[AJD] What are failed Master Brake Cylinder Symptoms?

[Tom in N Texas]

Dean,

Seems to me that you can easily isolate the problem between the master 
cylinder and external systems.

If it loses fluid, it's an external leak, master cylinder, wheel cylinder 
or lines. You can tell by pumping the pedal and noting if the level in the 
reservoir goes down, then look for a puddle and look for fluid in the booster.

If the fluid level does not go down, it's internal in the master cylinder 
or a soft flex line that expands or air. To see if it's soft flex or air, 
just pump up the pedal. If it will pump up (get a solid pedal), then it's 
soft flex or air in the lines. If the soft line bursts, that's a solution, 
too. If it won't ever pump up, the problem is in the master cylinder and 
you got a new, bad master cylinder.

However, you must also consider whether the master-cylinder/booster/pedal 
mechanical linkage  is OK. Another subject.

And you don't have to crawl under the vehicle, nor do a lot of plugging and 
un-plugging lines.

If you have a dual system (I missed the early post(s)) It's more difficult, 
but the logic still applies.

Hope this helps,

Tom in North Texas
------------------ YOU WROTE ------------------------
At 02:03 AM 7/15/05 , you wrote:


>Now that I reread the earlier messages, I know understand better what you
>were trying to tell me about plugging the MC to see if it operated properly.
>I didn't have anything here then to plug the outlet of the MC so didn't
>properly complete that test. Now that I have purchased a replacement MC, I
>now have two threaded plastic plugs, one of which I used to bench bleed the
>replacement, rebuilt MC.
>
>I didn't like the way the bench bleeding of the replacement MC went either.
>The instructions that came with the replacement MC said to manually compress
>the piston until it would only move about an 1/8". I never achieved that,
>even after several pumping sessions. I suspect I got it to stop at slightly
>less than 1/2".  I was troubled by that but it would hold pressure and stop
>at that point.
>
>I'm reporting this in case this information might be significant.
>
>Another thing that troubled me was I installed the MC on the firewall with
>the temporary plastic plug still in, per instructions, then removed the
>plastic plug and installed the brake line. Lost some fluid installing the
>connection. I should have tried the brake pedal with the plastic plug still
>installed but it didn't dawn on me to do so at the time. Dumb!
>
>But after I connected the brake line to the MC I did start pumping the brake
>pedal. No pressure at all. Pumped the pedal over and over, bled the line to
>the Vacuum Assist, pumped the pedal some more and finally got some
>resistance on the pedal. Almost identical results to when I first added
>fluid to the old MC. These new results immediately told me I was still in
>trouble. But I proceeded down the chain of hydraulic lines, bleeding as I
>went and then bled each wheel valve twice. Got air on three of the 4 wheels.
>But..... No improvement of the pedal drift down after initial resistance. No
>real change after bleeding the lines at all, which surprised me a bit.
>Actually the new MC wasn't performing noticeably better than the old one. A
>big disappointment.
>
>When I get some free time, I'll next block the outlet of the new MC and 
>see if it holds there with the plug in. If it does, I'll just work my way 
>down the chain until I isolate a spot causing the pedal drift. Don't know 
>how I will plug some of the fittings yet but I'll have to get creative 
>some how. Not sure the plastic plugs will work on the smaller fittings. It 
>is a male plug and in some spots I need a female plug.
>
>One other curious piece of data is, I am unable to detect any difference 
>whether the truck is running or not. For some reason I expected a change 
>due to the vacuum assist. Now, I can't remember what the difference was 
>when the brakes were working properly. This also has me a bit baffled.
>
>Why can't I have simple problems?
>
>Part of the problem I have working on this thing is I really get nervous 
>crawling under this truck. It took me months to get over having this truck 
>run over me a couple years ago. I got so close to being killed by this 
>truck due to a broken line it caused me nightmares for a long time and 6 
>months of very slow rehabilitation. I missed death by a whisker and I have 
>never forgotten it. Just have to get my mind to overcome that old fear. 
>Needless to say it is currently parked on a perfectly flat area with all 
>kinds of wheel chocks but it really is a challenge to crawl under this 
>thing. I'm probably never going to get over that near miss experience. And 
>probably shouldn't either.
>
>Have three busy, full schedule days ahead so probably won't get back to 
>this until Monday. Maybe a bolt of lightening will hit me and make all 
>this completely clear or no longer important!  :-) Or something I have 
>included in this message will trigger something from you folks that 
>identifies where the real problem is. I do appreciate all the input and 
>suggestions I have received. Thanks a bunch. I think I now have learned 
>from all your suggestions how to methodically work my way through the 
>components to isolate the problem. No different than most trouble shooting 
>procedures. At least now I understand the proper sequence.  I hate 
>kaleidoscope replacement of parts to find a problem. Made one mistake 
>already on the MC. Hopefully it is the last.
>
>Dean A. Van Peursem
>Snohomish, WA 98290

Tom in N Texas, KC5INU,  tdulin at pulse.net, "Nothing Runs like a JD MT Deere",
1949 John Deere MT, JD No. 50 Box Blade,  1954 JD No. 5 Sickle-Bar Mower,
1975 JD 214 Lawn Tractor, 1918-22 JD Dain Horse-Drawn Sickle-Bar Mower