways:

1. Less important: Maximum pressure when really pressing the brakes hard reduces pressure from the brake valve from a little over 90psi to about 60psi.
2. More important: Medium pressure (the sort we typically use to slow the truck with a heavy load in a normal stopping situation) and less than medium pressure is being reduced by my Q2 valve to 0psi. Zero. So no wonder the front brakes aren’t helping stop the truck at all.

Basically, the front brakes would only work at all in an emergency stop.

Is there a way to adjust the Q2 valve to allow more pressure through? Are my findings similar to yours?

I think I’d like to exchange the valve for some substitute that will still allow a decent pressure through when only medium pedal pressure is applied.