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burping master cylinder

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jvandyke

jvandyke

Luke Skywalker
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I guess I'll start a new thread on this.
I rebuilt my M/C earlier this year (sudden pedal softness occasionally). Then I did it again because it still didn't feel right, second time around I used a hone (very very sparingly). Bores looked and felt good. Used a new main seal on the brake side as I stupidly ruined the one in the kit. Just got a 3/4 cup seal from NAPA.
Brake side was very "tight" to the point were the spring would barely push it back out, "good" I thought.
Anyway....problem is brake fluid is getting pushed out through the M/C vented cap. I slipped something under the cap to seal it (although it's supposed to vent I know) when that failed, had more fluid come out again (enough to wet my floor mat).
What's up with that? Must be something about my rebuild. When pushing brake pedal with cap off to observe, a good "burp" is seen.
The "valve" in the rebuild kit was different, could that be in wrong or something? Tempted to put a well sealed non vented cap on it as the brakes feel good to me now.
 
Perhaps the adjustment on the push-rods needs adjusted. The Napa cup may have altered the throw distance of the piston. There are small holes in the bottom of the resevoire that the piston (and cups)must be aligned with properly otherwise you are circulating fluid in the resevoire with each pump!
 
That was my thought as well Mike. Push rods adjusted in correctly allowing the fluid to push back directly into the resivor. Cap must be vented.
 
Well, correct me if I'm wrong but the piston is pushed by the spring to the limit of its travel, push rod clearance (as per Haynes) is 1/32" which just insures the piston can travel its fullest (clearance is to be sure the pushrod isn't preloading the piston).
I don't see how, if you aren't pushing on the piston (you have the clearance), it can be misaligned?
And yet, something's wrong! I can just start adjusting things and see what effect it has.
 
Yes but you are not considering the adjustment on the push rods where they go into the clevis. That makes the push rod and thus the piston rest at a different spot and move within a different portion of the MC barrel.

OHhhhhhh, yep need somewhere about 3/8 of an inch thread showing for the piston to travel in the right area as there are two holes from the MC resevor to the barrel and they must be open and closed at the correct time in the cycle.
 
Okay, look at the last picture in the MC gallery here, I'll adjust that then, right?
https://gallery.me.com/jvandyke#100153

I changed both ways, seemed to make it much worse, like touch the pedal a geyser of juice shoots out.
 
You will see a burp when first pressing the pedal. That is when the fluid within the cylinder is being compressed, but before the supply hole is closed off. It should be slight. If it is burping too much, the supply hole isn't being closed off, or isn't being closed off early enough. This could be due to wrong size (length) of the seal, or the seal failing completely.

You are correct in the assumption that the spring pushes the piston to the limit of it's travel, and that if the push rod doesn't touch it, then it's ok. You just have to be sure that the spring IS pushing it back. You mentioned that it's tight and stiff to move.
 
Okay, must be something amiss with the internals then. I'll plan another tear down and see what I see.
 
I think you understand how that MC works now. Oh you can take the piston out without removeing the MC from the car.
 
I'm not sure I can get THAT piston out with MC in the car. That piston was tight in the bore! I'll try first though, if I can't not much time lost.
 
Piston should not be tight in bore but the seals should be snug.
 
That's what I meant, the piston as an assembly with seals was pretty snug on the brake side after I put in the new 3/4 cup seal off the shelf from NAPA. It was burping up fluid before that though.
 
There should also be a valve at the brake base (the clutch side does not have this valve). This valve could also be faulty or missing. I have also experienced the problem you describe when there was an almost imperceptible crack (hole) in the main cup (seal between the spring and the piston) which allowed the fluid to blow by!!
 
Still haven't addressed this thoroughly (driving time is rapidly dwindling as winter sets in, so avoiding down time at all costs). What I did tonight was a workaround. Earlier this year I had made a cap for the MC with a fitting to allow pressurized bleeding. I put this cap on with a tube coming out into the disfunctional washer bottle (in the highly unlikely event that fluid makes it all the way through the tube). To minimize fluid squirting up into it, I put a vacuum T fitting under the cap. Vigorously pumped the brake and saw no seepage anywhere. So now I have a mega vented MC. Still not sure what the real problem is, will tear down when the weather turns.
 
Weather turned. Pulled it off and investigated. I put some plugs into the outlets so I can play with it on the bench. Swapped main cup seal from clutch side to brake side and it seemed to have reduced the surge. I can only push the piston in about a 1/4" using my hand, no fade, so I think the MC is in pretty good shape. Just wants to pressurize the reservoir and leak out the vent yet. I'll put all new seals on (for the third time this season) and see what happens.
Here's a video my son and I made.
 
From your sig, I can not tell what your first name really is but..........

Van before you put new cups in that cylinder, and cups from the local parts store are fine. Check that bore with your finger to make sure there is no burr in there that is destroying your cups.

Otherwise I would suggest it was a fluke and you will be fine.
 
I did hone them lightly last time around and they feel alright. It is hard as a rock on the bench too (just pushing by hand) after the first 1/4" of movement I can't budge it by hand, maybe a lever (pedal) and foot power it would be different. I'll hunt down some more seals (which isn't as easy as it sounds) and see what happens.
Made list of winter projects on the Midget. Should be fun.
Convert to negative earth is on the list. Rewiring the tach is then in order, that's the only thing I'm a bit worried about.

PS
it's Jeff
 
Any good auto parts house should have a very large assortment of seals/cups. They are marked with size.
 
Yeah, you'd think. I had to hit three different ones last time I wanted a few. This time, at the NAPA that had them last time, I got funny looks and they struck out. Another NAPA by work, the guy had no idea what a 3/4" cup seal was, had to go ask an old guy. They didn't have either but could order in at $2 a pop and a few days. Silly. I'll hit a well supplied brake parts place and if that fails, I'll just get a rebuild kit from Moss or VB. Autozone is useless. I must not have found the right parts place yet.
 
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