Do I need the rear brake load sensor ?

[acesneights1]

I seem to remember tossing this on my old 83 K20 civvy. This one is shot and I have no idea where to get one or what it is even called. I am pretty sure the pre 81 trucks had the same braking system but without this thing.

 

[firefox]

I believe there was a thread recently about someone having problems
due to this being broken. People were giving him the finger because it appeared he was
hot rodding it or some such.

 

[jeeping88]

It's a brake load sensor/valve. Late 80's jeep truck had em. They increase braking when truck is loaded

 

[doghead]

Positive identification(including a GM part number) can me made by simply using the part TMs.

 

[Recovry4x4]

There are 2 related threads at the bottom of this thread that can be of assistance.

 

[Chief_919]

GM part # 15549248

 

[acesneights1]

Thanks for the replies. I read the threads at the bottom and pretty much what I remembered from my 83. I just got rid of it.
The pre 81 trucks did not have them and they worked just fine. Being that mine is lifted It probably won't be exactly right geometically anyway. Adios. It's sitting in the file next to my 6.5TD PMD...in the trash can where it belongs.

 

[doghead]

My 77 3/4 ton has one, iirc.

 

[acesneights1]

My 74 didn't. I cut up alot of mid to late 70's trucks and don't remember seeing any. Wonder what year they started. I had though after the bodystyle change in 1980 but I coild be wrong.

 

[Gunzy]

I believe they were part of a towing or camper package. I used to see a lot of problems with them reducing rear braking to almost nothing. A lot of people did just remove them and seemed to have better braking after. Of course if there is nothing in the bed (no load) and you have a panic stop you may find the rear end coming around. I had old Fords and they never used them and were easy to lock up the rear brakes. Just train yourself to modulate the brakes and you are fine.

 

[Recovry4x4]

I stripped the one off of my wrecker project. Plan on adding an adjustable with cab control as I've done in the past.

 

[Skinny]

If you plan on actually using your truck as a cargo mover, than I would probably keep it or install a manual one that was mentioned. I think I bought one off Jegs or Summit the last time I did it. Sure you can get away without it but dialing in your rear drums (especially drums since they tend to be on or off) is nice going from completely empty to fully freighted. Not a whole ton of money either IIRC.

 

[Recovry4x4]

As Skinny said, it helps with dialing it in. After a while you get a feel for where it works best. I could dial mine back in the rain to assist with rear wheel lockup.

 

[Sharecropper]

I seem to remember tossing this on my old 83 K20 civvy. This one is shot and I have no idea where to get one or what it is even called. I am pretty sure the pre 81 trucks had the same braking system but without this thing.

How do you know your's is "shot"? If it is adjusted correctly it should work properly, that is to say, it will increase the braking power to the rear wheels as weight is added to the bed of the truck. I provided a detailed explanation with photos on how to correctly adjust the valve, and also how to fabricate a longer actuating arm for lifted trucks, in my rebuild thread.

 

[Skinny]

How do you know your's is "shot"?

I think you could only arrive at this diagnosis if you pulled the drums, inspected the rear brakes, made adjustements, and verified with a road test. If your base brakes are not operating correctly, you can't blame it on the valve. I've seen lots of guys with Toyota bend them or make adjustments yet the parking brakes are dragging, the shoes are way out of adjustment, leaky gear oil, etc.

I don't think you really need to eliminate it even if you are going offroad unless you have some real big wheel travel out back. Then you may want to put a manual valve in. I did take it out on my crawler after a disc brake conversion, tube bed, and bigger tires. Nothing was stock and the rear would lock up because the valve was pretty wonky at that point.

 

[NPD732]

Here is a GM TSB on the subject:


GMC NUMBER: 88-T-151

GROUP: 5 - Brakes

DATE: August, 1988

CORPORATE NUMBER: 865010R
SUBJECT: REAR BRAKE SENSITIVITY - HEIGHT SENSING VALVE REMOVAL

MODELS: 1984-86 C/K 3500 AND 1987-88 R/V 3500

THIS BULLETIN IS A REVISION OF TRUCK SERVICE BULLETIN 85-B-79, DATED MAY, 1985, UPDATING FOR MODEL YEARS 1984-1988.
1984-1986 C/K 3500 and 1987-1988 R/V 3500 series trucks are equipped with a rear suspension height sensing device that optimizes the brake proportioning valve setting for the load the vehicle is carrying. The height sensing system adjusts the brake balance in vehicles subjected to a range of loading conditions. Occasionally, vehicle modifications by body builders or owners influence the height sensing device. If a vehicle has had rear suspension modifications that affect its trim height or the spring rate, and the driver comments about the brake modulation characteristics of the vehicle, it is recommended that the height sensing system be removed and the brake system be revised as described below:

Parts listed below are currently available from GMSPO.

1. Detach rear brake hose from lever and bracket assembly. Reference Figure No. 1.

2. Remove lever assembly and bracket from axle.

3. Install rear brake hose bracket (P/N 34897 and spacer (P/N 2355099) with a 3/8-16 x 1.38 bolt (P/N 358000 - shorter bolt removed in Step No. 2) in the upper cover hole. Reference Figure No. 2. Use Loctite 75 or equivalent on the bolt threads.

4. Attach brake hose to bracket with bolt from original installation.

5. Install a 3/8-16 x 1-3/4 bolt (P/N 9439637 - longer bolt removed in Step No. 2) through spacer P/N 14055556 (removed in Step No. 2) in remaining axle cover attaching hole. Use Loctite 75 on bolt threads or equivalent.

6. Disconnect brake pipes from height sensing valve and brake hose, and discard short brake pipe. Reference Figure No. 1.

7. Remove and discard height sensing valve, bracket, and bolts.

8. Carefully reposition the rear brake pipe, and connect it to the rear brake hose.

9. Bleed and test brake system per Section 5 of the appropriate Light Duty Service Manual.

Quote from a GM engineer who worked on the C/K platform:

By way of background it was placed there specifically as a "test beater" to pass the unladen portion of FMVSS 105 and FMVSS 208 brake tests.

We also used specially trained brake test drivers who we knew could achieve the best stopping distances, to help ensure passing the test.

Lastly, there was no limit to how many attempts we could make so we could also keep testing until we got the truck/configuration to pass.

I say all this to say: removing the valve should not be a problem. It was released to address this one (very specific) test situation and not for general use or customer pleasability.

Original link: http://67-72chevytrucks.com/vboard/showthread.php?t=582800

 

[Recovry4x4]

I've read that bulletin but couldn't find it for this thread. Had a friend that assembled wreckers and he always pitched them.