Question: '81 Master Cylinder

[ladams]

A couple of months back I replaced the MC, and all four callipers on my '81.
Since then, I have not had very good pedal and twice I have lost the rear brakes because of an empty reservoir.
I thought the MC was leaking out the back but have found that it is leaking around the cover gasket.
My questions:
1. Other than a bad seal in the top, is there any reason why it would leak? (ie. back pressure being caused by something that would make a normally tight seal leak?)
2. Assuming it is a bad seal, could this be the cause of the mushy pedal?
3. Could the problem just be bad MC in general? (it is rebuilt)

I'm sure someone has some answers.
Thanks

 

[wishuwerehere82]

If you notice the M/C seal, there is an accordion expansion bladder in each of the reservoirs. They will expand downward to keep air from entering the reservoir as the brake fluid is pumped out of the reservoir into the brake system. If you are losing fluid in either the front or rear brakes, those bladders will be fully extended in whichever cylinder is leaking.
If you are losing fluid from the reservoir, and those bladders are not extending, then the seal is leaking.
Check the vent hole under the rear of the M/C for traces of fluid, the shaft seal could be leaking.
If the bladders are extending as you run low on fluid, then the seal is good and your leak is farther down the line toward the calipers. Possibly a bad proportioning valve or flex line.

In short, any time you run out of fluid in the reservoir, you are going to introduce air in the system and will need to re-bleed the entire system from scratch.

You can wrap some paper towels around the M/C to help trace the source of the leak too. If it's leaking there it will be obvious in short time.

 

[ladams]

If you notice the M/C seal, there is an accordion expansion bladder in each of the reservoirs. They will expand downward to keep air from entering the reservoir as the brake fluid is pumped out of the reservoir into the brake system. If you are losing fluid in either the front or rear brakes, those bladders will be fully extended in whichever cylinder is leaking.
If you are losing fluid from the reservoir, and those bladders are not extending, then the seal is leaking.
Check the vent hole under the rear of the M/C for traces of fluid, the shaft seal could be leaking.
If the bladders are extending as you run low on fluid, then the seal is good and your leak is farther down the line toward the calipers. Possibly a bad proportioning valve or flex line.

In short, any time you run out of fluid in the reservoir, you are going to introduce air in the system and will need to re-bleed the entire system from scratch.

You can wrap some paper towels around the M/C to help trace the source of the leak too. If it's leaking there it will be obvious in short time.

Thanks, the rear bladder does extend, but I am still getting leakage around the seal.

 

[wishuwerehere82]

Thanks, the rear bladder does extend, but I am still getting leakage around the seal.

Then you are losing fluid out of the system somewhere else at a higher rate than the leak in the lid seal.
Even if it leaks fluid a little it would be sucking air into the leak when the brakes are applied.
So somewhere in the system the fluid is being lost faster than the leak can supply air to keep the bladder from extending. You follow me?

 

[ladams]

Then you are losing fluid out of the system somewhere else at a higher rate than the leak in the lid seal.
Even if it leaks fluid a little it would be sucking air into the leak when the brakes are applied.
So somewhere in the system the fluid is being lost faster than the leak can supply air to keep the bladder from extending. You follow me?

I get you, thanks very much for your help.

 

[ladams]

Really Wierd!!

Ok, I have searched and can't find any leaks other than around the MC. This is the wierd part, The rear res. is going empty with the bladder extending, and the front is overfilling which is where the leakage is coming from!!!!

Anyone had this problem? I am going to try a new MC because right from the start when I first installed this one I had a hell of a time getting fluid to the rear brakes and had to gravity bleed them over night.

 

[wishuwerehere82]

Seems to me that the master cylinder is leaking internally between the front and rear pistons.
There is a seal between them that seals the shaft.
The seal may be bad and allowing fluid to flow from one cylinder to the other. It sounds like it is pumping from the rear to the front cylinder and depleting the rear reservoir and overfilling the front.
Take it back and get a new one under warranty.

 

[ladams]

Seems to me that the master cylinder is leaking internally between the front and rear pistons.
There is a seal between them that seals the shaft.
The seal may be bad and allowing fluid to flow from one cylinder to the other. It sounds like it is pumping from the rear to the front cylinder and depleting the rear reservoir and overfilling the front.
Take it back and get a new one under warranty.

That is exactly what I think is happening, but being new at this I didn't know if it was even possible. I am a locomotive mechanic and only deal with air brakes, but it seems that some of the principles are the same. Also, unfortunately, being new at this, I didn't keep my receipt so I had to buy a new MC. (Live and learn) Luckily, my buddy got me a deal at his shop and I only paid $40 for a NEW one. I have it installed and so far so good. But, believe it or not, my damn headlights just sprung a vacuum leak!! What's next!

 

[ladams]

fingers crossed

I think I have my brake problem solved. I have a firm pedal and so far no leakage or change in MC reservoir levels. I am going to take the old MC apart to see if the seals were bad or what was wrong.

 

[LVMYVT76]

most part shops can look up when you bought the first m/c and warranty it.

 

[gmjunkie]

But, believe it or not, my damn headlights just sprung a vacuum leak!! What's next!

WELCOME to C3 Ownership!!! You'll get it Ironed Out,We've all been there!! Hang in there!!!

 

[ladams]

One last question...about MC

I have one last question... I see in the assembly manual that there is are two gaskets or seals between the MC and the booster. They were not on the MC I replaced and not on the new one. I don't see the purpose for them. Are they inside the MC already or should I put them on?

 

[wishuwerehere82]

I have one last question... I see in the assembly manual that there is are two gaskets or seals between the MC and the booster. They were not on the MC I replaced and not on the new one. I don't see the purpose for them. Are they inside the MC already or should I put them on?

They are probably the front shaft seals for the booster.
If you are replacing the same parts you took off, I wouldn't worry about it.

 

[ladams]

thanks for all your help!!