I am having a brake issue in the '79 C30 ramp truck. I have gone through, quite literally, everything in the past year. New rotors, calipers, pads in the front; new drums, shoes, and wheel cylinders in the rear; all new SS lines, new proportioning valve, new MC, and rebuilt hydroboost.
I got in it the other day to stretch her legs and it feels like something is dragging. The brake light comes on when I push the pedal (I believe that happens when the prop valve is pushing more to one output than another). The brake pedal is also slow to return (but that may be unrelated, the return spring broke and I've been on the hunt for a new one, no luck so far).
I am going to take the wheels off this week or weekend to get another look. If anyone has any thoughts for what I might check for other than something sticking/binding, any help is appreciated. Will probably try to bleed the brakes to see if that helps...and then try to find where air got in if it does...
The primary functions of a BPC valve are metering (i.e. by initially limiting the flow of hydraulic fluid to the front circuit until the rear brakes can make frictional contact with the rotating assemblies and the hydraulic pressure in the rear circuit begins to rise).
Once that design pressure in the rear circuit is met, the spool in the BPC valve overcomes spring pressure and shifts -allowing flow to the front brakes. Also at that point, the BPC valve becomes a proportioning valve and it's primary function is to limit flow pressure to the rear circuit. During periods of heavy deceleration, a certain percentage of the vehicle's weight that is normally carried by the rear wheels shifts to the front wheels. The resultant loss of downward force at the rear tires makes them more apt to lose traction with the road surface and skid/fishtail. But, because the proportioning valve is limiting the degree of hydraulic pressure in the rear circuit, the tires can continue to roll on the road.
And, without sufficient back pressure in either circuit, it also provides you with a brake failure warning light.
So, when you mention the following, it might contain some important info:
The brake light comes on when I push the pedal (I believe that happens when the prop valve is pushing more to one output than another). The brake pedal is also slow to return (but that may be unrelated, the return spring broke and I've been on the hunt for a new one, no luck so far).
1. Your BPC valve is new and I assume your plumbing is tight
2. Your rears were recently rebuilt (albeit without a return spring). Having only one return spring is really the same as having zero return springs.
3. Also, due to the above, your star adjuster on that side is probably useless. The drum/shoe clearance on the initial application is likely way over limit. It is re-established with the first stroke of the master cylinder, but that brings up another problem.
4. Weak or broken return springs: Proportioning valves maintain a residual pressure on the
drum brake system after the brake pedal has been released. If the return springs are too weak to overcome the residual pressure in the system, the shoes will drag against the drum. Your initial complaint.
So, when you first apply the brakes, three (bad) things are happening. The normal drum/shoe clearance has to be re-established every time (star adjuster).
And, because there is no tension being exerted on the wheel cylinder pistons by the return spring system, the necessary back pressure - that helps the spool stay centered within the BPC valve body - is slow to develop. Consequently, the spool shifts way over and trips the warning switch.
Oh, and without return springs, the shoes will tend to stay out against the drum - and cause a dragging sensation.
I might be totally wrong, but if it were me, I would start my search for trouble at the rear brake - the one that is missing a return spring.
