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Discussion Starter · #1 ·
OK, my 1998 K1500 has been losing brake fluid for years. The first sign was a dark spot on the front frame rail under the master cylinder. I've mistakenly thought that was spilled oil for years. Then I started putting two and two together. It's worsened to the point now where I can drive 10 miles up into the hills, cut a truck load of firewood and when I get back the previously full reservoir is empty. The front tank will be completely empty, the rear one will have half and inch left in it. That's after 20 miles. If I fill it, start the truck and cycle the pedal to the floor 20 times the rear tank fills with tiny bubbles making the fluid look white. It settles and clears after a few minutes. There are no leaks at any of the wheels. The bottom half of the booster is rusted and corroded on it's flanges.

I can't find any "wet spots" per se. I don't think brake fluid behaves or appears like oil or water though.

SO, does this sound like it needs a master cylinder rebuild kit? Does anybody make one? Should the booster be OK? Burning questions!

One of our local parts stores told me they don't sell a M/C rebuild kit "anymore". I detest Chinese stuff but I can't help but think that's all that is going to be available.

Any help would be appreciated.

The only thing good about this is that brake fluid is now cheaper than gas. How screwed up is that?
 

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2004 Sierra 3500 6.0L, auto, extended cab, cab&chassis, upgraded to 4wd
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First, you need to figure out where the fluid is going. It's not evaporating, so it's going somewhere. Perhaps it is going into the booster, then perhaps the intake manifold, but I would verify that before throwing parts at the truck.
Second, given that the MC has been run dry, it has to be bench bled, then the rest of the system bled.
 

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Discussion Starter · #3 ·
If I had to GUESS I would say it's leaking from the seal between the face of the booster and the rear of the M/C. Do they make a florescent dye for use with brake fluid? That would turn up pretty quickly.
 

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Not aware of any dye for brake fluid. I would suggest loosen the master cylinder to separate from the booster for inspection. If their is brake fluid in the booster then should replace both booster and master. Avoid buying a remanned master if you can avoid it. The booster most likely will only available remanned. Like others said make sure to bench bleed the master first.
 

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2003 Silverado 1500 LS
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First, you need to figure out where the fluid is going. It's not evaporating, so it's going somewhere. Perhaps it is going into the booster, then perhaps the intake manifold, but I would verify that before throwing parts at the truck.
Second, given that the MC has been run dry, it has to be bench bled, then the rest of the system bled.
Yep, this ^^^^^^^^^ all the way. You absolutely, positively 100% have a simple leak somewhere. You gotta find that leak, and focus on that.

When I have leak like this, I usually fill the M/C, then get a buddy to hold pressure on the brake pedal, so that I can look around underneath until I spot the leak.
 

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Discussion Starter · #7 ·
When I have leak like this, I usually fill the M/C, then get a buddy to hold pressure on the brake pedal, so that I can look around underneath until I spot the leak.
I tried that with the counter guy from the parts store. He couldn't see a thing. I cycled the brake about 10 times, right to the floor and HARD. Anyway, I'll pull things apart when I get the time. At 17mpg (imperial) I don't mind laying this truck up for a while.
 

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2003 Silverado 1500 LS
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As stated in post #2, it may be leaking out rear of M/C, then going into booster, then getting sucked into intake manifold.... so you'd never see any visible leaking. The M/C comes off in about 30 seconds by removing two nuts, so I would check that area for leakage.

17mpg Imperial gallon (4.8 US quarts) is not pleasant at all, especially with those Canadian prices (which have ALWAYS been high). I was born in Halifax, NS, so I visit all the time... until COVID that is. I had a '98 K1500 (which I absolutely loved) with the 5.7L and it got way better mileage than that. Made many trips to CA in that truck. I think you maybe need to spend some time on tune-up and maintenance.
 

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Discussion Starter · #9 ·
Chevy-SS, if you have any tips on tune up and maintenance, let fly. I've replaced the air filter, plugs and change oil regularly. I'm not electronics wizard so I'm not going to be swapping chips or dialing in my ECU but I would love to hear about anything that might bring that mileage up. Anyone else feel free to chip in too.
 

