At 10:03 AM 10/13/00 -0400, you wrote:
>http://british.nerp.net/car/mc.html
>In that picture, are you talking about the dowel pin he shows? My
>1980 master cylinder has that pin, and other then holding the
>secondary piston in place, I can't figure out what it's for. I do know
>it works fine without it there, and the replacement mc I bought
>came without the pin installed and works well.
The dowel is NOT the tipping valve. Actually I suspect
that the late-design MC does not have a tipping valve. Everything
I describe below applies to the older MCs with the rounded reservoir.
>But I admit to not fully understanding the Spitfire term tipping
>valve. I've heard it mentioned, but I haven't ever really understood
>it, and still don't.
>
>Normally when the brake pedal is pushed down, the piston
>immediately covers the opening to the fluid reservoir. Hence
>the little squirt you see in some reservoirs when the brake
>pedal is pushed quickly. As soon as the piston covers that
>hole, the circuit is sealed, and pressure can build up.
Yes, you are describing the same thing I was in my posting.
I suspect that the later Spit MCs work this way.
>If fluid is needed (say to replenish a leaking circuit), it can
>leak around the piston seal(s) in the master cylinder due
>to their orientation when the brake pedal is released to
>come back up. The seals only work one way, like a check
>valve. Otherwise, once the brake pedal is fully released,
>the reservoir is open to the circuit, allowing free
>equalization of fluid.
One key difference is this:
On the "conventional" MC there is low-pressure fluid behind the main
piston seal. This area is connected to the reservoir. Thus:
1: Any leakage past the seal is captured and returned to the reservoir.
2: If fluid wants to be sucked backwards past the seal (like you describe
above), there is a supply.
3: It is possible to have a fluid supply hold that is covered by the
piston.
BUT on the older Spit MC, there is NO such thing! Any fluid that leaks
past the main seal immediately drips onto your feet! Thus you
need a separate valve to supply fluid from the reservoir to the actual
working cylinder.
>So the description of a tipping valve allowing fluid into
>the master cylinder bore seems odd to me, and not
>in my 1980 master cylinder or some of the other
>ones I've had (no pre 70's though). I don't know if
>I've always been missing something or what. It's
>very confusing to me, especially using the word
>"tipping". What on earth is tipping, and why?
When the piston returns to its resting place, it "tips"
the tipping valve lever, which tips the valve's rubber seal from
its seat and allows fluid to flow between the reservoir and cylinder.
Note that a simple check valve would NOT work here. As
your brakes heat up in use, the fluid expands, and it must
be possible for it to flow back into the reservoir.
If it can't the brakes will lock up. (Ask me how
I learned this...).
You may ask: "Why did Triumph pick such a complicated
and failure prone design?" Of course, you
can ask this question of many parts of a Spitfire :-)
But it looks like (based on the pictures you mentioned)
that they finally switched to a better design for
the later 1500's. I have never taken apart a
new-style MC, so I cannot say for sure if it
avoids the problems I described for the older ones.
Doug Braun
'72 Spit
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