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Old 01-02-2010, 10:39 PM
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cabloom300 cabloom300 is offline
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How to tell if the master cylinder is bad?

I replaced about half the brake lines in my 76 highboy and after bleeding them i still only get a little brake pressure and its close to the floor. How can i tell if its the master cylinder or how do i find the problem? Thanks
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Old 01-02-2010, 10:49 PM
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I am not sure exactly how to tell but they are usually cheap enough to change if in doubt ($15.99 at AZ w/ core).

I changed all my lines and found that the best way for me to get all the air out was by gravity. I went to the back right and the worked my way up leaving the valve open enough to drip and kept adding fluid, it also flushed most of the old out and made me feel better...

Best of luck!
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Old 01-02-2010, 10:53 PM
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When it does stop close to the floor does it stay there or will it bleed down.
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Old 01-02-2010, 10:56 PM
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If you can eliminate the rest of your brake lines, hoses, wheel cylinders and calipers as the culprit then it's a pretty safe bet your MC is bad.
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Old 01-03-2010, 12:14 AM
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I don't recommend replacing any part simply because it's cheap to do so. Only replace something until you know it's bad. Are you able to pump the pedal several times to build pressure, or does the pedal go to the floor every time?
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Old 01-03-2010, 02:35 AM
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I don't recommend replacing any part simply because it's cheap to do so. Only replace something until you know it's bad. Are you able to pump the pedal several times to build pressure, or does the pedal go to the floor every time?
Must be instilled in me from the Navy, PMS. Preventive Maintenance Systems. I change just about anything that has got some age on it.
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Old 01-03-2010, 03:20 AM
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Originally Posted by cabloom300 View Post
I replaced about half the brake lines in my 76 highboy and after bleeding them i still only get a little brake pressure and its close to the floor. How can i tell if its the master cylinder or how do i find the problem? Thanks
Help us help you sir. Did you replace the Front brake lines or the rear? If the front, did you bleed them at both calipers? If rear, did you bleed them at both wheel cylinders? How did you bleed them? GRAVITY bleeding is NOT an option! It may get you close, but what about that air bubble in the brake line above the rearend housing? That exact issue caused a crash and got a co-worker fired recently...His.. defence.."I gravity bled them"

Have someone pump the brake pedal with the engine running at least 3-5 times, and have them HOLD it. When you open the bleeder, have the person pushing the pedal keep it pushed to the floor untill you tighten the bleeder, then they can let their foot up. Start at the left front, crack the bleeder untill clear fluidand no air is comming out. Repeat the pedal pumping action and Fill the master ceyclinder before it get's below 1/3 rd of the way empty. Repeat this proceedure to the right front, left rear, and finaly, right rear. If no fluid comes out of any bleeder, Email back to us..we can help find the issue
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Old 01-03-2010, 05:14 AM
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Repeat this proceedure to the right front, left rear, and finaly, right rear.

All good advice but you want to start at right rear and work your way to left front.
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Old 01-03-2010, 05:27 AM
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All good advice but you want to start at right rear and work your way to left front.
All jokes aside..WHY? I've read all the wives tales about bleeding brakes and tried them all... well, I'll let you explain the rear way first..not that there is a farts difference IMHO..the end result is the same. I'm probably wrong though. But since it was worth a responce from you... Older Volvo station wagons even requested bleeding the right front first, why?

Power Bleeding is the best way...even with anti-lock brakes.....front\rear, right left, I've done it several way's, the end result should be clean fluid with no air. Bleed once a year and see how long a few dollars worth of fluid keeps your lines\and cylinders\calipers working like new for many years.
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Old 01-03-2010, 09:40 AM
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If you press the brake pedal and it feels spongy like it's pressing back against your foot that's one way to determine your mc is bad.

There may be others like if it has an obvious leak.
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Old 01-03-2010, 09:51 AM
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You might try leaving the cover off, stand in the doorway where you can see the master cylinder and slowly depress the brake pedal, you should see the brake fluid swirl as you push the pedal down, too fast and it will squirt right out, if not, it probably bad....
Also if there is air in it you should see little bubbles rising in the first inch or so of pedal travel... This has worked for me in the past.. After doing most of the above described procedures to make sure the are bled as well as possible.
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Old 01-03-2010, 11:55 AM
starmilt starmilt is offline
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If you can pump it up and it stays as long as you hold it and does not slowly bleed to the floor it is good.
Changing a good master cylinder with a cheap rebuilt one just takes up your time and money nothing else.
On these rigs a lot of the time you will never get your pedal back where it needs to be until you adjsut your rear brakes. And I know it was there before but if the wheel cylinders are to far out you have to adjust them to get it back.
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absolutely no dodges.
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Old 01-03-2010, 02:06 PM
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First off i replace from the valve back with new lines. My bleeder screws and rusted BAD and couldnt seem to get them out. So just thinking off the top of my head i unscrewed the end of the new line and bled it like that. How effective is this? It could be i still have some air in them but the back seemed good. Do i need to bleed the front as well even if i just did the back ones? Need some help as im not to familiar with brakes. Thanks all
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Old 01-03-2010, 02:42 PM
starmilt starmilt is offline
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starmilt has a very good reputation on FTE.starmilt has a very good reputation on FTE.starmilt has a very good reputation on FTE.
Here comes paul harvey with the rest of the story. lol you need to get your bleeders operrational or replace your wheel cylinders.
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absolutely no dodges.
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one town car.sold
If it breaks because of my advice
you can keep both halves
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Old 01-03-2010, 04:16 PM
4x4 Bart 4x4 Bart is offline
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I agree.......X2
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Old 01-03-2010, 04:16 PM
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