"Soft" brake pedal when engine running

[new2yukon]

My 2001 Yukon has a firm pedal when it's not running. It's considerably softer when running. Why?

No known leaks.
New front calipers.
New front brake hoses.
New master cylinder. (This helped a little)
Cycled abs via activating on dirt roads and bled all 4 wheels again.

I really don't think my service work has changed the brake feel much at all. Pedal is similar to what it's been for quite some time but my other cars are much much firmer so it seems this can be improved.

Pads are 75,000+ miles old but still have life. It stops "ok" but there's no real brake bite feeling. New pads/rotors prob would help this but when idling those are irrelevant.

Wondering about air somewhere but the pedal is firm with everything off.

Thanks in advance.

 

[Scottydoggs]

you can try re bleeding the master, (pump the brakes crack the lines off the master as if it was a bleeder) then all 4 calipers again, if that dont firm it back up, you might need a tech 2 to do a proper abs pump purge of air.

 

[buckwild27]

Make sure you got all the old brake fluid flushed out. Reasons for spongy pedal are old water saturated brake fluid, wore out master cylinder and air in the system.

 

[OR VietVet]

You took time to replace calipers but did not replace 75k miles pads? Rotors, I can understand a little but I still would have done rotors too.
Sounds like some air still but if is exactly like before new parts, I doubt it.

 

[new2yukon]

You took time to replace calipers but did not replace 75k miles pads? Rotors, I can understand a little but I still would have done rotors too.
Sounds like some air still but if is exactly like before new parts, I doubt it.

I understand your perspective and agree but had to wait since they are in ok shape and I have header bolts to pay a shop to replace and an oil pan leak so just cutting costs.

The calipers were swapped because I couldn't get out the t55 slider bolts.

Looking in to maybe a vacuum issue and check valve malfunction but still learning and have to check on that too.

 

[OR VietVet]

The plot thickens.....after more shared info.

 

[buckwild27]

A side note your pedal will be firm with the vehicle off, because the vacuum booster is not helping you without any vacuum.

 

[exp500]

If tou have tried everytging else, cap off outlets on master cylinder. Bleed and test for firm pedal.

 

[rockola1971]

I understand your perspective and agree but had to wait since they are in ok shape and I have header bolts to pay a shop to replace and an oil pan leak so just cutting costs.

The calipers were swapped because I couldn't get out the t55 slider bolts.

Looking in to maybe a vacuum issue and check valve malfunction but still learning and have to check on that too.

So hydraulic system has been opened up since you "swapped calipers". You got air trapped in your ABS manifold. It will take a very long time of locking up the brakes to activate the ABS pump and then bleeding to get it all out, if ever. You need a bidrectional scan tool and perform an auto bleed which actuates the solenoids in the ABS manifold. Is your Brake light on in the cluster????

 

[new2yukon]

Make sure you got all the old brake fluid flushed out. Reasons for spongy pedal are old water saturated brake fluid, wore out master cylinder and air in the system.

I've purged old fluid prob every 24months so what was in there wasn't too old. So not sure.

 

[new2yukon]

So hydraulic system has been opened up since you "swapped calipers". You got air trapped in your ABS manifold. It will take a very long time of locking up the brakes to activate the ABS pump and then bleeding to get it all out, if ever. You need a bidrectional scan tool and perform an auto bleed which actuates the solenoids in the ABS manifold. Is your Brake light on in the cluster????

After more testing today, you're probably right. But the caliper swap and hose swap etc were an effort in routine maintenance and didn't cause this issue but somehow I got this issue but don't know when or why. I've just decided this issue needs fixing. The new MC did help because the pedal holds really good now with engine off but unfortunately didnt solve the running softness.

 

[new2yukon]

After checking the booster with pedal tests and engine cycling it seems to test as it should.

Booster check valve only allows air one way so that's good. I get the big "whoosh" when I pull it after a run cycle. So it holds vacuum.

Vacuum hose has good suction when pulled from the booster with engine running so that seems fine.

Sooooo...the "only" thing left seems like the abs has air in it??? Or its bad in some manner. As you guys suggested I think that's my last issue....need a shop to do that for me and hope that fixes it.

 

[new2yukon]

If tou have tried everytging else, cap off outlets on master cylinder. Bleed and test for firm pedal.

I would do that if this issue arose after I put in the new MC but this issue was prior so I really feel most if not all of this issue is something else.

 

[new2yukon]

I'm going to make a shop appt and will update this after I know more.

 

[rockola1971]

After more testing today, you're probably right. But the caliper swap and hose swap etc were an effort in routine maintenance and didn't cause this issue but somehow I got this issue but don't know when or why. I've just decided this issue needs fixing. The new MC did help because the pedal holds really good now with engine off but unfortunately didnt solve the running softness.

If you opened up the hydraulic system which you most certainly did when you changed out calipers.....that caused the issue. And that issue is air trapped in the system that you will never get out without a bidirectional scanner using the auto bleed feature. Is your BRAKE light on in the instrument cluster when engine is running?

 

[new2yukon]

If you opened up the hydraulic system which you most certainly did when you changed out calipers.....that caused the issue. And that issue is air trapped in the system that you will never get out without a bidirectional scanner using the auto bleed feature. Is your BRAKE light on in the instrument cluster when engine is running?

No brake light is on. This issue was prior to my caliper swap, but even so, the calipers are downstream from abs so I dont follow how that would even cause an issue like mine.

 

[OR VietVet]

No brake light is on. This issue was prior to my caliper swap, but even so, the calipers are downstream from abs so I dont follow how that would even cause an issue like mine.

Because air in the system can travel. What @rockola1971 is saying about a bidirectional scanner is correct. Otherwise, you will over think the problem and say things like, "I don't follow how that would even cause an issue like mine". You have asked for help. You are being given help.

 

[new2yukon]

Because air in the system can travel. What @rockola1971 is saying about a bidirectional scanner is correct. otherwise, you will over think the problem and say things like, "I don't follow how that would even cause an issue like mine". You have asked for help. You are being given help.

I know what you are saying but also know that this issue was prior to my caliper swap so that in itself can't be blamed. Yes, air rises but its also opposed by gravity and the fluid.

This could also have been a bad booster but its not. It could actually be a bad modulator and not even air from what I've been reading. I'm trying to do a proper diagnosis prior to concluding air in the abs system.

 

[OR VietVet]

I am not saying the calipers caused the problem. I am saying the task of changing the calipers likely put air in the system and has traveled from the calipers to the ABS system.

 

[Matahoe]

From experience a spongy brake pedal is with out a doubt air in the system. Either in master cylinder, ABS module or lines/calipers. Air in ABS module accumulator section would require bleeding at the module itself (somewhat challenging) or using a scan tool to activate the automatic bleed procedure.