Brake pedal goes to the floor...

[Ralph Bartels]

Looking for a little help with my brakes. I have a 1997 suburban k1500. The previous owner swapped in rear discs. I then, installed a NBS master cylinder and everything was functioning extremely well. The 10 bolt gave up the ghost a couple weeks ago so I swapped in a 6 lug 14 bolt sf. I removed the drums and put on a disc kit from lugnut4x4.com. I’ve bled the system, slammed on the brakes to engage the abs and then bled again. I even put on a new NBS master cylinder. The pedal is very firm when the engine is off but once I start it up the pedal goes to the floor before stopping the truck. I’m taking it to a shop in the morning to have them cycle the abs and bleed the system again. I’m hoping someone can give me some insight and maybe save me a trip to the shop.

Thanks!

 

[Gibson]

Having a dealer or a pro brake shop hook-up the scan to cycle the ABS and pressure bleed the system is the best money you can spend for safety and peace of mind.
And no,, discs do not "use lots of fluid". Drum brakes do have to move fluid, the master has to push enough fluid to expand-out the wheel cylinders before any braking action can take place.
With discs, the pads are already dragging on the rotors, and only move a few thousands of an inch, the "movement" of fluid in a tight system could be measured with an eyedropper,, the MC is functioning as a pressure transfer device.

 

[Ralph Bartels]

Thanks Gibson. I had our shop here in town bleed the brake system, they cycled the abs and all is well again!

 

[RichLo]

 

[Schurkey]

discs do not "use lots of fluid". Drum brakes do have to move fluid, the master has to push enough fluid to expand-out the wheel cylinders before any braking action can take place.
With discs, the pads are already dragging on the rotors, and only move a few thousands of an inch, the "movement" of fluid in a tight system could be measured with an eyedropper,, the MC is functioning as a pressure transfer device.

The piston in a drum system is perhaps an inch in diameter. It moves enough to take up the clearance between shoe and drum, which is controlled by the self-adjuster. The shoes may lightly drag on the drum.

The piston in a disc system is 2.5 to 3 inches in diameter. It has FAR more area than a drum piston, but--until it slides far enough on the piston seal to adjust--moves a fraction of the amount of the drum piston.

So you have a small piston moving a larger distance, and a huge piston moving a smaller distance. Either way, approximately the same amount of fluid transfer takes place--which is why the master cylinder bore is similar-sized.

 

[Gibson]

The shoes may lightly drag on the drum.

Yep,, however, the drums are round, and the shoes move in an arc,, because of this the shoes have to move a lot to accomplish any braking effort at all.
In discs the pads are already in full contact through out their entire area,, their actual movement is measured in thousands.
In a four-wheel disc system that is sized/set-up properly the only slack in the system is whatever is necessary for the pistons in the MC to close-off the compensating ports,, and that isn't much.
Actual fluid movement is almost nothing compared to drums.

 

[Schurkey]

You're not taking caliper flex, hose ballooning, pad compression, seal distortion, and wheel-bearing slack into account. Mostly, I think you're exaggerating the movement of the drum piston(s), and overly-conservative on the movement of the disc piston(s)

Remember, in a duo-servo drum wheel cylinder, only one of those pistons is actually moving. The other piston is held in place by the action of the rotating drum pushing the shoe against it's anchor.

Fluid movement is similar between drum and discs for the reasons I've already stated.

 

[Gibson]

Hose ballooning/bearing slack is applicable to both,, and yes, calipers do flex,, but you would be amazed at how much shoes have to flex,, the oblong shape of the shoes have to be "flexed" into a circle before maximum braking effort is reached, and that uses WC piston travel.
When I did the 4-wheel disc conversion on my K20, the pedal literally had no more than ~1inch of movement,, the braking was almost instantaneous in action,, once the compensating ports closed you were stopping, 'course I used stainless hoses and had all new rotors with virtually zero run-out, and all new wheel bearings.
We could go back-an-forth forever, arguing about tid-bits, to no avail, but the main thing I've been trying to point out is that so many people have absolutely no idea how hydraulic systems work,, and they keep repeating over and over again how discs "use much more fluid" and with discs "you need a MC to move lots of fluid",, and that is just a total myth that comes from not having a basic understanding of hydraulics and how pressure is transferred thru a non-compressible medium.