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I'm stumped...

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JohnnyO

JohnnyO

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Why does hydraulic fluid leak out of the top of my master cylinder filler cap?? 66 Midget, dual master cylinder (rebuilt by Whitepost Restorations last winter) new stainless steel brake lines, the brakes and clutch all work wonderfully, fluid filled to just below the bottom of the filler neck. But when I pumped the brake pedal and clutch pedal either in the garage or out driving it, I end up with fluid leaking out the top of my filler cap!!?? Any ideas??
 
Air, or badly out of adjustment drum brakes. If air, you pump fluid in, compress the air, let off, it all comes back. Brakes the same, you run the shoes out, fill the wheel cylinder with fluid, let off, it all comes back, especially if you have air in the system. It's all the non-compressibility of the air. How much pumping do you have to do to get "pedal"?
 
I have great pedal, on both the clutch and brake pedal, I don't need to pump at all. What I meant "I pump the brake..." was just in the garage to pinpoint where the leak was coming from. Rear brakes are adjusted good.
 
Fluid won't compress......so....if you step on the pedal, let go, and it comes back out, air. If you do it once, as in depress pedal and let off, you should see no overflow. Do it a couple of times, yes. If you have a leak OUT, it's probably leaking air IN. Get a 2X4, figure out length, cut it so it wedges between the front of the seat and the pedal.....adjust seat to get a hard pedal....then gut out and look for leaks. Bad seals in master, or wheel cylinders can allow air in. When they get old, worn, and deteriorated, they tend to collapse, often you need two hits on the pedal to get a seal (which pressure will do....pushing weak seals out to the walls).
 
More confused then ever now....Ok,I got an assistant to sit in the seat and work the pedals. Held the brake pedal down firm, checked for leaks, found none,master cylinder, lines,wheel cylinders, calipers... Same on the clutch side. I took off the filler cap and observed reservoir while the pedals were pushed in and out. When the brake pedal is pushed in a jet (eruption like, large wave) of fluid rises up out of the reservoir. But I don't observe any burst of air it seems to be all fluid rising. If you press the pedal slowly the rise in fluid is there too but not high enough to reach the filler neck. Pressing the clutch pedal with full force causes just a small wave (eruption) of fluid, not near enough to reach the filler neck.
 
I'll start by warning you that I'm not familiar with the later master cylinders, so this could be nonsense.

Below is a cross section of the master cylinder, based on my late Bugeye. The hole from the reservoir to the cylinder should remain between the two ends of the piston, but if the left end gets to the right of the hole, you will obviously get the kind of squirt into the reservoir that you describe when you push on the brake pedal. The manuals are notoriously vague on adjusting the master cylinder pushrods and such, so it's easy to get them wrong. Not sure if it's possible to get them that far wrong, though. Frankly, I doubt it, but it might be worth looking into, just in case.

I've seen this phenomenon on non-LBC master cylinders, but on the Bugeye, at least, I don't think it's possible to adjust it so that happens. But maybe it's possible on yours? I dunno.
 

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Steve,
I'll be adjusting my master cylinder soon, after reinstalling an entire braking system in my B/E.
If the manuals are vague, can you elaborate or point me in the right direction for instructions?
Thanks
Fred
 
If he's getting enough to come out the filler cap, it ain't normal.
And, if he's leaking fluid out (looking for a leak....) good chance he's sucking air in. Couple of strokes on the pedal with air in the system, when you release and it's supposed to recharge from the reservoir, it will squirt, quite a bit, and that should be your first clue.
 
Steve Mass!!!! You were right!!! And apparently it is possible to get the piston that far out of place. After seeing your diagram and explanation it dawned on me that 5 or 6 weeks ago I moved my pushrod back out of the cylinder thinking more total movement of the pushrod would be more movement of fluid. I obviously moved it back far enough to allow the piston to come back beyond the hole from the reservoir. I thought about it all night at work and when I got home this morning I went out and adjusted the pushrod back into the cylinder more and there you go, no more squirting!!!! Pedal pressure still feels great, maybe even a bit better. When the rain stops I'll give it a test drive for the final results. Thanks everyone for their help!!!
 
The manual I recall wants the adjustment screws to have 1/32" to 1/16" clearance before the pedal starts to more the piston from the rest position. Hard to determine, as one can not see the piston move. I have been able to look at the piston through the reservoir into the fill hole.
Scott in CA
 
Glad you solved it. I'm as surprised as anyone.

Well, as for adjustments--I can speak only for the Bugeye and probably other early cars having the dual master cylinder, and this is just what I figured out; it's not from the manual. Yes, the manual states the 1/32"-1/16" clearance, but there's actually more to it.

There are two adjustments: the bolt that is a stop for the pedal position, and the screw adjustment on the pushrod. The bolt affects not only the pedal position but the length of the stroke. On the bugeye, you need at least 0.4" of motion when you push on the clutch pedal, and you don't want much more than that. Then, the pushrod should be adjusted so there is just a little clearance when the pedal is at rest--that 1/32" to 1/16" figure.

For the brake, it's less clear, since the travel depends on brake adjustment. I've always just adjusted the stop so that the clutch and brake are the same height, then the pushrod for the 1/32" to 1/16" clearance. I don't think it's possible in the Bugeye to get the piston so you experience the kind of thing described here.
 
I buy that Steve. Good advise.
 
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