My brake light came on the other day and I cant figure out why. I was driving down the road when it came on. Now my ujoints are totally shot. So I have alot of vibrations. Not sure if something wiggled loose or not. I took the connector off the porlortioning valve and the light turned off. But im not sure what else that connector does so I plugged it back in. If I turn my key to Acc and pump the brakes they hold firm. While holding I start the truck and the brake pedel drops a little bit. I assume thats normal? Brake pedal does seem a little soft but both resivours are full of brake fluid. Should I just take the valve off and clean it?

 

Why are you thinking it's the portioning valve?

There are quite a few things to take into consideration regarding your brake system.
How's the overall health of the system..when's the last time you serviced the system, bled the brakes/changed fluid, pads/shoes, etc?

What's the condition of the vacuum line from the manifold to the booster?

Typically, a spongy feeling pedal would indicate air in the brake lines, bleed the system.

Faulty hoses could be an issue...any leaks?

How's the master cylinder mounting..is it nice and tight?

Lastly, it could be your master cylinder...any leaks?

 

I think the proportioning valve because my cluster brake light is on. Which is triggered by the proportioning valve when it senses low pressure correct? If not then what else should I look in to?

Master cylinder, wheel cylinders, pads, shoes, hardware, calipers, and lines are all brand new. Replaced them a few months back. The brake light just happened past week. My truck vibrates like crazy due to bad ujoints. Which is why I suspect something came loose to trigger the light.

Also changed the pcv valve and line from the manifold to the booster when I did the brakes awhile back. As I said my brake resivours are as full as the day I did the brakes.

 

The only thing that will trigger the light, other than a defect in the warning light, is a pressure differential between the front and rear brake systems.
If there are no visible leaks in the system and the master cylinder is full, check the adjustment of the rear drum brakes. If that checks out okay then I would suspect the master cylinder.
The warning light itself is an independent circuit and doesn't do anything else.
Try resetting the light, this will tell you whether the problem is the front or rear system.
Crack open a bleeder valve on the front and, with the ignition on, slowly press the brake pedal until either the light goes out or the pedal bottoms out. If the pedal bottoms out, close the bleeder and do the same on the rear. If the light goes out, close the bleeder. This will tell you that the problem is in the rear brake system.
When you're pressing the brake pedal, stop as soon as the light goes out. If you continue to push, the light will come back on.