Air...

[kwilson]

OK, so a few weeks ago, I posted in the master cylinder thread that we replaced some parts and my brakes were better. Well, I was wrong, sorry. After driving it for a while, I noticed that the problem continued, so I took it to another, very well trusted mechanic. After looking over the system, he agreed with the replacement of the parts that the first mechanic did ( I felt good about that decision), then we started some research. I got in the truck and applied the brakes, the fronts held tight, but there was no pressure whatsoever on the rear brakes (they should have been locked up tight). Next we started to check for pressure throught the system, to the rear wheels, since the fronts were working fine. At the block that is on the frame rails over the drivers side rear wheel, I hit the brakes, and he opened the bleeder valve, we got fluid, good. Next we went to the right rear wheel, he opened the bleeder, I hit the brakes, nothing but air!! same at the left rear wheel!! SO, we bled the rear brakes. On the test drive I nearly launched myself through the windshield, the brakes worked like new!

Now the bad news, 2 weeks later, and the brake pedal is going away again, just like it did originally. I've had my truck on the lifts, cleaned all the lines, and all the connectors, I'm not losing any fluid, yet I'm getting air in my brakes.

Any suggestions, anyone, help!! I do not want to have to bleed my brakes every 2 weeks. Thankfully, this week I have to rotate tires, so it will be easy to bleed (plus I have a vacuum bleeder)

IDEAS???

[dyogim]

If air is in your brake lines and you know for sure, there was fluid through out the system prior, there has to be some evidence of fluid leaking somewhere. Have you checked the proportioning valve above the spare tire? There's a fitting/valve up there to bleed the system. Check to see if there's any fluid in that area and check to see if the fitting/valve is closed all the way. Otherwise, all I can suggest is to start from each corner of the brakes and check the brake lines through out.

[kwilson]

The other day while we had it on the lift we followed each line individually looking for leaks and found none. The only proportioning valve I saw was actually over the drivers side rear wheel, but since I will be under the truck again this weekend, I will double check everything and keep you posted. Did I mention that the fluid level in the master cylinder has not changed and there has been no discernable fluid loss?

[dyogim]

Originally Posted by kwilson

The other day while we had it on the lift we followed each line individually looking for leaks and found none. The only proportioning valve I saw was actually over the drivers side rear wheel, but since I will be under the truck again this weekend, I will double check everything and keep you posted. Did I mention that the fluid level in the master cylinder has not changed and there has been no discernable fluid loss?

That's the one I was referring to. In anycase, can't explain it other than, there's air still trapped in the system.

Out of curiosity, when you had the brakes bled, were the rears done before the fronts. I've been told, you should bleed brakes starting with the closest brake to the reservoir. Then, work your way out.

[apimpdad]

Originally Posted by dyogim

That's the one I was referring to. In anycase, can't explain it other than, there's air still trapped in the system.

Out of curiosity, when you had the brakes bled, were the rears done before the fronts. I've been told, you should bleed brakes starting with the closest brake to the reservoir. Then, work your way out.

i have always heard that you start with the tire furthest away from the master cylinder!?!?!?!? I hate to say that and cause more confusion but does anyone know the correct way?

[kwilson]

when we bled them, we only bled the rears because the fronts seemed to be working fine, so we didn't touch them. The first thing we did was check the pressure by pumping the brakes to increase pressure, then opened the bleeder at the box over the drivers side rear wheel, and there was no air there, so our next step was to do the passanger side rear, and there was so much air that when I pumped the brake, and we opened the bleeder I could hear it from in the truck (with the windows open of course). After we finished that one we did the passanger side rear. They felt like they came right from the factory after that, so we stopped. I had heard, and it has been confirmed by several mechanic friends to do the farthest from the master cylinder first. When I do them this weekend, I will be doing all 4, starting with the rears. What concerns me now, because I now know there's air in the system, without being able to find any obvious leaks in the system, where the hell is it coming from? I'm thinking either master cylinder or the valve in the rear. I will be all over it this weekend and will keep you updated.

Thanks for all your help and support.

[ICON]

Proper way to bleed brakes is to start w/the farthest and work your way closer to the master cylinder so you start w/passenger rear and end at driver's front. You didn't say but do you have ABS??

On some vehicles you need a scanner to open the ABS valve in order to get all the air out. It's possible that if you do have ABS maybe that's where the air is coming from. Either you're getting it all out now or it getting in somehow. And that's where i'm scratching my head like you. What you might also want to try is jacking up the rear end so that hopefully any air will migrate to the rear lines and bleeding them will get all the air out. I have to put a new master cylinder on my buddy's quad and for the life of me i couldn't get it to bleed all the air out and build pressure. I spend about 3 hours trying and it's a super simply system plus i tried every trick in the book from any mechanic i could find. I finally gave it back to him and told him to drive for a while to see if the air will eventually make it to the top of the reservoir.

[dyogim]

Originally Posted by ICON

Proper way to bleed brakes is to start w/the farthest and work your way closer to the master cylinder so you start w/passenger rear and end at driver's front. You didn't say but do you have ABS??

Thanks for clarifying! I had not completed a full brake flush in a long while and couldn't remember if it was closest first or farthest first.

[kwilson]

Yes, the truck does have ABS, but we didn't have any problems bleeding them once before. So, when I do bleed them, I will be doing Pass. rear, driver rear, Pass. Front, driver front. Also, I will be using a vacuum bleeder, I've used it in the past and love the thing. The proper tools make life so much easier.

Again, thanks to everyone for their help, you guys are great. Beers are on me when you come to Buffalo.

[MEvang]

I don't think the way you are bleeding the system has anything to do with this. Matter of fact I believe you got all the air out when you bleed it.
My bet is something is heating up your brake lines. Like an exhaust leak. I have seen this a few times. If exhaust can leak anywhere near a brake line it will slowly heat up the fluid inside. At some point the fluid will boil and turn to air.
In one case I saw there was a small hole in a tail pipe that no one new was there. Two brake shops and a dealer never found it. It blew hot exhaust directly on a brake line. It took about 3 days and the pedal would go soft and air was always found in the wheel feed by this line.
It's a long shot but worth a very close look.
Mike

[kwilson]

I think I have an answer. today I pulled the master cylinder from the power booster, and the back of the master cylinder is leaking fluid into the powerbooster. THere's my leak, the rear seal of the master cylinder.

[MEvang]

That will do it!
Make sure you get any fluid out of that booster or it can ruin it.
Mike