Bubbles in my master cylinder

[Stout72]

After discovering that contamination virtually destroyed my brake system, I've replaced nearly every major component except for the plumbing. So far I've been unable to bleed all of the air out of the rear brake system. After going through about half a gallon of brake fluid during the bleeding process, I happened to notice that a stream of tiny bubbles would rise from the forward port in the bottom of the master cylinder reservior for the rear brakes each time my assistant slowly depressed the brake pedal. This is a brand new Bendix unit that I carefully (so I thought) bench bled on the truck.

Thinking that either the new master cylinder was bad or I screwed up in the bench bleeding process, I returned the master cylinder for a replacement... another new Bendix. This time I carefully bench bled it, while it was clamped in a vise, according to the mfg instructions. Same result. So I must be doing something wrong.

Suggestions or words of wisdom greatly appreciated. Otherwise, It's going to the brake doctor.

[barn9]

You say that you got all the air out of the rear brakes, but what about the front? From the info, I'd say there is a problem in the front brakes somewhere. Good luck!

[Stout72]

No, no. The front is fine. The calipers were easy to bleed. Sorry if I was misleading, when I said rear system, I meant rear brakes. It's the rear brake system I can't seem to get all of the air out of.

[cableguy0]

try to gravity bleed the rears open the bleeders and just leave it sit.once fluid starts to flow out the rears do the normal bleedin process

[Stout72]

I tried that too, Cableguy. I let the rears gravity bleed for quite a while, until I had a good dribble out of the bleeders. It's almost like the master cylinder is manufacturing air bubbles. This is my second BRAND NEW Bendix master. Can it be sucking air in from somewhere? I've got no brake fluid leaks that I can find.

[barn9]

Sorry about the first answer, I guess I wasn't reading close enough. You must have had a lot of air in those lines I'm guessing. Either keep doing the conventional bleed untill the air is out or maybe take the lines loose untill you get all fluid, then hook them back up and do the regular bleed. Someone may very well know more than me about this, but that is probably what I would do next. Good luck!

[Stout72]

Thanks, barn. The thing that troubles me the most is that it appears the master cylinder is pumping air into the system when the brake pedal is depressed. I'm sure that shouldn't be happening. Until I cure that problem, I'm 99.9% sure I'll never get all of the air out of the system... I reasonably sure I'll be adding air to the system. And all of that doesn't make the truck stop. It doesn't stop too well now as it is, and I've already got about a grand invested in new parts so far...

[DennyB]

Do you have the cap on the reservior (tight) when bleeding?

[1-PU70]

When you bench blead the master did you get all the air out. What I mean is did it stop making bubles. If not your probably doining it to fast. Bench bleed it very slowly or you will never get all the air out. The same goes for bleeding the brakes on the truck. When your helper is appling presure to the pedal and you open a bleeder your helper muust depress the bedal very slowly or you create bubles and will never get all the air out.

[Duallie]

A geyser is normal when depressing the pedal quickly. Air bubbles are not normal.
You aren't using an aluminum Quick Take Up master cylinder, are you?

[Stout72]

Thanks for your feedback in helping me to diagnose this problem.

Denny, I didn't have the cap on tight while bleeding, ie. didn't have the bales over the cap. I didn't think it was an issue, since the ports are on the bottom of the reservoirs, below the surface of the brake fluid. It made it easier keeping the master cylinder topped up with fluid, since I spent a lot of time running back and forth between the rear wheel cylinders and the master cylinder. I didn't even let it run close to dry, was very careful about that.

PU70, I was certain I got all of the air out while bench bleeding. Went by the numbers, according to the instructions in the box. In the bench vise, I stroked it very slowly, pushing the piston in no more than an inch, and waiting about 15 seconds between strokes to prevent foaming the fluid. Took forever. Went from the vise to the truck with it full of fluid and the bleeder tubes still installed.

Dualie, It's an OEM Bendix, cast iron, I believe, not aluminum. Heavy sucker. I get the little geysers when pressing the pedal, even slowly. But that one port is streaming up bubbles.
Frustrating!

Thanks for the comments so far. A few more and maybe we'll find the cause.

[Stout72]

ttt

[cacauba]

i'm stupped on the problem at hand, but to make your life easier you can get a set of "speedbleeders". they come in pairs and are like $12.00 but they have a check valve in them and they replace your bleeder screws. i.e. no helper. i know it doesn't help you in your current situation but it will make someone elses foot feel better. if your interested in can get you some info. stdcac28@shsu.edu

iv'e never used one but what if you get ahold of one of those vaccum bleeders so that your sucking fluid out instead of puching it out. just a thought but in don't know. good luck

chris

[Erl]

Stout72,
I am having simalar problems and have decided to purchase a vacuum type bleeding device. It hooks to the slave cyl and pulls the fluid through and into a reservor. I will be trying it out soon (coming UPS). They have them at Sears too ($30). I will let you know how it works.

[Stout72]

K5er, I've heard of the speed bleeders, saw them in the Summit catalog, and my local auto parts shop now carries them. They'd be great for shutting off before air creeps back into the slaves, but you still need a pair of eyes to watch for when the bubles are gone. Might be a good time to try then out since I'm not done with the bleeding process.

Erl, I considered the vacuum bleeder, but at the time I was a little flush for cash (I know... it's only $30 or so). I'll probably end up trying that next. Yeah, let me know how it goes.

A friend suggested having it powerbled at a shop. I suppose this is my next-to-last option.

Thanks for your suggestions.

[cacauba]

hey stout I just put a clear hose and used a coke bottel as a catch that way you can just lean your head out the door and see when you can stop.

chris

[Stout72]

Good News! Mostly...

I tinkered with it a bit yesterday evening and it seems I managed to get the air out of the rear brake system. All I did was gravity bleed the system (once again), this time by cracking each fitting from the master cyl to the prop valve, to the trailer brake unit, and finally the bleeder screws on the rr wheel cylinders, until I got a good steady drip from each one. Now the brake pedal acts nearly the way it should, by beginning to stop the truck at about half the pedal's stroke (I'd like it to be a little sooner). Now the truck stops without having to stand on the pedal with both feet.

I drove it around the block a bit experimenting with planned stops and panic stops. Worked OK, but I couldn't get the brakes to lock up. I may still have a little air in there somewhere. I drove it to work today for the first time in a couple of months, and the brake performed satisfactorily. After a few trips I'll bleed 'er again just to see if I've got air entering the system from somewhere, because...

I still get that small stream of bubbles in the rear brake reservoir when I step on the brake pedal. I know that's not right. I'm not sure if that can add air to the system or just affect braking efficiency, but I'm gonna run it that way to see if it eventually works the air out.

At any rate, I'm back on the road!

Thanks for all of your input.

Erl, I'll be watching posts for your progress... good luck!