Usually if it gets in the booster it stays there. Does not sound like you are sure where the leak is other than the general area. I would try to pinpoint where it is coming from. Maybe get someone to press on the brakes while you look for any signs of leaking.

 

We have a mechanic that usually does most of our work and he is perplexed as well. Like I said originally I thought it was master cylinder so replaced it the again thinking I had a bad one. Then tightened the bejeesus out of the lines to no avail. Then last shot was replacing the lines so... and the best part is it doesn't lose fluid so how can that be.

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If it's not losing any fluid then it's not a current leak...

Pull the master away from the booster and see if its wet in there. If it is, then a previous leak ruined the booster.

Also, you are sure it is, in fact, brake fluid correct?

 

Well it's clear so.... assuming it's brake fluid.

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Does it feel like brake fluid? It has a very noticeable feel to it.

 

Yes quite sure it's brake fluid. This whole thing started as i thought there was a leak in the drivers side line to the caliper. I replace it but then noticed the fluid under the master cylinder. I assumed it was bad so replaced it.

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Well the only next steps then are making sure the booster doesnt have fluid in it.

Then if there isnt, you still have a leak somewhere in the system. But if your not losing any fluid at all, then it isnt a currently present leak. If it is definitely leaking then you will notice a drop in fluid level obviously.

Also, making the fitting too tight will cause a leak as well. They are a flare fitting, so crazy tight isnt always answer. But stainless lines do often require being tighter than say if you used some nicop line to make them. The stainless is fairly hard so if the flare isnt perfect, it won't seal just right and leak.

 

Well "crazy tight" was the original lines as a last resort. I let out mechanic put the new SS lines on.

So I also added Powerstop rotors and pads (stock replacement) . I am still not real happy with the stopping power.

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Eh the brakes on the gmt800 trucks are pretty marginal at best anyway.

But if there is a leak they definitely will not have the same stopping power as they would if the system was totally closed looking should be. You lose pressure due to a leak obviously

 

Oh a side detail this notice of fluid under the master cylinder happened in October last year. So no fluid loss in 5 months.

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I had a master cylinder leak at the seal where the actuating rod goes in ... Have a friend who had the same & leaked fluid into his vacuum booster.

Wonder if the brake fluid you see is left over from your first bad master cylinder?

 

Well that was what we pondered but my mechanic though it would have been sucked out of the vacuum booster.

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They'll feel weird after you crack the system open if you haven't done an automated ABS bleed.
Not quite spongy but definitely not right either.