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SC brakes (stock) booster ‘issue’

I am trying to finish off the brake project I’ve been at for a bit (inclusive of pads and caliper clean/rebuild on all four corners, in addition to some other bits along the way...)

Today, I decided to try my new toy: a “motive power bleeder” to bleed the brakes. One of the first steps that is supposed to be done is: put the cap from the power bleeder on to the master cylinder resivoir cap, and apply some pressure (15psi, per the “motive” instructions). After fidgeting a bit with the cap on the resivoir; I found that any pressure that I was attempting to “build” would just leak out through hoses (rubber, cloth covered, blue and very dirty/frayed) that go from the bottom of the resivoir to the master cylinder. These cloth covered hoses are old and crusty, and I believe that they connect to the MC pressure switches, which do not seem to be seated very securely.

My question(s) is (are):

I can (pretty easily) replace these hoses with some new cloth covered ones, but wouldn’t the pressure just pass at the base of the master cylinder where (I believe) the pressure switches sit? Aren’t these all supposed to be more secure?

Is it time for a new MC? (I hope not; enter the scope creep....)?

To be clear, my frunk is dirty, but it is not soaked in brake fluid anywhere, my MC is not leaking. Also, I am *pretty sure* that my booster is working (albeit, not likely to be optimally), because when I start the engine while pressing the brakes, you can feel the vacuum engage the petal.









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'Inherited' car: 1979 911SC - getting it running right was a task, read about it here: http://forums.pelicanparts.com/911-engine-rebuilding-forum/722362-dads-911sc-i-am-finishing-rebuild-long.html
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Old 01-23-2019, 05:43 PM
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Here’s a photo from the other side of the resivoir.

Thoughts?
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'Inherited' car: 1979 911SC - getting it running right was a task, read about it here: http://forums.pelicanparts.com/911-engine-rebuilding-forum/722362-dads-911sc-i-am-finishing-rebuild-long.html
Other cars: 1993 Corvette LT-4/ZF6, polo green. 2011 Grand Cherokee Overland, Hemi 4X4 with some adds.
Old 01-23-2019, 05:46 PM
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I’m pretty sure that line coming off the reservoir is a vent line. To use the motive, you need to remove that hose and plug that nipple somehow.
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Old 01-23-2019, 06:32 PM
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The red (smaller diameter) cloth braided hose is the ‘vent’ hose, correct?
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'Inherited' car: 1979 911SC - getting it running right was a task, read about it here: http://forums.pelicanparts.com/911-engine-rebuilding-forum/722362-dads-911sc-i-am-finishing-rebuild-long.html
Other cars: 1993 Corvette LT-4/ZF6, polo green. 2011 Grand Cherokee Overland, Hemi 4X4 with some adds.
Old 01-23-2019, 06:56 PM
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I did the same thing as you once except my vent hose was in good shape. It dumped all my fluid on the ground through the end of the hose in the front left wheel well. I plugged mine with a short piece of vacuum line I had lying around and a short piece of solid aluminum rod.
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Old 01-23-2019, 08:19 PM
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Quote:
Originally Posted by Robey5 View Post
The red (smaller diameter) cloth braided hose is the ‘vent’ hose, correct?
Yes
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Old 01-24-2019, 08:09 AM
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So I just used my motive on this car for the first time. I disconnected that vent line, installed some clear tubing to the nipple which I then clamped with vise grips to seal it, and then used the motive to bleed. Worked like a charm. Only issue was the rate of old fluid discharge from the last caliper (driver's side front) was really, really, really slow, almost like there was a clog or something. So I depressed the brake pedal about half way to see if I could get the new fluid moving and that did the trick. It was still a slow flow, but eventually I saw the new fluid going out so I think I was successful! Good luck.
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Old 01-24-2019, 08:18 AM
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The blue hoses are what carries the brake fluid from the reservoir to the two chambers (front and rear) of the master cylinder. This allows the MC to keep its fluid topped off as the brake pads wear. Normally, these blue hoses only see a very small hydraulic head of pressure. The pressure bleeder increases this pressure some, but for sure zero fluid should leak from the blue hoses. The solution there is obvious.

Scott - your issue puzzles me. When working on lines or calipers, if you don't want to drain the system, if you depress the brake pedal a bit (or a lot), you prevent fluid from flowing from the reservoir into the MC. It is only when the MC is in its full "brake off" position, or pretty close to it, that the orifices which admit fluid from the reservoir to the chamber within the MC that needs fluid are open. If things were otherwise, when you step on the brake pedal you would push fluid back into the reservoir.

Or do you mean that you momentarily depressed the brake pedal, and then released it, at which time the flow was a bit better? How old are your rubber brake lines?
Old 01-24-2019, 11:05 AM
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i just used the motive on m 930. it worked great. best pedal I have had.

clamp the vent hose with vise grips.

remove the MC and replace the blue hoses if they are leaking.
you can also take the NC apart and clean and inspect it.

I like the motul fluid, either the 600 or 660.

test your brake booster (BB) with a hand vacuum pump. BUT, do it from the engine bay.
this will also test the long vacuum hose too.
the hand vac pump is a great tool to have. I not have 2, one also does pressure. I use the vac tool more than you would think.
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Old 01-25-2019, 02:45 AM
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Quote:
Originally Posted by Walt Fricke View Post
The blue hoses are what carries the brake fluid from the reservoir to the two chambers (front and rear) of the master cylinder. This allows the MC to keep its fluid topped off as the brake pads wear. Normally, these blue hoses only see a very small hydraulic head of pressure. The pressure bleeder increases this pressure some, but for sure zero fluid should leak from the blue hoses. The solution there is obvious.

Scott - your issue puzzles me. When working on lines or calipers, if you don't want to drain the system, if you depress the brake pedal a bit (or a lot), you prevent fluid from flowing from the reservoir into the MC. It is only when the MC is in its full "brake off" position, or pretty close to it, that the orifices which admit fluid from the reservoir to the chamber within the MC that needs fluid are open. If things were otherwise, when you step on the brake pedal you would push fluid back into the reservoir.

Or do you mean that you momentarily depressed the brake pedal, and then released it, at which time the flow was a bit better? How old are your rubber brake lines?
Thanks for your comments. Yes, all I did was depress the brake pedal about half of its stroke and then release. When I walked back to the caliper the fluid was moving. My thought was just to add a little pressure to the fluid in the same way as though I were manually flushing the fluid using the “pump the brakes” method. That seemed to do the trick.
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Old 01-26-2019, 05:15 AM
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