Sinking Pedal after Break Work ...

[LEAKYSEALS951]

Originally Posted by island911

pfft... /\ what do you know about leaky seals... oh wait.

I assume that the ABS system runs independent of the MC, but really don't know.

Does anyone have a hydraulic diagram for that system?

That's what I'd want to see. If changing out the MC would cause the ABS to need a special treatment (or even bleeding the brakes), seems like there'd be a lot of post about it from those who had been there/done that to learn from.

[chrismorse]

Originally Posted by aigel

A round of thorough pedal bleeding of all 4 corners in the right order produced zero air and pedal still does the same.

You guys are right, pedal bleeding does not require big strokes. But my helpers are not experts, so I am just glad they can do the procedure okay with me under the car. 1) put foot on pedal to build pressure 2) open bleeder 3) Pedal sinks towards floor and about half way down helper announces this 4) close bleeder 5) Pedal up - repeat. Key here, the way I learned it, is that the system needs to be under pressure whenever you crack the bleeder.

My money is on the MC, because an air pocket will give a springy pedal feel like an air shock. You compress the pedal but it can come back up. If it sinks, ever so slowly, that tells me the fluid is leaking past the MC piston seal.

Now what Jeff writes about the ABS unit is worrisome. On two fronts. Maybe I got air in it and that's causing my pedal effect but worse, if I replace the MC, I may get air in the ABS unit as well? I'd get a scan tool and do it all right, but very limited on time these days.

At this point it is either spending more time installing a new MC or throwing in the towel and just get some professional help this week.

What would you guys do next!?

G

I have almost no experience with abs, but a lot of standard brakes, power and straight manual. I'd go for the MC, probably about $100 - Dont go cheapest, do a reputable rebuilder or new - a bit of shopping, maybe Napa??? had good luck with my local guys.
chris

[brainz01]

I had this happen on my 997 turbo. Master cylinder ran dry while bleeding the brakes and got air in the ABS. Felt like the MC was shot, as that was how it was acting, but no: the ABS needed to be bled.

You need to get a scan tool that has ABS bleeding for your vehicle. Don't throw parts at it until you do this first. The scan tool bleeding process typically requires a pressure bleeder to work as it will cycle the valves in the ABS to get the air out.

Everything was back to normal on my car once I bled the ABS (with the tool).

Good luck.

[unclebilly]

Originally Posted by aigel

A round of thorough pedal bleeding of all 4 corners in the right order produced zero air and pedal still does the same.

You guys are right, pedal bleeding does not require big strokes. But my helpers are not experts, so I am just glad they can do the procedure okay with me under the car. 1) put foot on pedal to build pressure 2) open bleeder 3) Pedal sinks towards floor and about half way down helper announces this 4) close bleeder 5) Pedal up - repeat. Key here, the way I learned it, is that the system needs to be under pressure whenever you crack the bleeder.

G

You got it right. I’ve seen people not realize they have to close the bleeder before they let off the pedal.

[fastfredracing]

In lieu of a scanner to perform an ABS bleed, you can go out and find a nice gravel road, or parking lot , and make several aggressive stops . The goal here is to initiate an ABS event, so that the pump comes on it opens and closes the valves in the HBCU.
Then come back, and try to re bleed.
Worth a try before you have to go to a shop .
The most likely scenario , is that you have air in the Hydraulic unit. Id at least want to try a proper bleed, with a scanner before I tossed a mc at it .
If that does not work, then Id be looking at a master

[rusnak]

Have the helper pump the pedal several times, then hold it down hard like in a panic stop. Open the bleeder screw with the helper holding the pedal down. If there is air, it'll come shooting out like crazy. Put a block under the pedal to keep it from going to the floor.

[LEAKYSEALS951]

I love this place. A week ago, I had no idea changing the fluid/ over compressing the pedal could cause MC problems. I also didn't know the ABS systems could potentially require extra love if they got bled dry. Now- not only do I know that, I know I need to go skidding on a dirt road to fix the ABS! Get em' duke boys! Yee Haw!!!!

[aigel]

Hi Guys,

Believe it or not, but after driving for a while it seems to have gotten better. I want to emphasize that it takes a lot of force to make it budge, really have to stand on it. A smaller person would not even be able to get it to compress. So the truck certainly is driveable but of course I can't keep it this way long term.

The gravel road is a really good idea and I was wondering about that. I'll give that a shot and then try to bleed again. Maybe just a series of bleeds and driving will get it ironed out.

Great input on the ABS unit and MCs etc. I'll update the thread with how things progress. Thanks a ton!

G

[john70t]

Originally Posted by Jeff Hail

Need to get the bubbles out and past the ABS pump before and after the pump. If any air gets in the servo chamber/ accumulator you are not going to get it out by pumping the pedal.

The ABS section of the brake system cannot be bled without the use of the scan tool; there are no manual procedures for bleeding the HCU. Using the scan tool, select ECU VIEW, then select ABS MISCELLANEOUS FUNCTIONS to access the bleeding procedures. Follow the instructions displayed on the scan tool.

Think of it as two systems:

Bleed the calipers/ cylinders first.
Left rear wheel
Right front wheel
Right rear wheel
Left front wheel

Then bleed the abs system.

With the ABS bleeding completed, bleed the base brake system a second time using the same procedures that were outlined for bleeding the base brake system the first time. Once again, check the fluid level in the master cylinder and be sure that it is at the FULL level.

Bump.

[aigel]

John,

Any recommendations for a scan tool? Looks like the cheaper scan tools have these functions.

Thanks!

Georg

[908/930]

Something to check, are your brakes assisted by the power steering pump? Most diesels are, if so there is a hydraulic accumulator located near the master cylinder that can fail, it gives symptoms similar to what you are describing.

[aigel]

It looks like a regular brake booster. This is a 1500 with the Ecodiesel, so a while a diesel, a different animal! Thanks!

G

[aigel]

I ended up buying this tool to perform ABS bleeding procedure:

https://www.amazon.com/gp/product/B07XK1P8ZT

It did the job nicely in terms of walking me through it and we repeated it a few times on every axle. Then bled it all again, no bubbles anywhere and of course, the pedal still feels the same.

I have to stand on it like an ox to cause it to sink, so it isn't a safety issue but it bugs me. I have a bunch of recall work scheduled at the dealer in a few weeks and I'll see if they bite on putting a new MC in for me under my extended bumper to bumper warranty. If not, that will be my next project on this when I get around to it.

Thanks for all the input. I sure learned a lot and now am invested another 180 into the truck.

G

[svandamme]

huh? thought issue was sinking pedal, eg soft?
but you call it super stiff now??


any missing helper springs on the pedal box?

[1990C4S]

Sinking pedal almost certainly means a small leak (bypassing) in the master in my experience. It won't get better.

[john70t]

Originally Posted by rusnak

Have the helper pump the pedal several times, then hold it down hard like in a panic stop. Open the bleeder screw with the helper holding the pedal down. If there is air, it'll come shooting out like crazy. Put a block under the pedal to keep it from going to the floor.

This is very important for old cars and parts.

When the pedal is pushed fully to the floor:
-The piston/seal in the master cylinder bore travels to the end of the normally unused cylinder section.
-This section will be corroded and lumpy with rust after a decade or two.
-That action can break the high pressure M/C old rubber seal, resulting in a permanent sinking peddle with loss of brake pressure.

FWIW, the M/C reservoir gravity port and system is closed off with only an inch or two of pedal travel.
Sealed system then.