master cylinder gasket help

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snowcrow

snowcrow

2 Time dart owner
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My 76 dart swinger is leaking brake fluid from the gasket in between the fluid "pots" and the big cylinder on my power brakes setup. I took off the front most piece and it seemed like the gasket was fine but it was plastic and metal with 3 gaps in the metal piece with a plastic piece that was over it which seemed undamaged. It was shaped to let the gap in the metal breath or leak. My question is, it that normal?? Or should the gaps be facing up so it breaths not leaks fluid.
 
At first I thought you were referring to the rubber diaphragm under the cover. Then I realized you were describing too many pieces. You have to be looking at the spacer assembly between master cylinder and booster. Those cutaways are supposed to be on the bottom not just atmospheric vent but also to let the evidence of a fault reveal. You'll need to replace the master cylinder.
 
Yep, master cylinder rear seal is leaking.
They used to make rebuild kits for them but last time I checked they didn't have them.
A new master cylinder for these is pretty cheap. (about 20 bucks and a 10 core) at Autozone Part# M1707 for 76
 
Is the booster the bigger round piece? And the master cylinder is the piece with the "pots"?
 
if your booster is bad you'll either have to have it rebuilt or convert over to something else. I used a chevy booster and later 90's ram 1.25 master and my car stops like crazy! best part is the lines bolted right up!
 

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Oklacarcollecto said:
Yes. You will have to bleed the system after you change master cylinders.
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If the master cylinder is bench bled properly it can be bled into the system at the line connections. Bleeding at other points wouldn't be absolutely required.
To completely bleed away all contaminated fluid is good practice but it's not common practice.
 
805moparkid said:
if your booster is bad you'll either have to have it rebuilt or convert over to something else. I used a chevy booster and later 90's ram 1.25 master and my car stops like crazy! best part is the lines bolted right up!
Click to expand...

On his 76 Dart Swinger the master cylinder is readily available at the parts store so they would be no reason to have to get his rebuilt.



RedFish said:
If the master cylinder is bench bled properly it can be bled into the system at the line connections. Bleeding at other points wouldn't be absolutely required.
To completely bleed away all contaminated fluid is good practice but it's not common practice.
Click to expand...

I have yet to have the brake lines not get some air in them no matter how fast a person is since they lay flat when they are unbolted from the master cylinder. It is just a little but personally I want no air in my brake system.
 
Oklacarcollecto said:
On his 76 Dart Swinger the master cylinder is readily available at the parts store so they would be no reason to have to get his rebuilt.





I have yet to have the brake lines not get some air in them no matter how fast a person is since they lay flat when they are unbolted from the master cylinder. It is just a little but personally I want no air in my brake system.
Click to expand...


If you get the master loose and then tweak it up gently the lines can face up a little.:D
Thats the way I have always done it and never had an issue as long as the master was bench bled right.
 
Oklacarcollecto said:
On his 76 Dart Swinger the master cylinder is readily available at the parts store so they would be no reason to have to get his rebuilt.





I have yet to have the brake lines not get some air in them no matter how fast a person is since they lay flat when they are unbolted from the master cylinder. It is just a little but personally I want no air in my brake system.
Click to expand...

re-read the first couple words :glasses7:
 
Oklacarcollecto said:
:banghead: Since we are dealing with leaking brake fluid I had master cylinder on my mind and didn't even see the word booster. :banghead:
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No big, i did mine because the booster was shot, the check valve was toast, and the master was leaking internally :banghead:. im in under 200 bucks and a couple hours to install and bleed. Would have been faster but i was using a file!

OP have you checked your booster/check valve yet? if its the 3/4" nipple valve they are not made, and same for the boosters.
 
To summarize,
The "gasket" you refer to is not supposed to hold fluid, it is just a mechanical interface and maybe a dust cover (plastic part). The slots at the bottom are so you can see the brake fluid is leaking, and to keep it from leaking into your booster (bad).

After changing the master cylinder, I would bleed all the brakes, until you get clear fluid and no air bubbles. See youtube. You should be doing that every few years anyway, maybe 6 yrs in Utah. Or if you switch to silicone DOT 5, you never need to.

Since a newbie, don't use a regular open-ended wrench to loosen the tube nuts or you will probably ruin them. Get a special "flare wrench" which better surrounds the nut.

Don't feel bad that you thought that part was a gasket. On many newer cars it is a vacuum seal. My 84 M-B is like that. The boosters on our cars have an internal rubber bellows which forms that seal, which you will see when you remove the MC. Good in that it keeps leaking brake fluid out of the booster, but bad in that the booster is thicker.

A final newbie tip. Glycol is an excellent paint remover, so quickly clean up any spills. Also why your firewall is probably rusty below the booster. Another reason many classic car owners use silicone fluid.
 
BillGrissom said:
To summarize,
The "gasket" you refer to is not supposed to hold fluid, it is just a mechanical interface and maybe a dust cover (plastic part). The slots at the bottom are so you can see the brake fluid is leaking, and to keep it from leaking into your booster (bad).

After changing the master cylinder, I would bleed all the brakes, until you get clear fluid and no air bubbles. See youtube. You should be doing that every few years anyway, maybe 6 yrs in Utah. Or if you switch to silicone DOT 5, you never need to.

Since a newbie, don't use a regular open-ended wrench to loosen the tube nuts or you will probably ruin them. Get a special "flare wrench" which better surrounds the nut.

Don't feel bad that you thought that part was a gasket. On many newer cars it is a vacuum seal. My 84 M-B is like that. The boosters on our cars have an internal rubber bellows which forms that seal, which you will see when you remove the MC. Good in that it keeps leaking brake fluid out of the booster, but bad in that the booster is thicker.

A final newbie tip. Glycol is an excellent paint remover, so quickly clean up any spills. Also why your firewall is probably rusty below the booster. Another reason many classic car owners use silicone fluid.
Click to expand...
I thought glycol - as in ethylene glycol - was engine coolant (anti freeze). Are you saying to put anti freeze into your brakes? Maybe I'm missing something.
 
cruiser said:
I thought glycol - as in ethylene glycol - was engine coolant (anti freeze). Are you saying to put anti freeze into your brakes? Maybe I'm missing something.
Click to expand...
Old thread. (2014) In fact the original poster hasn't been here since July 2018. Ya brake fluid eats paint but I don't know about glycol.
 
toolmanmike said:
Old thread. (2014) In fact the original poster hasn't been here since July 2018. Ya brake fluid eats paint but I don't know about glycol.
Click to expand...
If you buy any gallon jug of antifreeze at the parts store, the label says that this stuff is actually ethleyne glycol, which has been used as an engine coolant for many decades. Brake fluid is entirely different, and is essentially a hydraulic oil used to operate the braking system. They're two entirely different things, and cannot be used interchangeably. If I learned that one of my A body buddies put glycol into his master cylinder, I'd take his keys away from him and keep him from driving his car until he fully and entirely emptied his braking system and refilled it with BRAKE FLUID.
 
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