2006 sierra denali brake problems

Just started today all the sudden,The brake pedal will go to the floor if you push hard on it to stop,and NOW it HAS to be pushed hard to stop,thats not normal. If the truck is off it has a good hard pedal and won't go down very far. I have no leaks anywhere that I can find.
When I hold pressure on the pedal and start the truck it will go down 1/2 inch or so and if I keep pressure on it,it will go almost to the floor. I know that the way to test the old booster were to hold pressure on gthe pedal and when youi started it,it would go down 1/2 inch or so.I also noticed some kind of canister next to the master cylinder with a plug with a bunch of wires plugged into it,any idea what that is, or what my problem is? Thanks.

No way to diagnose this on the net, what you see sounds like the abs unit, if you are not up to dyer type repairs, have someone look at it, many things can cause what you describe, I assume there are no leaks in the brake system and the m/c is full, any recent work done? Do you have hydroboost power brakes? BTW, we are almost neighbors.

No way to diagnose this on the net, what you see sounds like the abs unit, if you are not up to dyer type repairs, have someone look at it, many things can cause what you describe, I assume there are no leaks in the brake system and the m/c is full, any recent work done? Do you have hydroboost power brakes? BTW, we are almost neighbors.

I am fairly mechanically inclined, just lookiung for ideas where to start.There has been no work done in awhile.I did front brakes on it about 2 months ago besides that not much.How do I tell if I have hydroboost power brakes? Is there a way to test the master cylinder while oin the vehicle? Or is there a way to test the booster? Thanks.

Hydroboost runs off the power steering, no vacuum system. If Hydroboost,, it may be bad, very expensive unit too, if you seem to have no assist (not so much an issue with pedal height), then the booster could be bad.

Hydroboost runs off the power steering, no vacuum system. If Hydroboost,, it may be bad, very expensive unit too, if you seem to have no assist (not so much an issue with pedal height), then the booster could be bad.

My bad, I have easy to push pedal, I just have to push all the way to the floor.It is NOT a Hydroboost and it does have 1 inch vacuum line running to the booster.

I am putting on a new OEM master cylinder tomorrow night after work. Hopefully this gets it. I hope the old bench bleeder kit I have at home will fit this master cylinder,if I can even find it. I will let you know how I make out. Thanks for the help.

Careful here, try not to empty the entire brake system as you will need a special scanner to bleed the abs.

I do know its NOT the master cylinder.I put a new OEM on and no change.I will keep you posted.Thanks again.