brake proportioning valve malfunction?

[JohnJ2]

I only have front brakes on my '83 K1500. The rears only grip if I mash down hard on the pedal. If I understand it correctly, this is the opposite of the way it should work; I.E. I should get all four grabbing from the start, then at some point the rears reach their limit. It is no fun driving on slippery roads with a 600 pound load in the back.

When I first noticed this, I replaced the master cyl without thinking much. All four wheels bled normally, but there was no change in the way things worked.

Has anybody seen a case like this? Is there anything else I should be looking at?

Thanks for your comments

[VileZambonie]

The 1st thing to do is check your brakes. Once your brakes are all ok check the ride height proportioning valve. If ok move onto the master cylinder and combination valve. Check for pressure at the wheel cylinders and move forward.

[JohnJ2]

Thanks, VileZambonie. The linings are good, but I am not sure I checked adjustment. I must do that next.

You mentioned the "ride height proportioning valve" and " combination valve". I am unfamiliar with those terms. Do I have them on my truck? Can you explain them a little more?

[VileZambonie]

The pix don't show up but I can email you this document if you want it with the pix.

Number: 83-T-61

Section: V

Date: July, 1983
Subject: REVISED REAR SUSPENSION ON VEHICLES WITH HEIGHT SENSING PROPORTIONING BRAKE SYSTEMS - 1982-83 "C/K" 30
TO: ALL CHEVROLET DEALERS


1982-83 "C/K" 30 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 "fine tunes" 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 complains 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:
FIGURE #1 - REAR SUSPENSION


 

1. Detach rear brake hose from lever and bracket assembly. Reference Figure #1.

2. Remove lever assembly and bracket from axle.
FIGURE #2 - DIFFERENTIAL


 

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 #2) in the upper cover hole. Reference Figure #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 (PIN 14055557 -- longer bolt removed in Step #2) through spacer P/N 14055556 (removed in Step #2) in remaining axle cover attaching hole. Use Loctite 75 or equivalent on bolt threads.
FIGURE #1 - REAR SUSPENSION


 

6. Disconnect brake pipes from height sensing valve and brake hose, and discard short brake pipe. Reference Figure #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. Remove the brake combination valve per section 5 of the Light Duty Shop Manual.

10. Select and install a replacement combination valve, based on the vehicle wheelbase and type of rear wheels, from the chart below:

       Valve Part
       Wheelbase                Rear Wheel Type                Valve Code          Number

       131/135                  Single                         DD                  1257200
       131/135                  Dual                           EE                  1257203
       159/164                   Single                                            1257203
       159/164                   Dual                           P                  1257087


11. Bleed and test brake system per Section 5 of the Light Duty Shop Manual.

ALSO

These vehicles use either a two function or three function combination valve. Some vehicles use a height sensitive proportioning valve in addition to the combination valve.
The two function combination valve consists of a metering valve and a brake failure warning switch. The hydraulic brake lines are routed through this valve to the wheel cylinders or calipers. The metering portion of this valve assists in providing balanced front to rear braking by delaying full hydraulic fluid pressure to the front disc brakes until the rear drum brakes overcome return spring tension and the linings contact the drums. The brake failure warning switch portion of the valve activates the brake warning lamp when there is a loss of pressure in either the front or the rear braking system.
The three function combination valve consists of a metering valve, a brake failure warning switch, and a proportioning valve. The metering valve and brake failure warning switch operate the same as those in the two function combination valve previously described.
The proportioning section of the valve proportions outlet pressure to the rear brakes after a certain rear input pressure has been reached, preventing rear wheel lock-up.
The height sensitive proportioning valve provides ideal brake balance according to weight at rear axle. This valve is mounted on the frame and responds to changes in vehicle trim height in relation to rear axle load. Mechanical linkage connects the valve to a bracket attached to the rear axle.

[JohnJ2]

Thanks again. From the description I should easily be able to see if I have the height-sensitive valve. Time to jack it up...

[DnStClr]

Do the 1500's  have the ride height proportioning valve?

Don St.Clair
Culleoka Tn

[jeffm5150]

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 complains 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:

Thanks for posting this great information.  I have an '87 V20 4WD 3/4T and have fought poor rear brake performance ever since I replaced the drums with a TSM rear disc conversion.  I've never been able to find information about the 'ride height proportioning valve' but figured that's what it was when I saw it mounted between the axle housing and chassis.  Your posting references '82-'83 model years.  Any idea where I'd be able to find similar '87 model year information?

Thanks again!
Jeff

[Da67goatman]

I removed the height sensing components on my burb and just connected the lines and now my rear brakes are the first to lock up, just the way I like it.  For the mear $5 or so in brake fittings, I feel its the best upgrade I've done.

[jeffm5150]

I thought about doing that first.  I will try that and install an aftermarket adjustable prop valve if I don't like the rear lockup.  Thanks for the reply.

Cheers,
Jeff

[VileZambonie]

The 87 was the same.