Driving along in my truck then all of a sudden I went to hit the brakes and the petal went almost all the way to the floor and the abs and brake light came on. I hit the brakes once and then hit them again instantly and the petal goes right where it should and the two lights go out, and the petal will hold pressure right there without going down. My question is, could it be the abs thing or my master cylinder all of a sudden junked out? I was planning on pulling a gooseneck trailer tomorrow and will need the truck. Thanks for any help.

 

Possible master cylinder problem, but I would try adjusting the rear brakes first.

 

What I don't get is it instantly happened. I've never noticed any problems with the brakes before until now. Now the abs module in the rear of the truck located in the frame rail, could that allow brake fluid to not be applied the rear wheels or does that part not have any effect on anything. My other thought is that maybe the cold temps (0* F) are having an effect on it but it was working before in the chilled temperatures.

 

The whole purpose of the valve in the rear frame rail is to block flow to the rear brakes when pressures get high enough to lock the rear brakes. But, this would cause a harder brake pedal, as only fluid is going to the front. As the shoes get farther from the drums, it takes more fluid to take up the slack and may be exceeding the master cylinder capacity for a single stroke. Since the shoes have no presssure against them, total system pressure drops activating the brake warning system. The second stroke is happening before all the fluid returns, allowing the master cylinder to catch up and you get a good pedal. Cold weather makes everything work hard, but is probably not the root cause.