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post #1 of (permalink) Old 12-10-2005, 08:42 AM Thread Starter
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air getting in brake system

anybody have any tips/tricks to find out exactly where air could be getting into the brake system at? I have a 78 CJ w/o power brakes, and cannot seem to get a firm pedal. over the last several weeks i've tried the kid method (then her mother) with her pumping and holding, while I opened the bleeders, then I tried one of those cheap 1 person units, and finally I bought speed bleeders, but I can't seem to get all the air out. I can get it to where I have a decent pedal, but eventually it goes to the floor. This is getting very frustrating (not to mention a little dangerous) I'm not losing any fluid in the master cylinder, but one of the manuals said there could be a hole to small to let fluid out, but air could be getting in. if this the case, how do I find out where it's coming in at?
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post #2 of (permalink) Old 12-10-2005, 12:06 PM
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Re: air getting in brake system

Well.....if fluid is NOT leaking out, you are not replacing fluid except the little that comes out with bleeding then the master cylinder might be the culprit. Disconnect the brake lines.....use flare nut wrenches with penetrating oil, use rubber vacuum caps to cap off the lines, pull it off of the firewall, take the snap ring out, pull out the guts and inspect the side walls. Corrosion? Pitting? You might not be able to see wear on the rubber cups if it was even but eyeball them anyway. If you've never done anything with the m.c. and you've had the Jeep for a few years then get a rebuilt one. You used to be able to rebuild them yourself with new guts and some honing of the bore, but with brakes you don't want to mess around unless you can accuratrely measure the bore(and know the max. dia.). Bench bleeding the m.c.: Chuck the new one up in a vise. Get short brake lines with the fittings already on to fit the front and back outlets. Gradually and carefully bend them to come up and well into the reservoirs. Fill the reservoirs with brake fluid. Push on the plunger with a small hammer handle or anything that won't mare the plunger. Work it all the way in and out till you don't see any bubbles. Mount it on the Jeep. Remove your bench bleeding lines and install normal lines. Bleed brakes thoroughly. Good Luck.
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post #3 of (permalink) Old 12-11-2005, 11:21 AM
 
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Re: air getting in brake system



OK, lets try this...

IF...
You are NOT losing fluid,
You ARE gaining air,

WHERE...
Is the air?
Do you get spitting and bubbling at the wheel cylinders when you first crack the bleeder?
OR,
Do you have to bleed a stroke or two before the air shows up?

DO...
You get air at just one wheel cylinder,
OR,
Do you get air at all cylinders if you bleed long enough?
---------------

OK, now start from the top...

Not Losing Fluid,
That means air is getting into the system (obvious).
No such thing as a leak so small that air can get into, but fluid can't get out.
Brake fluid is under about 150 to 175 PSI when you mash the pedal, but when you let the pedal up, return spring pressure is only about 3 to 5 pounds...
Petroleum brake fluid is a very small molecule, and under 150 PSI it WILL find a way out...

(I'm sure we'll have a bunch of old wives tales to the contrary, but it's simply not physically possible...)

On the other hand...
When you apply pedal pressure, there is a hollow rubber cup, skirt side facing the juice, that trys to compress the liquid.
The liquid pressure expands the skirt of the cup, creating a seal so you can build more pressure.
The harder you push, the more pressure is built, and the more the cup skirt expands and seals.
When you let off the brake pedal, the skirt pressure drops, and spring pressure trys the force the rubber cup back to the atmosphere side.

If the rubber cup is old, has dried out, absorbed too much brake fluid and moisture, ect, it will let air pass into the cylinder when the liquid pressure drops, and spring pressure is making a low pressure cell inside the cylinder.
Low pressure cell will draw in atmosphere past the non expanded cup skirt...
And you have 'Air' in your cylinder...

Air immediately at any wheel cylinder means there is a leaking component at that cylinder.
Air that takes a while to arrive at the wheel cylinder is probably getting in at the master cylinder.
(and yes, disc brakes have cylinders, they just use 'O' rings instead of 'Umbrella' or 'Cup' type seals)

Also, when you are bleeding brakes, keep the master cylinder full, and keep your 'hired help' from getting too wound up...
All they have to do is push the pedal down EASY, and let it up EASY...

Lots of help gets excited and lets the pedal up too fast with low fluid in the master cylinder and sucks air into the system on the return stroke.
Going nice and easy with even strokes keeps this from happening when you can't be everywhere at once...

