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ok i finally got my truck running good last night, and everything was fine. I woke up this morning and started the truck and the brake light came on and the pedal went to the floor. I just replaced the master cylinder about 2 months ago. what would make it lose pressure and go all the way to the ground? any thing will help i have no clue. oh and it still has fluid in the resivoir so i have no idea.

Hold the brake pedal down while starting the engine. If the power brakes are working the brake pedal will go down somewhat further if the assist is working properly.

Or there may be air in the system that needs to be bled out. The light coming on means differential pressure between the two systems.

once i get it started it still goes to the floor though but it was fine last night. i dont know how it could have got air in the lines? what do you mean by when the brake light comes on its 2 different systems?

Sounds like you have a leak or an air bubble that was trapped made its way into the lines. Start at the MC and check the lines, look for leakage all the way down to around the calipers and the wheel cylinders in the rear. If you don't see any leaks, bleed the system. The MC could have failed again too, its not unheardoff for new parts to fail early. I'm guessing you have a leaking wheel cylinder or a bunch of air in the lines somewhere though.

The light comes on when the pressure in the front and rear is different by too much.

Was the replacement master cylinder a remanufacturered or brand new one? Sometimes remanufactured isn't remanufactured enough. The switch that turns on the brake warning light is in between the brake lines going to left front/right rear & right front/left rear. It acts like a single pole, double throw, center off switch. When the brake pressure in both sides are the same, it stays in the center off position. If pressure in one side is too low, the higher pressure in the good side turnes the light on, warning you of a problem, like air in the system.

so i could just have air in my line? also what would be the proper procedure to bleed the brakes? like what tire do i start with and so on and which one do i end on. I guess i could try bleeding the brakes and see if it fixes the problem. and for the master cyl. i have no idea, it didn't come with new resivoirs i know that. my guess is that it is a reman.

Wait, you've replaced the master cylinder and did not bleed the brakes? If that is the case you would have air in the lines.

The procedure is something like this (assuming you have a helper):

Start farthest away from the master cylinder first and move toward it right rear, left rear, right front, left front. You need a chunk of hose that will fit over the end of the bleeder and go down to some container to catch what comes out.

Find the bleeder, it is right near where the line goes into the wheel cylinder.
Get it freed up, also helps to just take out and clean it, blow the crap out with brake cleaner/air/small wire whatever it takes.
Then with the bleeder closed and the hose on it, have your helper pump the brakes a few times, then hold them down. Open the bleeder a bit for a few seconds, close it, tell your helper to let up the pedal and pump again. You should get some brake fluid out. Repeat about 5-10 times, more for the first wheel of each set (front or back).
Every couple of times, check the fluid in the MC resevoirs and top them off if needed. As you do it, you will get a feel for how much it uses up each bleed cycle, so check often at first.
You should start to get some pedal after the first few times. If you can find some clear hose to use it helps, you can see the air coming out and tell when to stop at each wheel.
Repeat for each wheel, moving closer to the MC each time. When you are done you should have gotten a good stream of fluid out of each one, and you should have good pedal height. Replacing the MC will sometimes require quite a bit of bleeding, because the air will have to travel quite a ways.

The book is pretty paranoid, saying you should have the motor running when doing this, and place a board under the pedal so it does not bottom out. On a new MC, the bottoming out thing is not nearly so critical though, and I've never had a problem doing it with the motor off either.

ASSUME! ASSUME! ASSUME, you know what it does to us, doncha 800xl? I feel like a dunce.

look i cant get fluid to come out of my rear tires but i can get fluid to come out of the front. what gives. also it comes out better when i have it cranked up but the pedal still goes to the floor more so when its running.??????????????

Did you pull the bleeders all the way out on the rear and clean them? Usually they just work, but mine have been clogged solid with gunk a couple of the times I've had to bleed them. You should be able to blow air through them. You might also have a clog in the wheel cylinder itself. It is tough to get in there where you can look inside the hole for the bleeder, but you might be able to get a small wire in there and knock stuff out.

Failing that, I'm not sure what the deal would be. MC could be messed up, proportioning valve stuck, a badly leaking rear wheel cylinder maybe? Does it use up any fluid just trying to pump up the brakes? That would point to a leak, but you should see it coming out somewhere if it were that bad.

Before bleeding brakes, you need to adjust them. This will put the wheel cylinder pistons alittle farther back in their bore, thus reducing the amount of fluid in the cylinder and making bleeding more effective.

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Certified pilots, looking down on people since 1903.

i dont use any fluid but when i pump them they dont pump up. and when i went to try to bleed the fron i just got a trickle out of them. could my brake booster cause all of this. my wheel cylinders arent leaking but they could be clogged. but i cant even pump the brakes up to get them to bleed, so use the 1 man pump bleeding mechanism.

also where is the poportioning valve located?

The proportioning valve is located on the left front fender well. Follow the brake lines off the master cylinder and it's the valve that the lines go into.

First thing first, if the booster was bad your pedal would be hard and it would not cause the problem you are describing. It sounds as if you have allot of air in the rear brake system. pump your brakes and crack the rear brake line at the master to check flow. If you have good flow go down the line and bleed the rears at the P. valve. It sounds as if the rear bleeders are clogged.

_________________
Certified pilots, looking down on people since 1903.

well im going to get another master cylinder cause one of the sides of the mc isnt pumping any fluid, but the other side is feeding it through. so im going to change it out and see where i sit. thanks for all the info and ill let yall know if its fixed or not. thanks

got the brakes fixed. thanks everyone for your input and knowledge. it ended up being the MC again one of the cylinders wasn't opening to allow fluid to my rear. so i replaced it and bled the brakes, even though i had a problem with my driver side rear, it was clogged. It eventually broke lose and everything is fine for now. again thanks and by the way 800xl good job on the block thing under the brake pedal i don't know if it worked or not but it cant hurt.