Brake Pedal to floor on Startup, brakes work fine

Hi, I know this topic has been beat to death, but I think mine is a little different, and I'm looking a fresh brain to pick. Here's the situation.

1997 K1500, 4x4, 5.0L, 230,000 miles. As of the last few weeks the brake pedal kept getting harder and harder to push down in order to stop. Assumed bad vacuum booster, but delayed fixing because of self-diagnosed case of laziness. Replaced booster with new cardone unit yesterday. Didn't disconnect any lines or anything else, just replaced the booster. Now the pedal is firm, normal feeling with the truck not running, but after it's started pedal goes almost directly to floor. Hardly any resistance, what so ever. Bled all 4 wheels, no change, and no air. Noticed wetness at rear of master, decided master could have been bad before, and was now leaking because of the added pressure from the newly replaced booster. Installed new master cylinder, bench bled, re-bled all 4 wheels, same result. The truck stops fine, in fact, quite well, at about half pedal travel. Took it down the gravel road and quick stopped ten times, abs kicks in and out, truck stops quickly, but the pedal still goes to the floor. I'm at a loss. Anyone have any ideas? At all?

More info...

Forgot to add, that if the booster is disconnected from vacuum, on start up, pedal stays firm.

I think I'm going to try a junkyard abs pump tomorrow, as I've read here that they can leak internally, which is what I hope is going on here.

More info...

Forgot to add, that if the booster is disconnected from vacuum, on start up, pedal stays firm.

I think I'm going to try a junkyard abs pump tomorrow, as I've read here that they can leak internally, which is what I hope is going on here.


with your model year it is very possible its the abs pump assy. report back how this all worked out.

with the booster when the vehicle has been off for a few hrs you should have a couple of normal brake applications with the engine off. this is because a good booster with a good vacuum check valve should hold vacuum on engine shut down.

if only at start up pedal goes to floor I would say a small leak.

place pressure on brake pedal and see if it holds for 5 minutes..

Update, replaced abs pump/whatever it is, no change. Truck still stops just fine, but pedal still goes to floor. Gravity bled each wheel a little over a cup of fluid, test drove and panic stopped a dozen times, abs kicks in like it should, still no pedal back pressure. I'm wondering, since I've replaced everything else, if one or more of the rubber lines is expanding when the added pressure from the booster kicks in. I've watched all the lines while someone else pressed the pedal, nothing visual. I am going to try and two person bleed each wheel next, I read somewhere you need to loosen the two 3/8 pipe plugs on either side of the abs unit a quarter turn first? That's what i'm going to try anyway.

Update, replaced abs pump/whatever it is, no change. Truck still stops just fine, but pedal still goes to floor. Gravity bled each wheel a little over a cup of fluid, test drove and panic stopped a dozen times, abs kicks in like it should, still no pedal back pressure. I'm wondering, since I've replaced everything else, if one or more of the rubber lines is expanding when the added pressure from the booster kicks in. I've watched all the lines while someone else pressed the pedal, nothing visual. I am going to try and two person bleed each wheel next, I read somewhere you need to loosen the two 3/8 pipe plugs on either side of the abs unit a quarter turn first? That's what i'm going to try anyway.

you need an asistant with bleeding . with air in the system you would get a soft pedal. with the ABS replaced you would need alot of bleeding.


since the brake pedal still drops to the floor when held firmly down I suspect a leak with the work completed so far.

you must inspect every inch of brake line, any wetness staining is most likely a leak.

I have on lack of help used a 2X4 cut to the proper length and using the front driver seat to apply pressure to the brake pedal then look for leaks on the lines / not dripping on the ground as it does not take much to cause this effect of pedal drop.

you must also remove and inspect the calipers for leaks.

you got drums rear did you remove the drums and check for leaks ?

Thanks for the quick reply. I've combed every square inch of the lines, no leaks. Replaced one caliper because of a broken bleeder, other one is clean and dry. Both rear wheel cylinders are also clean and dry. The truck has never had any brake fluid loss, until I changed the master cylinder (spillage) after installing the new booster and having the pedal go straight to the floor afterwards. Before the booster, everything was fine, just no boost.

After more head scratching, more bleeding, abs pump included, I swapped in another booster. Brakes are back to normal. Normal isn't super good, by any stretch, but normal for this kind of truck anyway. I have absolutely no idea what could have been wrong with the booster for it to act that way, except maybe something with the throw length, but everything looked the same from one to the next. Oh well, moving on anyway. Thanks jcat.

Thanks for feedback, I knew something was messed up!

Sounds like being setup with a bad part.

Interesting thread. Glad you got it 'fixed'......