So my buddy has a 05 GMC Envoy on my lift getting a suspension refresh. Part of the work is flushing the old black brake fluid out.
One of the problems we were trying to solve was very weak/squishy pedal feel. Now I've done about 100 brake bleedings in my day, but this one is confuzzling me.
The problem is that there's very little fluid coming out of the calipers. I'm talking that it takes at least 4 times longer than normal to fill the little fluid container on my vacuum pump.
I thought it was an issue with the vacuum pump, so we went old school (one guy on the pedal, one guy cracking the bleeder). Even with the old school method that normally shoots a stream of brake fluid all over the wheel well only results in a little trickle.
Now, I know this happens to all guys eventually, but this one seems a bit young?
So we tried it with the engine on, and I noticed that the engine audibly revs up (like a vacuum leak) after releasing the brake pedal. Is that normal?
I'm thinking that maybe there's an issue with the booster/master.
Thoughts?
Thanks!
Could be a vacuum leak in the booster for sure, though I'd suspect the soft lines are internally collapsed and restricting flow. Is the booster full of brake fluid? Bad M/C seals can cause it to leak into the booster, then the fluid gets sucked into the engine. Not likely (you don't mention smoke or loss of fluid), but possible I suppose.
Bleed passages in calipers are pretty small diameter, and they can get crudded up to the point they'll barely bleed. Same thing can happen to the bleed screws themselves.
If the fluid is black, it's likely never been changed, so it's absorbed a ton of moisture. This can cause a soft pedal. I wouldn't believe how bad it could be unless I experienced it myself on a 94 Probe that I bought in 06. PO did pads and rotors and the pedal still sucked. I flushed a bunch of clean brake fluid through it, and the pedal feel improved a ton.
AngryCorvair (Forum Supporter) said:
Bleed passages in calipers are pretty small diameter, and they can get crudded up to the point they'll barely bleed. Same thing can happen to the bleed screws themselves.
If the fluid is black, it's likely never been changed, so it's absorbed a ton of moisture. This can cause a soft pedal. I wouldn't believe how bad it could be unless I experienced it myself on a 94 Probe that I bought in 06. PO did pads and rotors and the pedal still sucked. I flushed a bunch of clean brake fluid through it, and the pedal feel improved a ton.
We did pull out all 4 of the bleed screws, and they looked pretty good internally and with the scientifically calibrated "blow on it" test, seemed to not be too crudded up. I can try to get a light in on the calipers' passageways, but I don't know how much I'll be able to see without dissassembly. All 4 have low flow, which is the really odd thing.
And yeah, we were hoping that just replacing the fluid would improve pedal feel. I know it made the same difference when I replaced the OEM fluid in my 2005 minivan in 2018...
02Pilot said:
Could be a vacuum leak in the booster for sure, though I'd suspect the soft lines are internally collapsed and restricting flow. Is the booster full of brake fluid? Bad M/C seals can cause it to leak into the booster, then the fluid gets sucked into the engine. Not likely (you don't mention smoke or loss of fluid), but possible I suppose.
It appears to be all 4 calipers having low flow, which rules out a soft line issue to me.
We haven't pulled the master away from the booster yet, but that's on the agenda for tonight. Fluid doesn't seem to be leaving the master itself, though. No smoke in the engine, no noticeable loss of fluid the past few months, etc.
Just ran across this video, and it seems like everything is functioning as, err... designed? Is that a word you can use on a GM truck? Either way, we probably just weren't being tenatious enough about getting air out. We didn't have the scan tool to activate ABS yesterday, either, we'll have one tonight.
Is this the sort of thing that can be solved by pressurizing the brake fluid reservoir and forcing fluid down the system instead of trying to vacuum it from the calipers?
I don't think pressure bleeding will do it because of the way the GM actuator works. You need either a Tech2 capable scanner to cycle the ABS when bleeding the brakes or do the old school trick like he describes in the video.
06HHR (Forum Supporter) said:
I don't think pressure bleeding will do it because of the way the actuator works. You need either a Tech2 capable scanner to cycle the ABS when bleeding the brakes or do the old school trick like he describes in the video.
Fair enough. We're going to attack it with more vigor and a Tech2 scanner tonight.
In reply to WonkoTheSane :
Pressure bleeding will probably work in conjunction with the scan tool, but if you can't activate the ABS actuator, the pressure bleed by itself won't work. If you have a pressure bleeder it's worth a try once you get the Tech2, keep you from a leg workout at least.
My caveman process is to gravity bleed, then go out and do a couple ABS stops, then gravity bleed again.
IMO, should not be necessary to actuate the ABS unless you've let the system run dry or are installing an ABS unit that's never had fluid in it.
In reply to AngryCorvair (Forum Supporter) :
You're right, i don't have to actuate the ABS in my Infiniti. It's strictly a late 90's - early 00's GM thing IIRC.
AngryCorvair (Forum Supporter) said:
IMO, should not be necessary to actuate the ABS unless you've let the system run dry or are installing an ABS unit that's never had fluid in it.
Yeah, I've never had to do it before, and I've bled plenty of vehicles with and without ABS, but this isn't acting normally.
Yep, that was it. Pumping vigorously with a tube into a container got us 80% of the way, cycling the ABS got us the rest. After that, letting it settle to get the last little bubbles out did it.
