1941 master cylinder - something's not making sense...

[Jeff Hansen]

All,

Please see the attached cross section of the 1941 master cylinder from the 1941 shop manual.  Take note of the bypass port shown.

I am replacing the master cylinder on the Sixty Special I am working on.  A rebuilt unit was included with the car which I am installing.  I disassembled the rebuilt unit tonight just to make sure all was well on the inside before I install it.  It's in great shape on the inside; the bore is nice and smooth with no pitting.  Upon inspection of the fluid reservoir, though, I noticed that the bypass port appears to be plugged.  Closer inspection of the unit shows it has likely been sleeved - but they didn't drill a hole in the sleeve for the bypass port.  "Not a big deal," I thought, "I can drill the hole."

On a whim, I looked at the unit I removed from the car - and its bypass port appears the same way; like it was sleeved and the hole was never drilled.

Am I missing something here?  I thought one unit not having that hole was odd, but to find two units in the same state?  Is this how all of them are being rebuilt?

Has anyone else encountered this or is it just bad luck on my part?  I welcome your thoughts.

Thanks,
Jeff

[Steve Passmore]

That hole Jeff is an absolute 'Pin hole'  Hard to physicality see it. I personally have never seen a drill small enough to drill it.  About 15 years ago had a 36 Packard that came with a re-sleeved master cylinder in the trunk. This was re-sleeved here in the UK so I fitted it. They drilled the hole with the smallest of drills but it was too large. When the cylinder rubber passed over this hole it would try to force itself into it shaving a tiny sliver of rubber off. Over a period of a few days of use there was enough missing for complete failure causing me to narrowly miss a brick wall and an oncoming car. It shook me up for days.   I never use them now, I always fit new. There's never that much difference in the cost.

Was the one you took off working OK?

[fishnjim]

I'd call the guy down at White Post Restorations as they resleeve wagner.

[Jeff Hansen]

Was the one you took off working OK?

Steve,

The brakes were not operational when I took delivery.  In opening up the m/c that was installed on the car, the fluid had solidified (is "gelatinized" a word?) and the cylinder bore had corroded and become pitted. I am hoping to use the rebuilt m/c Barry included with the sale.  I will do some checking with a flashlight tonight to see if the pinhole is really there in the rebuilt unit.

Thanks,
Jeff

[Jeff Hansen]

That hole Jeff is an absolute 'Pin hole'  Hard to physicality see it.

You're not kidding, Steve!  I did finally find the hole in my rebuilt m/c and in the original m/c.  The holes in both units were totally clogged with gunk.  I was able to clean each with the point of a pin, a single strand of 16 gauge wire and some rubbing alcohol to flush them.  That is one TINY, TINY hole!

I sure appreciate your input! 

Thanks!
Jeff

[Steve Passmore]

Love to know how they drilled them Jeff   You can see how the guys who did the cylinder here got a 1mm drill bit and thought it would be fine for the job but the rubber is under massive pressure when your foot is on the pedal and the hole has to be so small to prevent the rubber forcing into it.