brake pedal - no resistance until halfway down?

[dieselbegreat]

Brake pedal used to be high and hard. Truck sits most of the time right now. Now the pedal goes about halfway down before I feel any resistance and the brake light is on. Fluid level is fine. What happened?

[DmaxMaverick]

Time for a new master cylinder. Very likely your rear brakes aren't even working.

[Robyn]

Yup
My 84 Blazer did that last winter.
If you are careful you can prebleed a new cylinder and install it with the lines snug and then have a helper pump the pedal and crack the lines loose at the MC and get most all of the air out.
If its still a tad spongy you will need to bleed clear to the wheels.
Wont hurt to get the old fluid out anyway

[dieselbegreat]

Thanks for the helpful info. I 'll do that.

[gmctd]

If that doesn't fix it, try adjusting the rear brake shoes - the self-adjustors only work in straight downhill runs in reverse.

If shoes are way thin, replace them

[Robyn]

Great point.
Many of the rigs dont have a hole in the back plate to allow a spoon in to adjust them either

[dieselbegreat]

Okay, well replacing the master cylinder didn't fix it. The wierd thing is that this happened suddenly with the brake light coming on. Why would the rear brake suddenly go out of adjustment? Otherwise, when the pedal hits, the brakes seem to work fine. Also, to make matters worse, the bleeders are rusted and rounded off. I can't figure out how to get the things loose now other than to replace the wheel cylinders. Anyway, could it be the proportioning valve going bad? Maybe a return spring snapped?

[DmaxMaverick]

Either you have a leak or broken hardware on the rear. I wouldn't consider adjustment or wear a factor if it came on suddenly. Even when the P-valve fails, the MC will still make pressure, meaning the depression won't be any different.

[Iain]

I just changed the rear brake parking brake cables on my 1994 and did not preset the adjuster on the rear brakes fully. I ended up with much softer brakes than before, although I can still get them to lock up. I have no broken parts as I went through both drums carefully but I realize that i will have to set my rear brakes manually if I want the firm pedal feel again. The adjusters will automatically take up the slack (as noted above in GMCTD's post) but you have to drive backwards to do it. To take up a lot of slack might take a lot of driving backwards!

Anyways, check your slack adjusters and make sure your rear brakes are adjusted to where they should be. There should be a slight drag if you have the rear wheels off the ground and turning by hand without the brakes applied.

Iain