Seal of brake master, what function?

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15 May 2004
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What function(s) does have sela part no. 3 in the picture. It's for sure a dust seal. Does it also prevent the master from leaking fluid or are there other seals inside the master again leaking?

Reason: I've seen quite a lot brake masters of older NSX leaking on the booster damaging the paint which is neglegiable. What does a leaking brake master indicate? Can it be fixed with only changing the seal (part no. 3)?
 
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I am thinking it is a vacuum seal for the power booster:).

JMO, not really sure.

Regards,
LarryB
 
I am thinking it is a vacuum seal for the power booster:).

JMO, not really sure.

Regards,
LarryB

I think Larry is correct. My newly acquired 93 has a vacuum leak coming from the master cylinder and there is corrosion where it's bolted to the booster.
 
Thanks for your opinions.

Can a brake master be rebuilt or better go with a new one? The issue is only leaking.
 
What function(s) does have sela part no. 3 in the picture. It's for sure a dust seal. Does it also prevent the master from leaking fluid or are there other seals inside the master again leaking?

Reason: I've seen quite a lot brake masters of older NSX leaking on the booster damaging the paint which is neglegiable. What does a leaking brake master indicate? Can it be fixed with only changing the seal (part no. 3)?

1) it is not a dust seal, it is a front booster seal
2) do not rebuild the m/c cyl replace it

the purpose of the booster seal is to stop brake fluid from a leaking m/c entering the power booster and then being drawn into the engine, most power boosters will have a little notch (or vee) where the master cyl. mounts to it ,this is the relief that allows the fluid to leak out and strip the paint off the booster, piss you off ,then you replace the m/c
A power booster is just a can that is seperated by a diaphram &spring, when you start the car,(pedal released ) vacuum is allowed on both sides of the diaphram when you step on the pedal a valve (usually called an atmosheric valve) will close off vac. to the back of the diaphram and allow atmosheric pressure in ,because of the pressure differential vac. on one side & atmosphere on the other )you end up with a brake pedal assist. this is the basic idea
 
1) it is not a dust seal, it is a front booster seal
2) do not rebuild the m/c cyl replace it

the purpose of the booster seal is to stop brake fluid from a leaking m/c entering the power booster and then being drawn into the engine, most power boosters will have a little notch (or vee) where the master cyl. mounts to it ,this is the relief that allows the fluid to leak out and strip the paint off the booster, piss you off ,then you replace the m/c

Thanks Ralph! One open question though. If it's not a dust seal only can it cope with a m/c leaking? If I look at the design of the seal I'd say yes. When I've replaced it on my car the seal was just breaking into two pieces, means not working anymore which could explain the leaking. But if the source of the leakage is inside the m/c then it would be not worth it changing the seal only.

BTW: The SM says not to rebuild the m/c.
 
Thanks Ralph! One open question though. If it's not a dust seal only can it cope with a m/c leaking? If I look at the design of the seal I'd say yes. When I've replaced it on my car the seal was just breaking into two pieces, means not working anymore which could explain the leaking. But if the source of the leakage is inside the m/c then it would be not worth it changing the seal only.

BTW: The SM says not to rebuild the m/c.

three things kill the seal, brake fluid , hydrocarbons from the intake manifold vacuum (many boosters have a charcoal filter in the vacuum line to filter the hydrcarbons that are in the intake manifold that will migrate to the brake diaphram,seal,and valve , when the car is turned off) & Time (this takes us all out), brake fluid leaking out of the back of the m/c is indicative of fluid getting past the secondary seal in the m/c ,you have no option but to replace the m/c, also if the front booster seal is deteriorated you must replace it also.
The last thing you need is brake fluid in the booster, it will take out the diaphram in no time , plus any loss of intake vacuum on our cars (or any body elses built in the last 30 yrs )will be interrputated by the map sensor as a load thus increasing fuel delivery, i dont mean to get deep on this but this is the way it is !!!
 
Thanks Ralph! One open question though. If it's not a dust seal only can it cope with a m/c leaking? If I look at the design of the seal I'd say yes. When I've replaced it on my car the seal was just breaking into two pieces, means not working anymore which could explain the leaking. But if the source of the leakage is inside the m/c then it would be not worth it changing the seal only.

BTW: The SM says not to rebuild the m/c.

goldNSX,
If brake fluid is leaking from your master cylinder the source of the leak is inside the master cylinder. There are 4 seals inside the master cylinder (secondary pressure seal, primary - secondary circuit isolation seal, primary pressure seal, and a rear isolation seal). The rear isolation seal is the one that you will find as the cause of your leakage.

I would not recommend rebuilding the master cylinder because I would suspect there is bore wear and/or primary piston wear that eventually lead to the rear seal failure. So even if you replaced the rear seal it would happen to you again down the road. You will need to replace the master cylinder, booster front seal if is deteriorated, and possibility the vacuum booster if brake fluid got into the booster.

The designed function(s) of the seal #3 is to:
1. Provide a vacuum seal to the front shell of the vacuum booster.
2. Stop brake fluid from entering the vacuum booster if the master cylinder rear isolation seal ever fails.

Many of the auto manufactures in the 80's & 90's used SBR rubber for the vacuum booster diaphragm's. SBR rubber is not compatible with brake fluid so to prevent brake fluid from entering the vacuum booster most OEM's used a booster front seal that was compatible with brake fluid (typically EPDM rubber).
 
Bob, thanks for your explanation! That's what I wanted to hear. I've seen quite a lot to older NSX's with leaking brake master cylinders, even ones with below 60k miles. They don't seem to be wear-related, it looks like the seals gave in over time. I had this on mine too and changed it for a 97+ master but was still wondering why the seal is sold separatly.
Thanks guys! :)
 
Ralph, thanks to you too! Thanks to all! The new m/c comes with a new seal. My question was if the m/c is leaking is it sufficient to just replace the seal. Now, I know it's not sufficient. :wink:
 
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