Odd master cylinder issue

Jon O.

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1972 Chrysler Newport.
Yesterday, some genius pulled out in front of me and I had a hard stop. "BRAKES" idiot light came on for a second then went away. I pulled into a parking lot, and started simple. I checked fluid first. It is a dual master with power disc brakes. When I checked fluid, the reservoir for the rear brakes was almost empty, and the reservoir for the front was way full and spilling out of the top. I then got both of them to the same level. Went on a drive, plenty of stops. When I get home and check it again, the rear reservoir had lowered some, and the front reservoir had raised some. Ive googled this and found nothing at all. They are supposed to be two completely seperate sealed systems, right?
 
MM sounds like the rubber seal that separates these two reservoirs in the bore is bleeding fluid from one to the other under braking. I would say you are up for a rebuild kit, or simpler, put a new master cylinder in.
 
Agreed with Wolfen that it's time to replace.

But - where is the fluid going? You said 1 cavity was almost empty - that needs investigated and repaired. It could be that you have an external leak, and then the hard stop allowed the MC plunger to travel on a 'new' area of the bore, possibly a little rusty, and that allowed a seal to get a little damage and it is now bypassing (a new problem). Drum brakes can probably hold quite a bit of fluid in the dust and grime, might be able to collect some in the axle bearing bore, too. Check the backside of your rear tires to see if any driplines on them?

Also - keep the reservoirs mostly full, so that as lining wears you have sufficient fluid to keep the caliper piston bores full. I could be wrong, but I don't think the fluid level should ever be low enough that one reservoir can empty and fill the other one.
 
I agree with Wolfen. There is a bypass occuring somewhere internally in the cylinder, likely due to degraded seals. Thank the heavens it gave you a warning that didn't end in disaster. I had the same thing happen once, scary. Needless to say, don't drive it until it is fixed!
 
Agreed with Wolfen that it's time to replace.

But - where is the fluid going?
The fluid moves from one chamber, and somehow overfills the other. Aka, it moves from one side of the master to the other. I figured it was time to replace, just looking to learn something.
 
I had this exact problem with 65 Dodge (4 drums) that PO had converted (poorly) to dual master. Finally tracked down a pinch in the hard line down by the front passenger wheel. Replaced the line, and problem completely went away. I'd look for a block in the line somewhere . . .
 
Do you have a power booster? Since the master cylinder is leaking you could be leaking fluid into the power booster. If this is the case the the Power booster along with the master cylinder would have to be rebuilt.
 
The fluid moves from one chamber, and somehow overfills the other. Aka, it moves from one side of the master to the other. I figured it was time to replace, just looking to learn something.
I can agree with that, except I'd say if your rear-brake reservoir was empty and had filled into the front one, your front one was way too low to begin with. You could still have a small seepage somewhere, you could have 2 problems. It's your brakes, so make sure everything is good.

And to clarify - is it the reservoir farther from the booster that went empty?
 
I can agree with that, except I'd say if your rear-brake reservoir was empty and had filled into the front one, your front one was way too low to begin with. You could still have a small seepage somewhere, you could have 2 problems. It's your brakes, so make sure everything is good.

And to clarify - is it the reservoir farther from the booster that went empty?
I last checked brake fluid in september. At the time, both chambers were about 1/4 of an inch from the top. I only checked it recently because the idiot light came on for a second during a hard stop. The chamber farthest away from the booster was almost empty. The bigger chamber, closer to the booster spilled out fluid when I took the top off, much higher than the last time it was checked. Since I added fluid and made sure there wasnt air in the line, it hasnt moved much if at all. It took months for it to happen.
 
Pull the wheels and check the brake cylinders, hoses. I would definitely rand r that master cylinder. Rubber brake lines crack and seep over time and give out when you least expect. Go through the whole system from stem to stern and check. Double check. triple check. Pretty good rule of thumb I run by is "if I have a question about the part , Replace". Unfortunately the power booster can't be bought any more ,so that has to be sent off to be rebuilt. Good luck.
 
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