Brakes...bubbles in the Master Cylinder???

[skydiver]

Hey Everyone, I need a little brake advice. I have bubbles coming from the bottom of the rear reservoir in my master cylinder when I press the brake down. As you can imagine that because of this the rear brakes dont work at all. Can anyone tell me why this is happening? Is it a bad seal in the booster? The truck is a 72 c10 with 350 / 350 / 4 bbl carb with disc brakes in front and drums in rear.

Skydiver
1972 C10

[Classic Heartbeat]

Are you loosing fluid? WES

[Classic Heartbeat]

Or did you run tha master cylinder dry?

[skydiver]

I have check for leaks and found nothing. I have also bled the front and rear lines and there is no air in the lines. The only thing left that I can see is the master cylinder bubbling air. Only the rear reservoir does it, the front pushes fluid and the brakes work great.

skydiver

[skydiver]

I emptied the master cylinder last fall to get all the rust out of it and then filled it back up. It was dry then but I have bled the lines since then.

[truckdude239]

if the rear brakes dont work then something is wrong. may it be a wheel cylinder leaking in back or the mc is bad. there is noway for the booster to leak into the mc. you have air int he lines that is getting in there some how

[Stocker]

Originally Posted by skydiver

The only thing left that I can see is the master cylinder bubbling air. Only the rear reservoir does it, the front pushes fluid and the brakes work great.

My fuzzy memory thinks that on a disc-brake equipped '72, the front reservoir is for the rear brakes, and the rear serves the front brakes.... No?

[Steelawork'n]

I'm sure you already know this, but when you remove the mc for work or replacement, you can bench bleed it before you reinstall it. Saves time.

FYI...With the lid off the mc cannot proper pressurize the system. You're really not supposed to pump the brakes with the mc lid off. You should put the lid back on and bleed the mc and then bleed the rear and front brakes.

[Longhorn Man]

your master cylinder may well be leaking into the booster.
They are very cheap to replace from any parts store.

[skydiver]

Can anyone confirm if the front reservoir is for the rear brakes?

I really dont think it is air coming from the brakes up through the lines to the mc b/c there are no leaks anywhere. When I push the brake the bubbles come from the bottom of the mc and then when the brake is released the bubbles also come again. Like there is a seal that is not sealing and allowing air into the mc.

[Steelawork'n]

Originally Posted by skydiver

Can anyone confirm if the front reservoir is for the rear brakes?

I really dont think it is air coming from the brakes up through the lines to the mc b/c there are no leaks anywhere. When I push the brake the bubbles come from the bottom of the mc and then when the brake is released the bubbles also come again. Like there is a seal that is not sealing and allowing air into the mc.

Yeah, it's cause you got the lid off the master cylinder. It's not pressurizing properly. Brakes are supposed to be a closed system which means when you pump the brakes with the lid off, you allow air into the lines and thus the closed system is opened and no longer will it work.

[copywritten]

I thought that it was okay to bleed brakes with the lid off as long as the fluid level doesnt get to low. How can air enter through fluid? Air bubbles will eventually move to the highest point in a brake system (as long as there is enogh fluid in the master cylinder)

[LONGHAIR]

Just a vacuum pump. That thing sucked.

Now that is funny.........

[jayharrell]

Originally Posted by LONGHAIR

Now that is funny.........

i was wondering if anyone would catch that...

[89 crew]

A problem I ran into a while back was a rebuilt master cylinder. They had bead blasted it and eaten away the female connection where the brake lines screw into it. It would constantly pull air back in and it didn't leak. once I returned it and replaced with a different one all was good.

[70GMCer]

The front of the mc is for the front brakes. At least mine is like that, and everything is brand new.

[LONGHAIR]

Yeah, it's cause you got the lid off the master cylinder. It's not pressurizing properly. Brakes are supposed to be a closed system which means when you pump the brakes with the lid off, you allow air into the lines and thus the closed system is opened and no longer will it work.

That's not really the case. The system is "closed" because the fluid is hygroscopic....(readily absorbing and retaining water) This not only destroys it's heat transfer properties, but it causes rust with-in the system. This is why they always tell you to use fresh fluid from a sealed container.
The cap is "vented" above the seal to allow for fluid drop in the system. As the pads wear (in a disc system) the caliper holds more fluid, taking it from the master cylinder. The gasket/seal in the cap is like a diaphragm, allowing the fluid to move w/o being "open" to the outside.
The master cylinder is not "pressurized" in the reservoir. It gets some "back-flow" when the brake shoes are pulled back by the springs. That is why you don't see much movement in the front reservoir of a disc equipped truck. The fluid really doesn't back-flow much.
The pressure is created by the piston. When the piston is fully retracted, it exposes it's bore to the tiny hole in the bottom of the reservoir. This allows the drum's wheels cylinders to push the fluid back, when you release the pedal. It also allows the discs to increase their fluid volume as the pads wear.

