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Master cylinder madness

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jvandyke

jvandyke

Luke Skywalker
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So I waited on yet another master cylinder kit and rebuilt yet again and now it's worse! Had a good feeling pedal during bleeding, took off for a test drive and by the 2nd time I hit pedal (to slow down for the end of the driveway) NOTHING. All the way to the floor.
I guess I'll tear it down again to see what happened then it's off to Whitepost or someone for a bore and sleeve. Unless you think a 7/8" would be an alright replacement?
Am I missing something? I can pump it back and get some braking but it only last a push or two then nothing. Air somewhere wouldn't have those symptoms would it?
At least the car ran GREAT after the tuneup. Man what a difference, too bad I can't STOP. Wow am I bummed. Oh well.
 
I talked via phone to a guy and during the conversation I realized maybe I honed with too fine a stone, leaving the walls too smooth. Is that likely?
Clutch feels better than ever BTW.
This guy has a decent 7/8" he said I can rebuild and try out and if works I can buy it from him (for a great price). Nothing to loose trying I guess.
What's the downside to running a 7/8", seems like it'd provide a bit more push power and change pedal feel slightly, no big deal?
 
Another thought, one of my pushrods is slightly bent, maybe this is applying the force "skewed" slightly, causing the main seal not to seal all that great?
bentrod.JPG


I doubt it. I'll swap pushrods but well, I just can't see that mattering but what do I know.
 
Too smooth? I assume smooth is better than scored. I was worried my stone left things too scored.

I'd smack that push rod on an anvil to flatten it back out just for prospertity.

I ended up honing my brake side almost to the point of leaking, but it's been holding tight. My brakes are squishy like they've always been, so I think it's time for new rubber hoses or stainless steel ones. My pedal doesn't squish to the floor though.
 
I guess you want cross hatching in the bore for good sealing, not a polished surface, I guess.
That push rod won't straighten! I tried smacking it with a big hammer, it laughed at me.
Yeah, stainless lines sounds good.
I can't understand how my brakes could be fine all summer then go soft and it not be something simple, I don't know. I think I'll try my friend's 7/8" if that doesn't work I'll get the 3/4" sleeved.
 
Have you checked the cups? I had a similar problem, and there was a super-fine split in the rubber that caused similar symptoms to yours. Another thing to check is the little plastic valve that goes in the end of the bore.
 
When I took it apart a week ago, I found a tear in the brake side main seal. This had only had 50 miles on it or so. Now it's doing the exact same thing. I'll take it apart again tonight if I have time and if I find the new cup chewed up as well I'll conclude that bore has rough spots.
There's just not that much to go wrong with these things I don't think. It needs sleeving or replacing.
Whitepost will sleeve it (and bead blase and paint) my MC for $160 or so, then I can rebuild it myself and will probably be done forever, but I'm planning on trying a 7/8" bore one that is available locally as that would be far cheaper and faster. Only concern is how the brakes/clutch will behave with a slightly larger bore master. I imagine I won't notice the difference.
 
Going from 3/4 to a 7/8 master gives a 1/3 increase in the amount of fluid pushed for a given pedal travel. So it will take 1/3 more pedal force for the same stopping power. Of course the pedal will only have to move 1/3 less distance. I have used Whitepost several times with perfect success. If you let them do the complete rebuild, they will guarantee it for life.
 
So the clutch would require more force as well? Right now I used a nut behind the slave's pushrod to extend it slightly. Might the use of 7/8" bore on the clutch side cause problems then? My clutch felt better than ever on the last and very brief (no brakes) test drive. I hate to resolve one issue (brakes) and create an new one (clutch problems). Maybe just removing the nut will compensate for the increased "push" on the slave?
 
Tore it down again. Brake side main cup seal has a hole exactly like the last one. I must conclude the bore has a bad spot in it beyond the reach of my finger.
 
A hole? If the damage is anywhere but the perimeter (bore contact) it could be the brass washer cutting into the rubber, I had that happen too, but not so much a hole as a slit! I have used both bore sizes and did not notice a great difference though they were different!!
 
:iagree: If it's a hole anywhere but at the outer edge then something else is poking it in there. If the bore is rough enough to tear or damage the cups that quickly then it would be quite obvious to the eye. My Sprite is nowhere near that point yet so I am just speaking from many years of general experience rather than spridget specific. But I'm sure the principle still holds. The reason they went from 7/8 to 3/4 was to reduce the pedal force required with the disc brakes as it takes more force to operate the discs than drums. finally, I'm not certain but I think the clutch cylinder stayed the same with either brake master. As an aside, Some time ago, I changed my wife's Super Beetle to front discs without changing the master and the difference in pedal is really not noticeable. I still think you will feel a 1/3 increase in required force though.
 
Re: Master cylinder madness--mystert solved

It's a tear on the perimeter. See picture.
It is indeed the bore, it's pitted down deep.
I think I"ll try a 7/8" one and if I don't like the feel of it, then I'll have mine resleeved. In any event, I'll keep good track of it and it's unique 3/4" pieces.
tornseal.JPG
 
Re: Master cylinder madness--mystert solved

Got the 7/8"s and the rebuild kit from a friend, but no joy as the pushrods he had were the same length as mine and that's too short. I put it all in anyway thinking maybe I could adjust it out but with the 3/4" pushrods adjusted to their maximum I run out of pedal travel at the stops before any pressure is generated.
So I need to get some 7/8" pushrods, right? 4 3/16" is what I need I think.
Anyone willing to part with some?
 
Re: Master cylinder madness--mystert solved

Got the correct rods, got the 7/8's installed. Brakes are good. Clutch side needed the nut behind the slave pushrod still. Pedals are certainly firmer and a bit more push is required but I really like it better this way. Feels good. Clutch feels like it's a bit "higher" in pedal travel and I like that too, it seems to engage/disengage mid stroke instead of near the bottom.
Hope it lasts a while!

Recent tune has this thing running better then it has in a long long time (certainly since I've owned it). It's really a whole new car. I only had time for a quick run and I'm really anxious to get it out again tomorrow!!
 
Re: Master cylinder madness--mystert solved

Enjoy :banana: :driving: :banana: !!! re: hope it lasts a while...it will! at least until a little air softens it (eventually!), but it sounds like even with that, you are ahead of the game with that set-up! Have fun!! :thumbsup:
 
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