Height-sensing Proportional Valve question;Differential mounting bolts

[quicky422]

It's a 1986 Chevrolet C20 350.  The rear axle has been swapped out, maybe for a 1-ton or greater but not sure.  I don't own the vehicle, it's my mom's, so I'm going by her info.

The rear passenger brake line broke at the rear t-valve. That valve bolts to the height-sensing proportioning valve linkage and that linkage attaches to the rear axle.  Well, the two bolts that fasten the linkage were missing, causing the steel brake line to stress and eventually brake, as far as I know?

So, I removed one of the top differential bolts and tried threading it into the holes where the linkage would attach.  I was able to get about a turn on them and they would give some good resistance, indicating a possible cross thread due to a difference in thread count.  I tried both holes and it didn't work!

I need the correct size bolts in order to attach the linkage as intended; they seem to be a different thread than the rest of the differential bolts.  As far as I know, the valve still functions, or at least it doesn't cause any problems!  It looks like it wasn't attached for some time but she hauls heavy round bails(sometimes two crammed in the bed and one towed!!) and now a bigger cattle trailer so I'm a bit worried about the trucks ability to stop and then this happens, a brake line fracture!

I had to do some searching to verify what that brake linkage was doing attached to the rear axle.  I guess more of that is aimed toward heavier vehicles and trucks and I'm sure it's just sensors monitored by the computer but that's my first encounter with this system!

Sorry, I tried to keep it brief but informative.  Thanks for any information!

[74 C-10 Shorty]

Sounds like you have a 1 ton rear end my 86 dually had one of those levers, there's a small tool  to get the right adjustment on those, I'll have to check I may still have that tool if I do you can have it... Bolt will be either course of fine thread, what ever you have use the opposite.

[bd]

Tech thread:  Height Sensing Proportioning Valves

Discussion thread:  Anyone know what these are for??

[74 C-10 Shorty]

And he needs to have one of these to adjust it properly or it won't work right, and I can't seem to find mine anywhere..

[Blazin]

All the diff cover bolts are the same thread. If yours are not someone has put the wrong bolt in at some point would be my guess. I would re tap them to match the rest. I don't recall exactly but i think they are 3/8 coarse thread.
My $.02, I would ditch the load sensing valve. They are for people that are not bright enough to change their braking habits when they have a load in the bed, or on the bumper, such as trailer tounge weight.
Also there is no computer sensing anything. Just a valve, open or open more with weight in truck.

[quicky422]

Thanks for the information!

For now I'll just get that brake line fixed and see if it operates the way it did before.  It looks like it's been disconnected from the axle for some time and she's never complained about the rear wheels locking up!  I thought I would try and secure that linkage and t-valve in hopes of preventing another line break at the least.

I'll mess around with the bolt sizes a little more when I fix the line and post back what I find.

I meant to say all the newer vehicles probably have sensors instead of a mechanical link.

Thanks again

[Irish_Alley]

new vehickles use the speed sensors for abs. but without it hooked up i would think it would be like having no weight on the bed since the unit would just be sitting there with no pressure on it.