[skydiver]

Longhair, so would air be entering the rear reservoir from the booster or the lines?

[og350]

i have a 71 with front disk and rear drum the side closest to the cab is the front and the side away from the cab is the rear on mine. i had bubbles in mine with the cap off too. i just bled everything and the prop. valve! that helped alot. good luck!

[jayharrell]

I had the exact same problem about a month ago with my MC and rear right cylinder swap. I had the ol lady pump the brakes and i could see one little bubble pop up from the rear reservoir piston whole. It turned out to be just a little air in the lines.

Mine is LWB and the rear lines were completely dry.

So it took ALOT of bleeding and pumping, letting it sit, Driving around to shake and rattle what air that was left in the system out. I tried one of those one man cheepo bleed systems. Just a vacuum pump. That thing sucked.

After driving it a day and then re-bleeding a final time the bubbles went away and the pedal wasn't squishy any more.

[skydiver]

Brakes still a little soft. I am going to try to bleed them again tomorrow. Cant believe how how bleeding is needed. Good thing the wife is patient.

[jayharrell]

LOL.. watch out, if she pumps too much she'll have a strong left leg able to kick you in the butt when needed.

Let us know how it turns out.

[LostMy65]

I used the forum search - typing key words: 'bubbles top master cylinder' and this thread popped up. Mine is doing the same.
It's a brand new mc and I have went thru quite a bit of effort to bleed the brakes. Still getting little bubbles trickling up from the larger reservoir. 66 chevy with 79 donor drivetrain.
79 mc, 79 discs front, 79 drums in back. I can get the front to gravity bleed, but not the rear.
I've used a mityvac pump, and I can get it to draw from the front, but not the rear.
However, if I run clear tubing from the rear bleeder valves into a clear jar half full of fluid, I can get it to bleed by pumping the brakes.
I guess I'll give it a few days and try bleeding again. It's just odd. When I bled the brakes when I switched out to the 14 bolt, all went fine, and I had solid brakes with little effort. This time, putting discs up front, with a new mc, I'm struggling to get a good bleed.
Like I said, I'll give a couple days and then try again.
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[jocko]

It sure sounds (to me) like the master was never bled after you drained it to get the rust out. You can bleed the brakes at the bleeder valves on the wheels all you want and it won't make up for not bleeding the master. If that was never done when you drained the master, that is the first thing I would do - and then go bleed at the bleeder screws again. (on a normal brake job, you shouldn't have to bleed the master because it never went "dry", but if you purposely drained it or installed a new one, it needs to be bled). "Bench" bleeding the master cylinder can be done with the master on the truck. It can (ok, it will) be messy, so make sure you throw some towels and stuff under the master to protect any engine compartment paint if you're worried about it. Really just need 2 fittings/lines to run from the outlet ports up into the master fluid (submerged) and pump the brakes about 1 inch (only), and SLOWly, until you no longer see the bubbles rise in either reservoir. Then reinstall the lines, top off the master, replace the cap, and then re-bleed at the wheels starting with the left-rear, right-rear, right-front, then left-front. (sorry, I know you know how to bleed the brakes at the bleeders).

Anyhoo, anytime the master is emptied, it requires a master cylinder bleeding first before the wheels can be bled for all to be truly "airless".

As for which res is front/back - I can't help there - I know that on the drum/drum setups, the front reservoir is for the rear brakes and vice versa because the MC's internal design provides for the forward proportioning (i.e. no prop valve required on a drum/drum). On a disk/drum, generally speaking, the larger reservoir is for the disk brakes and the smaller res is for the drums. But, if you have same size reservoirs on a disk drum setup (and the proportioning valve provides all the proportioning) then I don't know which is which. My gut feeling is that most disk/drum setups are "opposite" of the drum/drum arrangement - i.e. most disk/drum setups should be setup just like 70 GMCer said - front(usually larger) reservoir is for front brakes and rear (usually smaller) reservoir is for the rear brakes.
Hope that helps.