Brake bleeding trouble... still
 

Some of you may recall my other thread from a few days ago: http://67-72chevytrucks.com/vboard/s...d.php?t=527289

Still have a soft pedal and getting air in the rear lines, but now have more info. The MC seems good -- with the rear line disconnected, the pedal is nice & firm. Hook up the rear line and the pedal goes soft about 1/2 way to the floor.

This tells me the problem is in the rear brake lines, which I did nothing with when installing the new rear brake shoes. Trapped air? I have literally pumped several 12-ounce cans of fluid (several hundred brake-pedal strokes) through the rear bleeders with no change -- still get lots of air bubbles coming out with the fluid.

I have tried teflon tape on the bleeder threads to help insure no air is drawn in past the threads. And I got a bleeder hose with a one-way check valve so I can pump the pedal continuously without opening & closing the bleeder valves.

Still no joy. I'm about ready to take it to a shop and have it power-bled, but I just hate to. This is a simple system and I should be able to bleed it.

Anyone got any new ideas?

Re: Brake bleeding trouble... still
 

Your getting air into the system from some where. Power bleeding will not solve this unless the tech can I'd the issue and fix it, which maybe your best bet at this time.
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Re: Brake bleeding trouble... still
 

Yes, definitely getting air in. Seems like pedal pressure would drive fluid out wherever the air leak is, but I can't find any sign of a fluid leak....

Re: Brake bleeding trouble... still
 

I'd start over.

Bench bleed the MC, install. Use a clear tube connected to bleeder valve.

- Starting with the furthest line away from MC
-- Pump pedal 5 or so times. Pressing the pedal as far down it'll go each time.
--- Hold the pedal all the way down
---- Release the bleeder valve just a little.
----- Do not allow pedal to bottom out while valve is opened.
------ Stop short of that, then repeat until air bubbles are gone.
------- Ensure lines/valves are tightened good.
-------- Repeat process, next furthest away.

Do all 4
Blowing out the lines, you introduced a lot of air to begin with.
If you still can't get rid of the air, you have a leak somewhere letting air in.

Re: Brake bleeding trouble... still
 

There are some connections that you can look at. Work yourself backwards along the brake line until you see a connection. If they check good, check the connections at the rear rubber brake hose on top of the rear axle. If the hose hasn't been replaced, it needs to be.

Another possiblity is to find some way to block the lines with plugs starting at each rear wheel to see which side firms up the pedal.

Re: Brake bleeding trouble... still
 

Thanks for the replies so far, guys.


litew8: I did all of those things, and then I did them all again -- some more than others. Got a few bubbles out of the MC the second time around, so I bled the snot out of it. It's good now, for sure. Only thing you wrote that I couldn't do was 'repeat until air bubbles are gone'. ;)


68gmsee, I didn't replace the rear hose yet simply because I'm already over budget and it's a spendy little devil. I may bite the bullet and do it anyway. Those connections have not been touched, probably since the truck was built, and there's no sign of leaks.

Your idea of isolating the problem gave me another thought. Since I have a hard pedal with the rear line off of the MC, I'm going to try disconnecting the rear line from the prop. valve to make sure it's good. Maybe I can find other connections to cap off and pinpoint the problem.
I haven't given up yet -- I'm determined to fix this bugger without taking it to a shop.

Re: Brake bleeding trouble... still
 

ok if you have shoes in the rear make sure they are adjusted out far enough.if they are not you will have to push more fluid to the rear to get a feedback at the pedal hence the pedal going further down..the rear shoes should be adjusted out until you can barely feel friction if the wheels are mounted if they are not and you have new drums then adjust them until you can barely slide the drum on. hope this helps.over the years this has been the solution many times in my shop with customers doing their own brakes.

Re: Brake bleeding trouble... still
 

just re read the first post something else that will cause air in the rear lines but not show a leak is a poor seal on the resovoir on the rear mc piston.their is no pressure there so it wont show/leak when you apply pressure but it can suck in air under vacuum again hope this helps

Re: Brake bleeding trouble... still
 

The reason I recommend replacing all the rubber brake hoses is that some of us have had problems with the isides collapsing and swelling -in some cases preventing fluid from passing through. The problem is that you cannot see the failure from the outside.

Auto Zone front are about $19 and rear is about $13.

Re: Brake bleeding trouble... still
 

i do agree with the hose issue being a potential problem but i like to alleviate all other issues first

Re: Brake bleeding trouble... still
 

I put new rear shoes on and adjusted them until they just start to touch the drums. Had a lot of clean-up on the adjusters... I don't think they've worked in a long time!

I don't quite understand this, could you please clarify? The front reservoir feeds the rear brakes; I don't know if air can even reach it? It would have to somehow get past the rear reservoir (which feeds the front brakes, which are good).

Well, for the time being anyway, there's plenty of fluid flowing to both rear brakes, so it's not collapsed (yet). My local NAPA wants $49 for that rear hose, and when I looked online at Autozone wants $49, and Oreilly's wants $48 (both stores are an hour from me). And yes, I did replace both front hoses.

edit: checked online hose prices again

Re: Brake bleeding trouble... still
 

Replace the MC, go from there. If it persists, you have air entering the system from somewhere.