Excited help will not hold the pedal to the floor. They will push it to the floor, mash on it a while, then start letting up before you can get the bleeder screw closed...

Going slow and easy up and down will keep you from throwing the safety valve into fits, and having to reset it half a dozen times before you get what you want for wheel cylinder pressures...
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post #4 of (permalink) Old 12-11-2005, 11:26 AM Thread Starter
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Re: air getting in brake system

thanks for the tips. for all I know this may be the original master cylinder. I've had the jeep since the early 90's and I never changed it, so it's probably due.
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post #5 of (permalink) Old 12-11-2005, 12:09 PM Thread Starter
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Re: air getting in brake system

TR thanks for all the in depth info. I really appreciate it. not sure where I got that " air could get in, fluid wouldn't get out" from, quite possibly a figment of my imagination, as I looked in my haynes manual and didn't see it there. I'm going to start with the m/c as it's at least 15 years old, then go from there.
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post #6 of (permalink) Old 12-12-2005, 08:28 AM
 
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Re: air getting in brake system



I use brake fluid as penetrating fluid when I need to soak something for a long period of time.
The brake fluid molecule is very small and gets into places I've never seen penetrating oil get into.

(especially civil war and older gun parts that are still made of iron, it's the ONLY way I've found with some iron parts)

Bubbles at the wheel cylinder (immediately expelled when bleed) means wheel cylinder, Bubbles that take a while means master cylinder, and they have to be pushed down the lines to the bleed point.
If your brakes (Master cylinder & wheel cylinders) are 15 years old, it might be a good idea to replace both while you have the jeep down.
If you are a few bucks up, do some of the brake lines also, I'm sure they are tired by now...

Kits require tools like small bore cylinder hones and sometimes special snap ring pliers, but you will learn something.

If you are short on time, replace the cylinder with something from discount auto parts stores and bleed, you are done.

All brake lines require a double flair on steel lines, and this is more of an art than a science. Double flair require a special flairing tool,... Hard on the hands too.

'Bench Bleed' a master cylinder before you install it.
Use a wooden dowel rod or brass drift pin, DON'T use a steel screwdriver to push the piston for the bench bleed.
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post #7 of (permalink) Old 12-12-2005, 10:32 AM
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Re: air getting in brake system

"""""I use brake fluid as penetrating fluid when I need to soak something for a long period of time.
The brake fluid molecule is very small and gets into places I've never seen penetrating oil get into.
"""""

Good trick, gotta try it.
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post #8 of (permalink) Old 12-12-2005, 12:07 PM
 
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Re: air getting in brake system



It's almost impossible to find true 'Iron' now.
It's all alloyed with something, carbon removed then another carbon added, ect...

Before the American War Between The States, the industrial revolution had started, but steel was still rather hard to make.
The Union made steel production a priority (rightly so!) and the old style iron ore type iron went the way of sailing merchant vessels, unshod horses and men wearing spats...

Raw iron is so rare now that the local collage archeology department couldn't find three large nails for an electrical experiment after 8 months of searching...
The next year one of the kids in the class was working part time for me and mentioned it, and we found him some real iron for his project...
And now the Collage has been back every year for more iron!
(no good deed goes unpunished!)

I use the iron for replacement parts for some vintage firearms, repairs on some antiques, ect...
--------------------

Anyway, brake fluid seems to get into places penetrating oil simply will not go in iron parts.
It also seems to dissolve iron rust and keep it in solution long enough to get it out of even tiny cracks and joints.
For small parts, I use an 'ultrasonic' (cheap vibrating jewelry cleaner) and brake fluid to get dissolved rust out of really small places, and it seems to work fine, but you have to have PATIENTS! It takes lots of time... This isn't an afternoon fix-it job...
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post #9 of (permalink) Old 12-17-2005, 01:18 PM Thread Starter
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Re: air getting in brake system

Just wanted to update: Finally got around to putting in the rebuilt master cylinder yesterday (it really bites being a shadetree mechanic this time of year) you guys were dead on. got a nice firm pedal, went for a test ride without incident, checked it again today and it's fine. hopefully this is the end of it. thanks guys.
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post #10 of (permalink) Old 12-17-2005, 03:46 PM
 
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Re: air getting in brake system


Glad to hear you laid this one to rest!

Now, on to the 96,000 other things that need done!
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