I can pump my brake peddle when the engine is off and it will get tight. If the engine is running I lose that tightness and pumping wont bring it back. I pretty much have to push the peddle all the way in to start slowing the truck down. Air in lines or booster or what? This in turn makes it to where my stupid brake light sensor doesnt turn the lights on til I have it all the way down.

 

What year truck? engine? trans? Have you inspected the brakes otherwise? A functional booster will allow you to pump away the reserve vacuum with the engine off, making the pedal hard, because you no longer have its assist, then once vacuum is applied it assists you in your efforts to push it down, booster sounds good. either you have a leak, air in lines, rears out of adjustment, a bad master or a combination. Start with your rear brakes, they are often neglected, pull off the drums, make sure the wheel cylinders aren't leaking, axle seals aren't leaking, and the shoes are within specs and dry, if that checks out, spray it all down with brake cleaner, make sure they are adjusted and then check the rear hose isn't leaking then move to the front. Make sure no hose leaks, pads and rotors are within specs, calipers aren't leaking. Then check the fluid level in the master, make sure its full, if no ails are found thus far, bleed the brakes, you may have an abs bleeder to do too, if you have rear abs. if still nothing then probably a master. Since it does get tight and it doesn't seem to bleed down when off, I'd say probably not a master and more likely a rear brake issue.

 

Sorry I thought I added my truck info. 1995 F150 5.0 V8 unknown tranny. Have not done any looking around yet because I am new to working on brakes and have no idea where to start.

 

That's quite all right, we love to help here, and we love pictures, if in doubt, take a picture of your brakes and one of us will gladly give you our two cents.

 

I went and tested them again to make sure I was describing it correctly. What I found is it is rock hard when off with what I feel is average travel of the pedal. When On it definitely goes soft, doesnt make it to the floor but travels far before getting any real resistance. I put it in gear and at idle speed I cant stop the truck by pressing it down slightly. Not sure if that info changes anything or not just throwing it out there

 

My F350 was doing the same type of thing, turns out the brake booster (vacuum like yours) was bad. Steve83 calls it an over-active booster.

Here are a few threads with info:

Replaced [some stuff], brakes still suck.

Adjusting stop lamp switch - FSB Forums

 

Thanks for the information. Maybe I didnt look hard enough but is there a way to test the booster? I would hate to buy one and it not work.

 

Your symptoms point to a bad booster IMHO. You have proved the hydraulics are good, the only thing left is the booster.

How long has this been going on? Was there any service work before?

 

Just bought the truck a couple weeks back so unsure about service history.

 

Boosters usually don't make for a low pedal, that requires the pedal going to the floor for it to stop. Rears are very important and often neglected, remember, if they're even just out of adjustment, that requires that much more pedal travel before they touch, work and stop.

 

As far as the stupid brake light switch, does that need proper resistance to activate and since my pedal loosey goosey its not flipping on with light touch?

 

The brake light switch is spring loaded, yes it needs some resistance to operate correctly. The issue you have is a little difficult to properly diagnose. If the pedal feel and height is good with the engine off then you are probably looking at a booster issue. If you pump the pedal a few times, engine off or on, then the pedal is solid then you have a hydraulic issue such as air in the lines or a master cylinder that is bleeding down internally.

I messed around with a similar problem for quite some time before finally figuring out it was a bad booster. Before tossing on any parts it is suggested to inspect all the brake components especially the rear brakes and all the rubber hoses. Never forget to cover the basics before going too wild with the checkbook and needless parts.

 

I'm not trying to call you out or start a flame war or anything, but as far as I understand, air in the lines gives a soft, mushy, low, whatever you want to call it pedal, and that's because in a closed hydraulic system, the fluid does not compress, but air does, so if you have air, instead of the brakes being applied with more pressure, you're just compressing any air pockets or in the case of a master, fluid is bleeding internally, allowing the pedal to go lower, without actually applying the brakes any harder.

Like I said, I'm not trying to call you out or anything just kinda want to know how that's possible, it doesn't make sense to me, but then again I learn new things every day, and some of the stuff I already know, I find is incorrect. And like you say, the description is not exactly complete, is he referring to pedal effort, varying resistance in pedal effort, braking effectiveness vs. pressure applied, etc.

 

I do not have any issues with anything you have said. What I stated that you put in bold is exactly what I have found with hydraulic brakes. If you can pump the pedal a few times and the pedal "feel" returns to normal it's either air in the system or a leaking master cylinder. Perhaps I should have gone a little further to say the pedal may then start to drop again as you keep applying pressure.

We're on the same page IMHO, it's how each other is interpreting the statements.

Before I sent the OP down the bad booster bunny hole theory I wanted to make sure all the basics have been covered.