Suspected brake master issue

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cbez
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Suspected brake master issue

Post by cbez »

As yall know, 'new' parts were thrown on my car 10-20 years back and then sat. I suspect the seals in my master are feeling their age.

First thing in the morning, I'll get a very soft pedal, that pumps up.

Then the brakes feel great. Randomly driving I'll get a longer, soft push that immediately pumps up on the next press. 95% of the time feels like a perfect pedal.

Rear drums are adjusted. Pads are all good. Fluid is fresh and bled a few months ago and remains high in the reservoir.

I'm thinking slow internal leak in the master that loses pressure overnight?
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Re: Suspected brake master issue

Post by Habitat.pat »

There really isn’t any residual pressure in the brake system. First thing I’d (re)check is rear adjustment.

Also, when it has been sitting overnight, see what the pedal feels like with the parking brake on, then with it off. The two pedals should be close to the same.

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cbez
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Re: Suspected brake master issue

Post by cbez »

That was my first thought too but I adjusted them Friday and still have the issue.

This morning was worst it's been, 3-4 totally soft pumps before pressure came up.
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Re: Suspected brake master issue

Post by jrusso07 »

Check the cylinder end under the dash/firewall to see if it is wet with fluid (under the rubber boot). I had one fail in the manner you describe - was a small leak on the outer seal. If it wasn't for the fluid on my shoe I would never had looked.
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Re: Suspected brake master issue

Post by zippy67roadster »

If you end up needing to rebuild or replace it I have the rebuild kit for the dual masters and new custom direct bolt in masters for the early and late cars. https://sincitydatsuns.com/store/c3/Bra ... Parts.html
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cbez
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Re: Suspected brake master issue

Post by cbez »

Image dry externally as far as I can tell
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Re: Suspected brake master issue

Post by redroadster »

A good test is push firmly on the pedal ( only like braking quickly )for 20 sec then
Let up some so only lite pressure but see if it slowly goes down if the seal is ballooned it will show here
Or a cly leaking
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cbez
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Re: Suspected brake master issue

Post by cbez »

redroadster wrote: Mon Nov 22, 2021 9:32 pm A good test is push firmly on the pedal ( only like braking quickly )for 20 sec then
Let up some so only lite pressure but see if it slowly goes down if the seal is ballooned it will show here
Or a cly leaking
no feeling of dropping with this test.
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Re: Suspected brake master issue

Post by JT68 »

I would say you either still have air in the system or one of the internal seals is compromised.

Internal seal failure without external leaks is very rare, so I'd wager air in the system.
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Re: Suspected brake master issue

Post by Solex68 »

+1
JT68 wrote: Tue Nov 23, 2021 9:56 am I would say you either still have air in the system...
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Re: Suspected brake master issue

Post by Curtis »

JT68 wrote: Tue Nov 23, 2021 9:56 am I would say you either still have air in the system....
+2 Had this more than once. Now use the speed bleeders.
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Re: Suspected brake master issue

Post by 70MTRoadster »

My story is about like yours. I replaced mine about 10 years ago too. I can push mine down the first time and she goes to the floor; second push is fine and is fine for the rest of the drive. I have become used to stabbing the brake pedal once in anticipation of needing braking and never had a problem. My take is that the cup(s) on the piston need a quick stab to flare them out so that they seal. After the above, I can push and it holds forever. Just my 2 cents.

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cbez
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Re: Suspected brake master issue

Post by cbez »

Well here is a video is my morning pedal.

I'll definitely bleed it before I buy an expensive kit. I guess I'm just more used to air in the system feeling bad all the time instead of soft and then good. Bear in mind I had bled it and had a good pedal for hundreds of miles so it'd have to be a leak that let air in but no fluid out.

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Re: Suspected brake master issue

Post by Solex68 »

My recollection from a 311s post was when bleeding the brakes, go very slow when depressing the pedal for bleeding. I believe it was to take 10 seconds to go through one pedal press. That worked for me. Depending where the air is, it can take a while. Also, I would focus on on the front brakes as that is likely where the issue resides.
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Re: Suspected brake master issue

Post by Curtis »

Solex68 wrote: Tue Nov 23, 2021 1:03 pm My recollection from a 311s post was when bleeding the brakes, go very slow when depressing the pedal for bleeding. I believe it was to take 10 seconds to go through one pedal press. That worked for me. Depending where the air is, it can take a while. Also, I would focus on on the front brakes as that is likely where the issue resides.
Those darn front calipers.
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