I spoke too soon, brakes still FUBAR

DAMMIT!
Ok, here we go. Took her out yesterday for a healthier test drive (previous drive was <2 miles), maybe 10 miles and my gosh what an adventure. I though
I was a gonner...........................
Here's what we have now:
1) LOTS of air in the rear lines with virtually no pressure 2) Front calipers will NOT let go after pedal application until 30-45 seconds later as the pressure bleeds off. You can actually hear them "creaking" as the pressure bleeds off.
I likely cooked my wheel bearings as they were smoking when I pulled back into my driveway. I bled another 2 quarts through the rears last night and was still getting air (total of around 4 quarts pumped thru the rears). There is no air in the fronts. There are no leaks in any of the rear lines.
It MUST be the combination valve. It's the only thing that would have the ability to let air into the rears and hold the pressure in the fronts. I think one of the internal seals took a dumper. I picked up a new combo valve today (AC Warehouse 45 minutes north of me) as well as some front wheel bearings (plus another 3 quarts of brake fluid) and am heading out as we speak with a 12-pack, 1/2 a pizza, two packs of smokes and a really bad attitude.
Doc (who is EXTREMELY PISSED OFF at this point in time).
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Doc. Have you checked your steel lines for corrosion holes anywhere. It could be sucking more air in as the fluid goes by.
side

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Have you checked your steel lines for corrosion holes anywhere. It could

I believe this would be very unlikely to happen because these lines can have hundreds of PSI pressure on them.
I have thought about this one for some time. In the days of front and rear drums master cylinders had a residual pressure valve at the Master Cylinder outlet. This was to keep the wheel cylinder under a slight pressure to keep them from sucking air. I don't know how it could happen but could the brake lines to the Mstr Cyl been switched. Most Mstr Cyl have different threads for front and back but not all. I wonder if the fronts being locked up without pedal pressure is due to the residual valve that is/was intended for the rear drums and the rear drums are sucking air because of the lack of residual pressure? If that is not the case it must be the combination valve
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I'm not sure if your bleeding with the pedal or using a suction tool like a MityVac. I think bleeding by operating the pedal is probably better in this situation. And not with the bleeder open and the hose imersed but by opening the bleeder and shutting it as the pedal is operated. I know this takes two people though, but it can dislodge stubborn air. b
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wrote:

This is probably not related but it comes to mind. A guy at work told me of such a problem that he had and it came after he had replaced his master cylinder. Anyway after a pile of hair pulling and another master cylinder it turns out that there are some O-Rings that need to be changed that weren't and they are either inside or underneath (wish I'da paid more attention). Anyway, he took the master sylinder off again and replaced the O-Rings and all is OK now.
I asked the obvious. Why would the O-Rings leak air and not brake fluid and he didn't have an answer.
Probably not related. Just a thought.
-- Regards Gordie
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Do I remember you saying that combo valve was leaking at the nipple a while back? maybe air is sucking in through there getting to the rears...?

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Im just catching up to your problem so I dont know all the details, BUT. Hows the truck running, does it sound like it has a Vacuum leak (Power brake booster could be shot). Or you could have some rust under the bleeder valve preventing an air tight seal, Look for rust on the face of the bleeder valve. How do the wheel cylinders look inside the drum, are they leaking? Is the master cylinder the type with the plastic resevoir? If so hows it look. You have an air leak somewhere, bleeding it out with multiple gallons of brake fluid is not going to help untill you find the leak and seal it. Good luck and dont throw a wrench thru the windshield (it wont do you any good but it will make you feel better)

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Ok I found your old post. Double check your pedal height. It could be that when the power booster was replaced the actuating rod became maladjusted and is pushing the piston in the master cyl. too far in causing your air leak

