"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.