mysterious brake issue

[packotobacco]

I totally agree that the problem most likely lies in the MC Or Booster. My booster seems to be working fine though I can't seem to find many specifics on diagnosing a booster systematically. the check valve works and the membrane seems good according to the diagrams I've found, i don't really know what else to check on the booster so i did some testing on the master cylinder. Tried to level the MC but not even close and tried to bench bleed in car using plastic plugs with short return lines but kept getting bubbles . Bench bleed was successful out of car, solid when capped and Nov  bubbles or leakage so i reinstalled it with return lines still in and no bubbles in car. However once i reconnect to bleed the lines i get more bubbles in the larger reservoir even after bleeding all air from the lines. Does any one know if its normal for the MC to suck air back in from the lines. And even after working bubbles out for quite a while i still get bubbles in just the larger reservoir and it never diminishes. I've read i might fix this with teflon on all fittings it's cheap and sorta makes sense,  gonna get speed bleeder screws while I'm at it. I'm hoping this info is enough to tell me something definitive about either components badness. Since having replaced everything at least once already I'm looking for ways to be sure a parts bad before replacing. I'm told my testing proves the MC is ok. Is that true or is there something else to test? What can i do to test the booster fully? Preferably without burning gas for vacuum. Could the fittings really suck in air but never leak a drop? Seems impossible but I'm getting air from somewhere and it's keeping my MC from staying full. And lastly can a booster issue send pedal to floor with no stiffness or stopping power? Thanks again to the whole den, sorry for the long posts and thread. Just trying to be thorough

[mudkicker715]

Booster is working from what i hear. Teflon on those fittings is not a solution or a fix. Speed bleeders are useless. Also when i bleed the system i just gravity bleed it. Unless a line is pinched it will work awesomely.

I still say replace the master.

[eagleman]

Well replacing the master cylinder may solve the problem . But if its leaking into the hydro its just a matter of time before it goes too. Best save yourself some time and just replace both now and be done with it.

[packotobacco]

I just replaced the master in september with a brand new, not rebuilt, unit. I see nothing to indicate that it isn't working properly as sponginess could be air in the lines. When capped the master gets firm and doesn't sink at all. And just to clarify it doesn't leak at all, i had one leak but it was on and off the car in the same week and i never even turned it over. I have replaced my mc four times for four different reasons and my soft pedal always returns after like a week. I'm all for replacing the mc but i need to know I'm not wasting my time and money as i am short on both. May i ask why exactly you think it needs replaced? I'm unclear because you seem pretty positive but i don't understand what lead you to that conclusion

[AMC1]

I can easily see your frustrations. However if I read the thread correctly you know what the problem is - in that the MC is needing some refill of fluid. Sounds simple but you have to find where that fluid is going. I wouldn't be overly concerned about air entering the system as finding out how that fluid is getting out of the system. If you have a friend to help I would remove the tires & drums - bleed the system & then have him pump on those brakes. You then starting at the MC - with dry fingers follow every inch of fittings & lines. Good luck 

[packotobacco]

Since last post I've gone over the lines completely and located that rear rubber line. The rubber section has no cracks or leaks even under pressure. I still can't find any issues anywhere in the lines calipers or cylinders. To be safe i checked the flares and found one that looked a bit ragged so i re-flared it and the two at the master. Afterwards i re bench bled and bled the lines  till all bubbles stopped and was pleased to see that the pedal got firm and i had no bubbles in the MC. Pumped the pedal and held it while starting and it sank. I understand that it's supposed to sink but i seem to lose all pressure with the vacuum assist on. When running i can feel pressure after a few pumps but it dissipates very quickly. When off the pedal gets firm again and i see no new bubbles. It seems as though the MC internal seals are allowing fluid past but only with the vacuum assist. I would swap it right now but  have changed it for this exact reason and had no improvement. I'm at a loss, is this definitely the MC? Could anything else cause this? Has anyone else experienced this and fixed it?

[1985amceagle]

Reading through the thread I also agree with the others on the master cylinder ruling. The booster is mechanicay linked in between the pedal and master cylinder. And a diaphragm and vacuum helps increase the force you are applying with the pedal. So the fluid is either going past the seals in the master, or you have a leak somewhere. What brand master cylinder are you using? I've had bad luck with some store brand items one was a hydraulic clutch master cylinder, at times the clutch pedal would be ok, other times it would go straight to the floor. Put on three of the same store brand before switching to a name brand, which havent had any trouble with since. I would check what the warranty is, maybe your parts supplier will get you another one, or credit for another brand depending on if you are in the warranty period yet. Also have run into a bad batch of parts before, so even name brand stuff can fail prematurely.

[AMC1]

I thought you were losing fluid but if you're not then like the others say you're probably best in replacing that MC. Good luck.

[packotobacco]

 So yesterday i capped the ports to the master cylinder and put some serious pressure on the pedal. It was solid enough but i thought i felt it sinking super slow and everyone seemed to be sure it was the master so i replaced it again. Dug out my receipt and used the warantee, bench bled carefully so as not to hurt the seals, then I capped the ports and connected it to the booster. Pumped the pedal and held while starting and sank and stopped 1" off the floor exactly like it should. So then i bled the lines one end at time just in case. Spent a long time bleeding after bubbles stopped again just in case. Held the pedal while starting again and it sank almost to floor with little pressure. Just like before with engine running i only get any resistance from the pedal after i pump it up and then it fades to bottom again. NO leaks anywhere, NO bad seals,NO bad booster, NO fluid loss, but still NO pressure. HELP!

[packotobacco]

I forgot to mention that when i was pumping the pedal my rpm's climbed slightly with each press. If i remember correctly thats normal but thought I'd ask

[AMC of Houston]

It is the booster.   Your rpm increase tells me something's not right with it (leaky diaphragm or some other internal vacuum leak.

[AMC of Houston]

Get a vacuum pump and see if the booster holds vacuum.

[packotobacco]

I hooked my mightyvac to to the booster and it held vacuum. And after turning the engine off i get 2-3 vacuum assisted pumps even hours later. I was under the impression that booster failure causes hard pedal. Could you please explain how the booster could cause my soft pedal.

[AMC of Houston]

Afraid I can't answer that one (not that familiar with booster guts).   I just know I had a similar issue on my Sundancer, and a new booster cured it.

[packotobacco]

So i checked my pedal last night and found that air had worked up into the master. I bled the lines three different ways and got pure fluid for a long time on each bleeder. Air must have come from the lines so I'm gonna try purge the air from the top end by applying vacuum to the reservoir. It's the only method i haven't tried yet so wish me luck.