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I have had a similiar experance with a car kept losing my rear brakes like you I tried everything and what it was in the end was a hair line crack in the flare at the wheel cylinder and it wouldnt leak any fluid but would suck air just a little. Brakes would be good for a day or 2 and I would have to bleed them again. I just started changing lines one at a time till it was good and it was the first line I replaced so I was lucky I couldnt see the crack till I bent the line with some visegrips then it was easy to see. Line was dry no sign of leak at all!
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, Re: I spoke too soon, brakes still FUBAR Sat, Sep 27, 2003, 6:38pm (CDT+1) From: snipped-for-privacy@hotmail.com (John) wrote in......
I have had a similiar experance with a car kept losing my rear brakes like you I tried everything and what it was in the end was a hair line crack in the flare at the wheel cylinder and it wouldnt leak any fluid but would suck air just a little. Brakes would be good for a day or 2 and I would have to bleed them again. I just started changing lines one at a time till it was good and it was the first line I replaced so I was lucky I couldnt see the crack till I bent the line with some visegrips then it was easy to see.
Line was dry no sign of leak at all! ...........
Seen it happen myself.....after tightening lines on cylinder swaps.
good input,
Scrib
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...
I spoke too soon, brakes still FUBAR Group: alt.autos.4x4.chevy-trucks Date: Fri, Sep 26, 2003, 8:42pm (CDT+5) From: snipped-for-privacy@noway.com (Doc)
DAMMIT!
Ok, here we go. Took her out yesterday for a healthier test drive (previous drive was <2 miles), maybe 10 miles and my gosh what an adventure. I though I was a gonner...........................
Here's what we have now:
1) LOTS of air in the rear lines with virtually no pressure
2) Front calipers will NOT let go after pedal application until 30-45 seconds later as the pressure bleeds off. You can actually hear them "creaking" as the pressure bleeds off. I likely cooked my wheel bearings as they were smoking when I pulled back into my driveway. I bled another 2 quarts through the rears last night and was still getting air (total of around 4 quarts pumped thru the rears). There is no air in the fronts. There are no leaks in any of the rear lines.
It MUST be the combination valve. It's the only thing that would have the ability to let air into the rears and hold the pressure in the fronts. I think one of the internal seals took a dumper.
I picked up a new combo valve today (AC Warehouse 45 minutes north of me) as well as some front wheel bearings (plus another 3 quarts of brake fluid) and am heading out as we speak with a 12-pack, 1/2 a pizza, two packs of smokes and a really bad attitude. Doc (who is EXTREMELY PISSED OFF at this point in time).
=============Doctor, with all respect, I know you been fight'n this thing, and you've thrown every thing at it but the sink.... (including....by my count....close to a case of beer)
1) If that thing has good pedal with the rear hose clamped down, you really oughta peel the dust boots back on the wheel cylinders and make sure you don't have "seepage" from the internal seals. I have seen it many, many, many times. Guys bleed and bleed and bleed, without getting all the air out.......or all the air comes out and a good pedal is achieved (for awhile)...and the prob all along is a wheel cylinder "sucking air".
2) The front calipers "MAY" be seizing from doing all the work , getting hot, and expanding the fluid causing the pads to apply. You may also have the master overfilled....not giving the fluid anywhere to go when it gets hot and expands, and again applying the front pads.
3) Here's the deal...........
Post repair, pre-test drive = good pedal feel
Post test drive Hot rotors, pads applied till cool off a) pads doing all the work... b) fluid expansion due to heat... c) overfilled master, not letting hot fluid expand d) binding claipers e) restricted hose (hoses)
Spongy Pedal a) brake fade (fluid boiling) b) air in system Air in both (?) rear cylinders...no air at front a) out of round drums, combined with weak cylinder seals allowing air to "suck" in. b) master cylinder bore "rear" seal failure c) master cylinder reservoir to bore seals bad (at rear seal) d) "pin" hole in "over the axle" steel line (lines) e) bleeder screws not seating at rear cylinders f) poportioning valve front seal bad
I know you've got the kitchen sink ready to install already, and you have the entire NG eagerly wait'n to see if that's the fix.
but............
" if " it isn't..........common sense can negate a majority of the above causes, you being the one working on the truck and knowing what you've replaced, and what the chances are of the respected parts being bad.
a suggestion to a fellow technician of high calliber...........if the sink don't fix it, and you still have the same results after the test drive, as soon as you park it......1) clamp that rear hose and see how the pedal feels 2) pop open a front bleeder and see if the caliper releases on its own.
with all respect to the Doc, the NG is waiting the results of the free brake seminar......
Scrib Abell (brake tech) ~~passes the Doctor a beer~~
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