Changed EVERYTHING, brake pedal still goes to the floor.

[SonnysSweetTea]

So I have a 1998 K1500 5.7. To start this off, I got a brand new brake booster 3 months ago before all of this which still works perfect. Now a few weeks ago i figured i'd take a shot at the caliper upgrade, swap a tahoe police pkg(8 lug k2500) calipers on and everything bolts up good even got some severe duty pads to try. No problems swapping, changed the rubber lines while I was under it, all went well. Took the truck out for a test drive, brakes still worked good but not great, had a bit of air trapped somewhere but the brakes could lock up and activate the ABS no problem. Took the truck down a dirt road to cycle the ABS like I've heard from some people. Activated it a few times at 40 mph, ABS works good. Heading back to the house the pedal was almost to the floor before the truck would slow. Obviously the master cylinder decided to go. It was original at 260k. Not bad.

Head to the parts store, swap a reman in, bench bleed, gravity bleed, then full bleed normal sequence RR-LR-FR-FL, solid streams, pedal solid when off, fire the truck up, pedal goes straight to the floor. No big deal just a junk M/C. Repeat this 6 more times with 6 more junk reman M/Cs. The last reman I put in I also took my truck on a trailer to the dealer and had them cycle the ABS with the Tech2 while pressure bleeding. There old m/c adapter was leaky so it didnt pressure bleed perfect but we vacuum bled the entire thing. Still the same thing, pedal goes to the floor when the truck is on. Decide to just do the NBS M/C swap, so I got a brand new one (not a reman) based off an '02 Tahoe. Bench bled it. Swap it in, carefully bend the lines back because it's a shorter M/C, hooked up the lines with one being an adapter. All good so far. Getting pissed off, I yank the drums off, shoes are beyond worn, and the one shoe is 1/2 inch from even touching the wheel cylinders. Replace both wheel cylinders, rear rubber line and got new shoes. Drums are still plenty thick no grooves or warping. All the hardware still good condition, all nice and tight, pushed the e-brake a few times to adjust the shoes properly.

Pretty much everything is new or in great condition still. Now that I have the NBS M/C with the newer style round cap that screws on instead of just popping on. I take the truck back up to the dealer and have them Truly pressure bleed it again with a better style adapter. Cycling all the ABS dump valves front/rear while under pressure, and doing the automated bleed procedure. All four bleeders all have perfect streams no air coming out at all. Almost 2 quarts of fluid just to be sure there is Absolutely No Air. Hop in the truck, pedal is super firm at the top, perfect pedal till you cant press any harder at 1/3 of the way down. All seems well. Fire the truck up one last time, pedal goes straight to the floor. It will stop the truck but your at the floor before you stop and when you bottom out, you still roll another 6-8 feet before a complete stop.

I'm stumped on what else there is. I've checked every line and connection and there's no leaks anywhere. M/C fluid level never drops, no leaks down the hard lines. I checked the booster rod and used a lathe to make a 1/4" longer rod which keeps the M/C 1/4" off the booster so it's not the rod adjustment. Checked the booster check valve, holds vacuum perfect, no sound. Literally every solution I've seen on these forums I've tried. I'm trying everything to keep the ABS module and try any last option, but other than getting a used module from a junkyard, I'm out of ideas. The last resort is just pulling the ABS module out and running a Wilwood adjustable proportioning valve and "T" the lines, bypass the whole ABS junk.

 

[east302]

How tight were the drums when you put them back on?

 

[SonnysSweetTea]

How tight were the drums when you put them back on?

It's not my first time doing drums. I had brand new shoes so obviously they are tight at first but you back the adjusting screw until the drum can slide on and then push the ebrake pedal a few times till it auto adjusts to where it needs to be. With the truck off the ground, put the truck in gear and not even a 1/16th inch of pedal travel and the drums lock. No problems there. Next idea?

 

[454cid]

I wonder if there is some valving that's opening up on the ABS when you start the truck. If I'm coming to a stop, and hit a hard bump my ABS will release pressure from my brakes, without turning the pump motor on.

