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MG Midget and Sprite Technical - burping Master Cylinder

Looking for ideas on the cause of the following issue

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.

It's the early 3/4 MC in a MKI midget
J Van Dyke

BTW
Here's the dirty old MC before I cleaned it up. Last picture is reinstalled. Notice the bent pushrod.
http://gallery.me.com/jvandyke#100153&view=grid&bgcolor=black&sel=0
J Van Dyke

You'll have to click the MC gallery yourself I guess. Sorry
J Van Dyke

wrong kit??? wouldnt be the 1st time...

there is a vent inside the cly at the back, that transfers fluid back and forth (or some such non-scence) it may be blocked

might try dropping in an alkaseiser..or a pepsid AC into the master resivoir


how much have you spent so far on the master cyl. repairs altoghter....when you get to $700.00 (easy to do) Go to tilton enginering, and just get a really nice aftermarket set-up, esay to fix, cheap to repair,....oh wait thats right...install it and forget it...seriously I did have to sevice my tilton set-up recently after almost 3 years....yeah, it got a little dust on top of one of the caps, and I had to wipe it off with a rag, took me like 12 sec....WHEW!!, What a tough brake job day that was.

prop
Prop

OH!!!, you got one of THOSE master cly. cant help ya there

A bent pushrod,mmmm, have you checked to see if its still covered under the fatory warrenty

yeah, Im a bad boy.

prop....UP turned nose in the air...prop
Prop

So far I've got about $20 into it. One rebuild kit, some fluid, lots of time. I didn't count the cost of the hone, another $15.
I'd send it off for sleeving but I'm not convinced the bores are the issue. Actually it stops alright and no fade, not that ALL the air is out either I guess.
Just not too thrilled with the leaking! I'm sorely tempted to cap it with a well sealed, none vented cap and see what happens. I had put a paper gasket inside the cap (blocking the vent) but eventually it got through that too. It's almost like the reservoir is pressurized. I still can't imagine how the fluid works its way through that tiny little vent hole enough to drip on my foot.
J Van Dyke

J. Definitely a problem somewhere, if I am understanding you correctly. Your problem, as I understand it, is that brake fluid is being forced out of the reservoir through and around the cap covering the filler port on top.

If the fluid is to be forced out, it must be under pressure from some source. Since the design of the master cylinder does not have any form of pressurization applied to the reservoir (the fluid drains down into the cylinder bores only under the force of gravity) there is a problem in either the master cylinder, or how it has been assembled, if the reservoir is being pressurized. It should not be happening. It should not be able to happen.

Add in your bent pushrod. This is a fairly strong piece of steel measuring somewhere between 1/4" and 5/16" (from memory--do not have one at hand to measure). It takes a significant amount of force to bend a piece of 1/4" steel rod by applying force along the center line of the long axis. I would think that the force necessary to bend the pushrod might also be sufficient to damage interior of the cylinder bore. (I know you honed it and did not see any pitting. But, you might not pick up a crack running in line with the long axis of the cylinder bore, nor one at the end of the bore.)

If the cylinder is damaged, the situation begins to make some sense. When the brake pedal is depressed, the fluid in the cylinder bore is pressurized and the pressure is transmitted through the brake lines to front calipers (if so equipped) and rear wheel cylinders. This is the only source of pressure available within the system. Thus, in some fashion, the pressure applied by the leg, against the pedal, to the pushrod, thence to the fluid through the piston, is being partially diverted into the reservoir, forcing the fluid in the reservoir out the top.

This is the only explanation which makes sense if the fluid is actually coming out the top. If the fluid is leaking down the pushrod and onto the floor, it is a problem with the seals to the master cylinder bore (and fluid level would drop, in the reservoir, from a leak via either mechanism).

You need to find out, exactly, where the leak is coming from. If it is from the seals, it may be possible to resleeve the cylinder or to rebuild it again with a fresh set of seals and have it work properly. If it is actually coming from the reservoir cap, there is an internal problem within the master cylinder which needs to be found before any decision can be made.

All of this is an intellectual exercise--I have never seen a problem such as you describe. But, it is the only thing which fits your observations.

Les
Les Bengtson

That makes sense. It did not do this before the rebuild though. The brakes were "fair" when I got the car. Just occasionally soft (which is bad). No leaks. Could be the internal main seal on the brake piston was leaking fluid past, therefore not a great enough amount of pressure to reveal the MC defect. Now that I've got it sealing better, the "defect" (if there is one) is showing itself? It's weird.
J Van Dyke

I went out just now with my son, 8 year old not-to-thrilled helper (sick of pushing pedals for dad). I wanted to observe as the pedal was pushed, pretty significant discharge of fluid straight up, I'd guess it was coming up the hole it's supposed to drain into the bore through. Since it's done it after two rebuilds, the odds of my making a stupid mistake (seals in backward or something) twice is pretty remote? The valve was just a plastic thing that sat on the end of the spring, that looked fishy.
J Van Dyke

I'm getting some feedback elsewhere that it may be pushrod length and piston position. I have a feeling the spring isn't pushing the piston all the way back, not getting good travel? Sending juice up the fill hole. Guess I should pull it out and rip it down.....again.
J Van Dyke

This thread was discussed between 17/10/2008 and 18/10/2008

MG Midget and Sprite Technical index

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