Master Cylinder Question

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Sep 22, 2015 | 06:59 PM

gsimon767

Thread Starter

Instructor

Joined: Mar 2013

Posts: 180

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From: Chesterfield Mo

Master Cylinder Question

I know it's a dumb question but what is the expected level of brake fluid in a 75 power brake master cylinder when full? There are no marks or indication of where the level should be.

I ask because my brake system is all restored but leaves a small amount of brake fluid on the garage floor after driving the car. The master was done by White Post. The booster was rebuilt by Booster Dewey. The cap is original but has a new rubber membrane. When I run my hand under the master I get a little brake fluid on it. It seems like fluid is leaking arround the cap even though it is held on very tightly. I am wondering if I have the master too full? The fluid is about 1/2 inch below the lip at the rear of the master.

Sep 22, 2015 | 07:31 PM

SH-60B

Melting Slicks

Joined: Mar 2008

Posts: 2,989

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From: Meriden CT

The level you have is fine. I'd suspect a leak at one of the fittings. Clean and dry the outside of the master, then step on the brake pedal and see where it's wet.

Sep 22, 2015 | 07:37 PM

C3Hawk

Racer

Joined: Jan 2014

Posts: 334

Likes: 6

I keep the brake fluid 1/4" below the top in the master cylinder of my '75. I did at one time replace the cap and cap gasket as a set to stop the leaking. The sealing surface at the top of the master cylinder has to be clean, too. (Careful when cleaning not to get anything in the fluid!)

Sep 22, 2015 | 08:19 PM

BlackC3vette

Burning Brakes

Joined: Jan 2012

Posts: 943

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Quote:

Originally Posted by C3Hawk

1/4" has been the standard. Even if the gasket is new, any garbage around the sealing surface will cause a slight leak.

Sep 22, 2015 | 08:29 PM

Jud Chapin

Race Director

Joined: Sep 2001

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From: Wellington, FL

St. Jude Donor '11 thru '25

Quote:

Originally Posted by SH-60B

The level you have is fine. I'd suspect a leak at one of the fittings. Clean and dry the outside of the master, then step on the brake pedal and see where it's wet.

Be sure to check the rear of the m/c also for leaks where it meets the booster.

Sep 23, 2015 | 08:33 AM

gsimon767

Thread Starter

Instructor

Joined: Mar 2013

Posts: 180

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From: Chesterfield Mo

Quote:

Originally Posted by Jud Chapin

Be sure to check the rear of the m/c also for leaks where it meets the booster.

Master is newly professionally rebuilt. It would be very unusual if there is a leak at the booster. I may unbolt it to see if it is dry at pushrods.

Sep 23, 2015 | 08:36 AM

gsimon767

Thread Starter

Instructor

Joined: Mar 2013

Posts: 180

Likes: 21

From: Chesterfield Mo

Quote:

Originally Posted by BlackC3vette

1/4" has been the standard. Even if the gasket is new, any garbage around the sealing surface will cause a slight leak.

The sealing surface looks like new. When I take the lid off there is fluid on sealing surface which makes me suspect the gasket.

Sep 23, 2015 | 11:00 AM

71 f/w driver

Racer

Joined: Oct 2008

Posts: 278

Likes: 5

From: illinois

Quote:

Originally Posted by gsimon767

The sealing surface looks like new. When I take the lid off there is fluid on sealing surface which makes me suspect the gasket.

I bought a new m/c, cap and seal it did leak at the real of the m/c with new cap and seal. Replaced it with the old exiting cap and seal NO MORE LEAK!!!!!!!!!! go figure. Mike