Assuming you get a hard pedal again when you shut the truck off, try pulling the ABS fuse and see what happens.... I don't even know if that valving would be electric or not.

 

[SonnysSweetTea]

I wonder if there is some valving that's opening up on the ABS when you start the truck. If I'm coming to a stop, and hit a hard bump my ABS will release pressure from my brakes, without turning the pump motor on.

Assuming you get a hard pedal again when you shut the truck off, try pulling the ABS fuse and see what happens.... I don't even know if that valving would be electric or not.

I pulled all the plugs from it basically disconnected everything from the module except the brake lines and it still does it. Only when the truck is on.

 

[east302]

Was the MC level when bench bled? I wonder if you could have a small air pocket trapped above the outlet connection. Maybe try jacking up the truck to bring the MC level and cracking the fittings to bleed. Or disconnecting it from the booster, letting it angle down slightly and then gravity bleed at the wheels.

 

[Dubs]

I think those calipers use a higher pressure than factory. You might want to research that. You may have to get a different proportion ing valve and use a bigger line to them to hold the pressure where it needs to be.

 

[Hipster]

I checked the booster rod and used a lathe to make a 1/4" longer rod which keeps the M/C 1/4" off the booster so it's not the rod adjustment

I'm not fully understanding what you did here. A spacer between the 02 m/c and the 98 booster?

Just a few thoughts. Calipers that have bigger pistons are going to take a M/C that can push a bigger volume. You bought one for an 02, but an 02 what? Different vehicles are going to have different leverage ratios on the pedal inside or even same model vehicles with different brake systems may have a different pedal with a different leverage/travel ratio. The distance between the pivot mount and the booster rod mount. This can affect the piston travel in the M/C. Too much and tearing up the seals or too little not building enough pressure? 6 bad M/C in a row? IDK, sounds more like the set-up or the methods.

Air can get trapped in the line above the rear end or in the ABS . You can push a lot of fluid out the wheel cylinders before you even get to the air. I have cracked lines at the distribution block on the rear. When I used to do frame swaps we used to have to go through the bleeding process a couple times before we had enough pedal to send it to the service dept. I used to start them up and go through the process again with a helper working the pedal gently most times to even get any pedal.

 

[SonnysSweetTea]

I'm not fully understanding what you did here. A spacer between the 02 m/c and the 98 booster?

Just a few thoughts. Calipers that have bigger pistons are going to take a M/C that can push a bigger volume. You bought one for an 02, but an 02 what? Different vehicles are going to have different leverage ratios on the pedal inside or even same model vehicles with different brake systems may have a different pedal with a different leverage/travel ratio. The distance between the pivot mount and the booster rod mount. This can affect the piston travel in the M/C. Too much and tearing up the seals or too little not building enough pressure? 6 bad M/C in a row? IDK, sounds more like the set-up or the methods.

Air can get trapped in the line above the rear end or in the ABS . You can push a lot of fluid out the wheel cylinders before you even get to the air. I have cracked lines at the distribution block on the rear. When I used to do frame swaps we used to have to go through the bleeding process a couple times before we had enough pedal to send it to the service dept. I used to start them up and go through the process again with a helper working the pedal gently most times to even get any pedal.

No spacer, I said I tried to make a longer rod just to see if the old one was short but it held the m/c off the booster so any length adjustment is rendered useless. I kept the factory rod that came with the booster in. These boosters aren't adjustable. And I specified it's a m/c from an 02' Tahoe. And yes they are the same because I cross referenced both the silverados the suburban and the Tahoe master cylinder part numbers and they're exactly the same, it doesn't vary unless you have hydroboost setups. And there's literally no way there's air in the system with a shop pressure bleeder machine that's got almost 20psi on the m/c and a vacuum line on each bleeder and they're perfect streams, going through almost 2 gallons of brake fluid. All while cycling the abs with the tech2 and opening each dump valve individually.

 

[Dubs]

Those calipers use a higher line pressure than the 02.