'01 2500 HD spongy brake pedal

[Mickeymotormount]

Hey guys. I am probably beating this topic to death around here but here goes anyway:

My 2001 2500 HD Extended cab longbed 4X4 with the 6.0 engine and 4L80E trans has been suffering what I call a spongy pedal. The truck stops fine but it just seems like it takes excessive brake pedal travel in order for it to do so. I have driven later (07 and up) 2500 HDs with the same hydro-boost setup as mine and the brakes actuate with minimal brake travel compared to my truck. The pedal feels firmer too.

I have replaced the master cylinder, hydro boost unit and power steering pump and used a scanner with an auto bleed function to actuate the ABS valves during the bleeding process. All to no avail. Still have excess travel and what feels spongy to me.

Am I missing something during the bleeding process?

Is it me or is this a common problem with the GMT 800 series line?

Thanks to all in advance.

[hatzie]

T400 owners upgrade to the T800 series master cylinder. This is a problem with your truck.
Sounds like you may have air trapped somewhere in the system.

Did you bench bleed the master cylinder to push all the air out before bleeding? The brake pedal will not push the piston to full depth to eliminate air pockets in a new master cylinder.
I use a pressure pot bleeder to push fluid through the system. This helps a great deal on vehicles that use the ABS pump to meter and proportion the brakes.
You can DIY one or buy one. At this point the Motive Products bleeder is about the same price as the parts to build one.

What are you using for power steering fluid?
Did you bleed the power steering and hydroboost system?

Another thing to look into is your tire size. If you're running larger than stock tires and haven't re-coded the ABS computer for the larger tires it can make the brake pedal feel somewhat odd.

[Mickeymotormount]

Hatzie, thanks for the reply.

To answer your questions:

The master cylinder was bench bled. It took about an hour but all air bubbles were out before mounting.

New A/C Delco OEM power steering pump and A/C Delco power steering fluid. Hydro boost unit was used but I flushed it before installing. My original Hydro boost unit was leaking so I figured I would replace it.

I am running 17" tires. Over all diameter of 31". Standard width tire on GM 8 lug aluminum alloy rims from a later GMC truck.

For bleeding I made a special cap with an air fitting that snaps onto the master cylinder. I pressurize the master cylinder with about 5-10 psi from a compressor to push the fluid through the system. This works quite well as I can go from wheel to wheel and bleed the entire system while monitoring the level of fluid in the master cylinder.

When I bought the truck about 5 years ago, I replaced the hard lines with stainless steel ones since the originals were rotted out. All new rubber lines on 4 corners and rebuilt caliper on the corners as well.

Since the system was opened to replace the hard lines, I figured I must have gotten air into the system, or at least the ABS unit. So I bought a scan tool to actuate the ABS unit while bleeding. I have bled multiple times but with same result.

Peculiar thing: If the ABS is activated while driving on ice or something slippery, the pedal will firm up momentarily and then return to it's spongy state.

[hatzie]

Sounds like you still have an air pocket somewhere in the ABS pumps.
You probably already did this but I'll say it anyway. You want to bleed from the longest to the shortest brake line run from the master cylinder. Usually this is RR, LR, RF, LF.

Since you already have a modified cap you can attach a clean weed sprayer to it.
I usually load the sprayer body with 1/2 or 3/4 of a gallon of my chosen brake fluid.

[hatzie]

The actual diameter at the tread is what makes the biggest difference to things. 245/75R16 or 30.6" tires came on my truck. I'm running 265/75R17 or 31.6" diameter now. There are a bunch of different possible stock tire sizes listed in the ABS computer. 265/75R17 are not among them but there are some that are the same diameter so I re-coded the ABS to a tire that matches the diameter of the new ones.

1" in diameter doesn't sound like much but it drops your final drive ratio noticeably.
I played with my RPM and gearing spreadsheet to come up with some numbers to illustrate what's happening.
1" larger tire drops your effective gearing
4.10:1 axle gears, .73:1 OD, 245/75R16 tires sets your engine RPMs at 2145 going 65mph.
4.10:1 axle gears, .73:1 OD, 265/75R17 tires sets your engine RPMs at 2002 going 65mph.
I then fooled around playing what if with the axle ratio in the spreadsheet to see what lower axle ratio would get the 245/75R16 tires to run at 65 at nearly the same engine RPMs as the larger tires with 4.10 gears.
Here's what I got...
3.83:1 axle gears, .73:1 OD, 245/75R16 tires sets your engine RPMs at 2004 going 65mph. 2RPMs is pretty close.

Add somewhere around another 1/4"-3/8" on the diameter and you essentially turn 4.10 axle gears into 3.73 gears without cracking open the diffs. That's a pretty serious difference.

I'd check the tire size in the ABS computer.

Your ABS motors or pumps may be fouled up but I would guess it's a combo of the larger tires without recoding the ABS computer and an air pocket you still haven't gotten out through the bleeders.

I usually do an ABS bleed under pressure then I crack open the bleeders and draw off a pint of fluid from each corner.
Since the system is full of fresh fluid... if it looks clean and it's not aerated you can re-cycle it back into the pressure pot.
Command another ABS bleed under pressure and draw off another pint at each corner.
Rinse and repeat til you've cycled a gallon or so through each bleeder.
I bet you'll get a small slug of air or ten out one of the rear bleeders.

[Mickeymotormount]

One thing not mentioned was that I repowered the truck 18 mos. ago with an updated 6.0. The PCM was flashed for the engine upgrades, as well as tire sizes to keep the speedo in check. I would assume this flash did not go as far as calibrating the ABS module for the tire upgrade.

Also, in the winter time, I run a standard set of 16" steelies so my aluminum rims don't take a beating from the winter brining of the highways.

Regardless of what tire size I run, the brakes act the same.

I guess I may have to dive into investigating or investing in a new ABS module/unit. And then take it to the stealership to have it bled.

[hatzie]

The PCM and the ABS are completely separate on this programming.
I'd try pushing more fluid through. You'll probably find there's an air pocket in the rear lines at the ABS module or just after. It takes a fair amount of fluid to push the old cruddy fluid out the back when I flush mine.

[Mickeymotormount]

Will do. Thanks for your help. Hopefully I can get out any air that may be trapped in there.