Re: Brake bleeding trouble... still
 

post a pick of you master and res i want to see which one you are useing

Re: Brake bleeding trouble... still
 

I thought the MC was good b/c I have a hard pedal when I disco'd the brake line and plugged the port.... it only goes soft when I reconnect the line. Maybe bad anyway, I dunno at this point... :confused:

Sorry I can't post a pic, but I'll look around for one like it and post that. It's the stock OEM setup on my '72 K20.

edit: I found a pic that's a bit hard to see but it looks like the same setup I have.
http://67-72chevytrucks.com/vboard/a...1&d=1332789134

Re: Brake bleeding trouble... still
 

I'd take the tape off the bleeders to start; they bottom out to seal. You need to keep the pin in the end of the prop valve from moving while bleeding; it may want to push out. If you have a vaccum pump they work great to suck fluid to the rear to start the process; I think you have air in the line to the back and its jerking you around going from side to side as you bleed. This has got me twice! I used a cheapy hand pump to draw fluid first to the passenger rear, then drivers (make sure system is sealed at the 3 other brakes). Once the line is free of air, and you're sure the prop valve stays centered, they'll bleed out quick.

My Blazer didn't have enough brake to stop the rear wheels in drive at idle on jack stands! Seems like I used 35 gallons of fluid and still had air in back; it took a while with the hand pump but I eventually got some BIG bubbles that must've been moving back and forth. I went through the SAME thing on a customers '68 GMC too.

Re: Brake bleeding trouble... still
 

Thanks, Tony. The tape was suggested to help prevent air from entering past the threads during bleeding. So far the pin seems to be in place -- it may have moved but I pushed hard on the pedal several times which should center the prop valve. I hooked up the wire tonight and the light is out. Unfortunately I don't have a vacuum pump, maybe I can borrow one or I'd have to buy one.

Re: Brake bleeding trouble... still
 

Try bleeding the "T" where the rear lines split to go to either side. Found lots of air there when I did mine. Just my $ .02

Re: Brake bleeding trouble... still
 

Stocker you might be money and time ahead to take it to a shop who has a pressure bleeder.
Quick and easy. Or you may be able to rent one from your local parts store.

Outrageous prices on those hoses in your area too.

Re: Brake bleeding trouble... still
 

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Re: Brake bleeding trouble... still
 

Attempt Gravity Bleeding first (after removing that tape). Go to the plumbing section at Home Depot or Lowes and get the smallest diameter 12" section of clear tubing that will fit over the bleeder. Slip it on and let it drain without letting the MC go dry. If its a bad MC you can get the air out and it will still have a soft pedal. Start at the LR corner of the truck because it is actually furthest from the MC. If or when you get the bubbles out of the LR move to the RR, you should only have to drain that one for a minute to clear from the "T" out. If you can't get the bubbles out you are taking in air along the line to the rear. I doubt this is the case because if you take in air you will let out fluid and a wet spot would indicate your leak. If you still have a soft pedal put an MC on it. RockAuto.com has both the hose and MC you are looking for for less than $20.00 each. My money is on a bad MC.

Re: Brake bleeding trouble... still
 

Subscribed.

Just be advised that should you decide to replace that rear hose, depending on what you have, the replacement hose is likely to be at least 3 " longer than original. My original measures 14.5" overall length ('72 C-10, SWB Coils). The replacement that I ordered (Wagner) was 20.5" long! I'm sending it back for the Centric hose which, at 17.5" is the closest I could find among any of RockAuto's vendors, or local autoparts stores.

Re: Brake bleeding trouble... still
 

Spent hours yesterday gravity bleeding (drip, drip, drip) and didn't notice any bubbles coming out but it's such a slooooww process.... and afterwards, the pedal was still soft. Bled again by pumping the pedal and cracking the bleeders and still got lots of air.

This morning I installed the 2nd new MC (bench bled very thoroughly), still getting lots of air from the rear bleeders after pouring a few more cans of fluid through the system. Crawled underneath with a flashlight looking for any sign of leaks or damage -- nothing. Remember I never touched the rear lines beyond the prop. valve, and I replaced the rear shoes but not the wheel cylinders.

I have made no progress at all so I'm headed out now to talk with a shop or two. Just have to get the truck there with no rear brakes, and I mean none at all. With the pedal held down firmly, I can rotate both rear drums easily by hand. At least the parking brake works....

Re: Brake bleeding trouble... still
 

It's not the m/c and gravity bleeding won't do it; gravity won't pull air down. The air in the rear lines will rise as the fluid drains around it. The pump I use to start the procedure cost me 12 bucks, and I'm in Canada so it should be about $2.65 there.

Re: Brake bleeding trouble... still
 

Sorry, I came in late in the game. I read your thread at work while waiting on other things. I'd be interested to see what happens with the pressure bleeding. If you have air in the system it will bleed out unless new air is being introduced into the system. You should have seen improvements if you were bleeding air out. If you do go with the pressure bleeding have them hold pressure on the system for a while before cracking a bleeder to see if they can find a leak.

