Brake problem

[junkyardpartseeker]

Having problem somewhere in my brake system. My parking light on my dash sometimes comes on and sometimes stays on. My pedal is also feeling like there might be air in the lines. I checked all pads and they are good. Also bleed all brakes, no change. Could this be telling me my booster is going out?

[CT-Truckster]

i would guess master cylinder. try holding your brakes and see if it feels like the pedal slowly goes down toward the floor board.

[RockHQ]

Probably caused by the brake proportioning valve, my blazer did it for some months before i got really interested in it and did something. Turned out rear brake cylinders leaked and the master cylinder reservoir ran out of brake fluid which caused the proportioning valve to trigger the light since the proportioning didn't work because of air in the rear circuit. The proportioning valve can also stick and cause the light to come on.
3rd possibility is that your brake shoes aren't set close enough to the drum, so that the brake pistons have to move too far out before they meet the drum. That would also trigger the light.

[junkyardpartseeker]

I put onnew cylinders 4 months ago and new shoes. They all look fine. What is this brake proportioning valve and where do I find it?

[apstguy]

It is under the hood. Follow the brake lines from the master cylinder to a valve that has 4 brake lines and a wire coming from it. It is below the fan shroud. It keeps the front to rear ballence of braking power to a set amount. Correct me if I am wrong but our trucks are set at 70% front 30% rear. Something around there!

[junkyardpartseeker]

How do I test it, I checked hose at booster and there is vacuum.

[apstguy]

Sometimes you have to "reset" that switch if bleeding the brakes was done wrong or other misc. strange things. Here is the snippit from my Chilton's manuel:

Centering the switch:
_________________

Whenever work on the brake system is done it is possible that the brake warning light will come on and refuse to go off when the work is finished. In this event, the switch must be centered.

1. Raise and support the truck.

2. Attatch a bleeder hose to the rear brake bleed screw and immerse the other end of the hose in a jar of clean brake fluid.

3. Be sure the master cylinder is full

4. When bleeding the brakes, the pin in the end of the metering portion of the combination valve must be held in the open position (with the tool described in the brake bleeding section installed inder the pin mounting bolt). Be sure to tighten the bolt after removing the tool.

5. Turn the ignition key ON. Open the bleed screw while an assistant applies heavy pressure on the brake pedal. The warning lamp shoul light. Close the bleed screw beforethe helper releases the pedal.

To reset the switch, apply heavy pressure to the pedal. This will apply hydraulic pressure to the switch which will re-center it.

6. Repeat step 5 for the front bleed screw.

7. Turn the ignition off and lower the truck.

>>> If the warning lamp does not light during step 5, the switch is defective and must be replaced.

[junkyardpartseeker]

I replaced all brakes approx 4-6 months ago. The warning light coming on and it feeling like air in my lines by pushing on the pedal has only been happening this past week. Yesterday, I checked wheel cylinders, pads, shoes calipers, and all lines. I noticed no leakes. Today, I took off the vacuum hose from the back of the carb to booster and there was vacuum present.