i have a 97 2500, and every morning and a lot of times when i lightly hit my brakes they lock up the rear brakes. My shoes and drums are good but i heard the proportioning valve may be causeing this if its bad. Would my truck have one? and if so where is it and what does it look like?
thanks mitch@jc

 

Definition of prop valve: The valve allows normal fluid flow during moderate braking. The valve only controls fluid flow during high decelerations brake (read heavy pedal) stops, when a percentage of rear weight is transfered to the front wheels.
this means the prop valve does nothing during light brake applications but transfer normal pressure to rear brakes.
That said, there is a metering valve that holds front caliper operation till rear shoes are expanded and touching drums. It is below MC and part of Pres diff switch. This should not be problem. Grabbing may be result of inadequate rear brake adjustment and is very common. May also be due to lining contiimation.
Locations: Height sensing prop valve if installed is located left side rear axle.
Metering valve is located below MC with pres diff switch. Residual check valve is built into fitting on front section of MC, if equiped.
Hope this info helps.

 

If you live in a wet or humid area the rear drums can flash rust overnight and cause lockup until the rust is worn away.
Only cure I know of is to be aware of it and use your brakes in a noncritical place every morning to clean them up.
If it's only doing it in the morning highly doubt anything mechanical is wrong.

 

My truck does exactly the same thing, if I breathe on the pedal the rears, when cold, lock right up. I ride the pedal for a bit to heat up the drums and dry them out, but the lock up symptom will return a few times during the day if it remains cold and wet. I don't like it, it has been quite scary a few times.