Re: Brake bleeding trouble... still
 

I have ran into this air problem before on a truck when the piston pins did not get installed into the brake shoes correctly.

Sucks pulling 3/4 drums back off, but maybe a picture of each side would show something that got missed?

Re: Brake bleeding trouble... still
 

Wish I could post pics, but have never figured it out (I am technologically challenged! :lol:). I was thinking of that possibility a couple hours ago and pulled the drums off just to check. Everything looked good to me and both sides looked the same. (I did one side at a time, also had a manual for reference).

Now I'm wondering if a bad wheel cylinder could somehow cause the problem....

I also just tried opening the rear hose so I could block off the wheel cylinders and then test. Could not break the connection and it's soaking in penetrating fluid right now.

Re: Brake bleeding trouble... still
 

I went through all of this 3 or 4 years ago after I bought a BRAND NEW master cylinder from Advance Auto. Replaced everything, bled the brakes numerous times and still no luck. Gave up and had the local Midas power bleed the brakes and a block from their shop the brake light came on again and only had front brakes. The easy fix was to go to NAPA and get a re manufactured master cylinder, install and bleed, which was pud (as it should be) with a good master cylinder.
The moral of my story is new Chinese or Mexican master cylinders are JUNK.
Good luck to you.

Jeff

Re: Brake bleeding trouble... still
 

This may be silly, but; did the brakes work before all this?

Re: Brake bleeding trouble... still
 

Excellent question! Brakes worked very well for several years -- truck was my DD until I retired 3 years ago. Short commute so typically driven just a couple thousand miles a year. Lately it has been mostly sitting until I need to haul something like lumber or firewood.

A couple months ago I did my spring woodcutting and noticed there was a problem. The pedal was soft & sunk near the floor (but not every time). Pumping the pedal 2 or 3 times would usually firm it up (but not always).

I figured it was probably a bad MC and a few others agreed that was probably it. When I pulled off the old MC, fluid poured out of the back of it, so it had been leaking past the seals.

That would have been the end of the project but I wanted to purge all the old fluid and start with all new, so I blew out the lines. And I had been wanting to replace the OEM front hoses so I did that.... then figured it would be a good time to check the rear shoes. They weren't too bad but had been installed wrong, with the long shoe in front, so I got new shoes and cleaned up the adjusters.

It all seemed good until I tried bleeding the system.....


More info than you asked for, sorry. Short answer is yes, they worked well until about 2 or 3 months ago.

Re: Brake bleeding trouble... still
 

I have nothing more to offer I guess, you're on the money with the m/c swap and I've been through the rest of these symptoms to the letter-including the frustration and google searches. Get the air out of the rear lines and your good to go.

Re: Brake bleeding trouble... still
 

Thanks to everyone who has replied! The problem persists but this thread has been very gratifying for me. The interest shown and suggestions offered by fellow board members has been terrific. If my front brakes will work with such a soft pedal, it's going to the shop soon -- and I will update this thread when the problem is found & fixed.

Re: Brake bleeding trouble... still
 

have you tried those speed bleeders, the ones with a ball and spring in them. Seems like if your still getting air out, and not pulling air in, your on the right path...hate to see it go to the shop, post #6

good luck

Re: Brake bleeding trouble... still
 

^^ Yes, purchased for this project.... lots of air still coming out so it must be getting sucked in somewhere. Nobody hates to see it go to the shop more than I do, but I am no longer making progress -- so it went to the shop this morning. Can't wait to see what they find.

Re: Brake bleeding trouble... still
 

weird...curious to see the answer, thanks for sharing. Valuable info. on here

Re: Brake bleeding trouble... still
 

same here would like to know.

Re: Brake bleeding trouble... still
 

so.... where was the air coming from? :smoke:

Re: Brake bleeding trouble... still
 

Bump for curiosity's sake.

Re: Brake bleeding trouble... still
 

The shop called this afternoon but we were gone, and got home just after they closed. Drove by the shop and the rear brakes are apart and both wheel cylinders are removed. That may be a clue.... :)

I'll call them as soon as they open tomorrow.

Re: Brake bleeding trouble... still
 

stocker: did you blow out the lines with air pressure before you put the shoes on the rear, or after, the wheel cylinders may have a piston or seal that is almost out or cocked a little, this may let air in if so, i would look real close at the rear wheel cyl. and make sure everything is ok back there, looks like you have tried everything else, would not hurt to look at this, may save more $$$ on fluid...hope you the best on this i hate when we get snake bit and can't figure things out.....

Re: Brake bleeding trouble... still
 

Both wheel cylinders removed, jives with -

Brake cylinder,
http://lh3.googleusercontent.com/pub...DTWCO9vNfFpxCA

"As the piston retracts after the brake is released, it no longer has that outward force on the cups and there may be a
brief moment of negative pressure inside the slave cylinder. This is the time when air might sneak past the cup seals even
though no outward signs of fluid leakage is present. This applies to rear drum brakes only."

You had put teflon tape on the bleeder valve threads to ensure no air passed, but you couldn't have
done anything (short of replacing/rebuilding the cylinder) to have known the guts were on their way to Chevy heaven. :)