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2003 Silverado 1500 LS
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A few tips that come to mind:
1) Swap out all driveline fluids for high quality synthetics
2) Tire air pressure - run moderate to high pressure (just not low)
3) Fuel injector cleaner - good stuff, add to gas every other tank
4) Fuel filter - replace
5) Spray clean the MAF (using correct spray)
6) Check engine carefully for any vacuum leaks
7) Cap & wires were always sore spots with mine, make sure no arcing (check at night)

Above assumes you are already using high quality synthetic engine oil.

#7 could be the culprit for you. If you have just a tiny misfire, you won't feel it, but it will absolutely hurt mileage.
 

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Discussion Starter · #11 ·
I looks like the reservoir is leaking where it joins the cylinder. I had the cylinder apart, all the seals look fine. Cleaned everything out and treated the grommets with Rubber Renu. Getting the reservoir back on the cylinder is a pig. The grommets want to push down into the MC if you install them first but then if you put them on the reservoir nipples, they don't push into the holes. Solution was to epoxy them into the dry MC, leave it overnight, put black permatex on the nipples and push them down into the grommets (which were by now cemented into place). While doing all this experimenting the thought was, "what's the worst thing that could happen, I would have to buy a new MC" which is perfectly fine.

I found that the fluid has been leaking, dripping down on to the frame horn and then migrating back along the driver's side frame right to the rear end. I'll be washing that off since it's corrosive.

Old vehicles,,,,,,,,,,,,,,,,,,
 

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Discussion Starter · #12 ·
Well this has gone from bad to worse. After removing and cleaning up the MC and starting to bleed the brakes I found that the problem is a holed steel brake line running through the driver's side of the frame, just about beside the gas tank. About 6" forward from the back end of the gas tank. It jets out when the pedal is pressed and shoots up to the underside of the channel of the frame. What to do.

I have some corrosion issues on the arse end of this truck I would like to address. I have a practically new bumper so the crusty one has to come off. The axle and it's hardware are thick with scaly corrosion and now the brake line. I'm thinking I may thank myself for going to the trouble of taking the box off. Any thoughts on that? Tough job? I'm thinking it's not too bad. Then I would likely replace all the brake lines back there just to be safe. I would appreciate any feedback.
 

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2004 Sierra 3500 6.0L, auto, extended cab, cab&chassis, upgraded to 4wd
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You might also check the front lines as well... I don't know much about your generation truck, but for the GMT800's, it's common for the lines to rust along the frame rail above the fuel tank and where they run along the frame rail from the driver's front wheel to the abs controller under the frame.
 

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Thanks Dave. Can a guy buy stainless tubing and make his own lines? I'm keeping this truck long term.
When I replaced the brake lines on my truck, I got a set of pre-bent nylon-coated steel lines from GM (which didn't include the two lines going across the rear axle :-( ).

From what I've read on various forums (but don't have any direct experience myself), people doing the job recently with plain tubing seem to prefer using copper-nickle brake line tubing, as it's easier to bend to shape without damaging the line, easier to form the double-flare at each end, and can't rust, compared to using stainless steel.
 

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2015 High Country Duramax
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Yup 100% use nicop line. It is a dream to work with. Flares beautiful, easy to bend, and doesn't rust.

Stainless is good, but it absolutely sucks to flare and bend. So only time I recommend stainless is if it's a pre bent set, or you want the look of them on a show vehicle.
 

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Discussion Starter · #19 ·
Thanks all of you. I think I'll look at yarding the box off today. I've got to do it myself so I'll have to rig something up for that. Thanks again. It's nice hearing about something that's going to make a job easier.
 

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Discussion Starter · #20 ·
Got the canopy and box off and the frame looks pretty good. No holes, though the original paint is now a layer of scaly rust. That's coming off nicely with the air chisel. What rattle can paint do you like for a frame? I will be applying a rust converter of some sort so something that would work with that. My local sells Permatex products so without looking right now, I expect that may be what I will use for rust control.
 
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