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I hate asking this for fear what I will find out. I cleaned out the treadle vac master cylinder reservoir. Dead center 12 oclock position on the top of the cylinder there is a chip missing in the cylinder wall where the piston sits. Looks like a little key way passage Its even with the drain hole in the top of the master cylinder reservoir and right in front of the compensator value. Tell me this is suppose to be there.

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I don't recall that notch being there but it's been awhile so others can better answer. The whole sealing area looks rough though so if that is part of a seal surface for the cup, would wonder.

One thing I am curious about is your rebuild kit. If new, what kind of cover gasket or instructions did you get with it. There was an issue we found with gaskets covering the relief port on later units. Some problems result if that happens because of the Packard cover not being for the gaskets they were sending. Some history starting about here:https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=61166#forumpost61166 Was wondering what the vendors are doing now -- shipping the same gasket & advising to rotate it, cutting it or what.

My kit came from Max Merritt. Haven't checked it yet, I stopped when I saw the chip.

To be clear, the piston (plunger) only "sits" in the rubber hydraulic seal. There is some question as to whether a phenolic washer, used in that stack of seals, actually supports the piston (plunger) rod. However, the rod should never touch the cylinder in normal operation.

The lip with the chip out of it is merely a stop for the rubber cup seal. The plunger seals are held in a fixed position - never travel beyond that point. The plunger moves through the seals. So, it's the condition of the wall of the counterbore ahead of that lip that's critical - as is the surface of the piston rod - to sealing.

The chip appears to be in a thin portion of the casting, between the counterbore for the plunger seals and the bore for the compensator port valve. That valve makes its seal against the the top of a fitting screwed into the casting, sealed with an o-ring. Make sure there are no cracks in the casting that would cause leakage between the plunger bore and the reservoir, including the drain/vent passage.

Hard to tell from your photo, but if the counterbore is badly pitted, it may require sleeving by a rebuilder or machinist who is competent with this design - provided the casting is otherwise sound.

In addition to the shop manual instructions for overhaul, have you reviewed Ross Miller's excellent "HOW TO"?

The lip with the chip out of it is merely a stop for the rubber cup seal. The plunger seals are held in a fixed position - never travel beyond that point. The plunger moves through the seals. So, it's the condition of the wall of the counterbore ahead of that lip that's critical important - as is the surface of the piston - to sealing.

I agree except that the way the cup is shaped & oriented, fluid pressure would tend to push the edges of the cup out to the counterbore and in toward the piston shaft making a tight seal. With that notch, is there enough or is the phenolic washer enough to prevent that same fluid under pressure from being able to be pushed around the top outer circumference and then behind the cup seal? I don't remember anything except a smooth surface but like I said, a fair time ago.

The chip-out doesn't quite reach the wall of the counterbore.

One might try repairing the chip with one of those aluminum weldng rods, but would need to have a fair amount of experience with that type of work and be able to make some sort of coppper dams to keep the repair material out of either bore. even then, it might need some delicate grinding with a Dremel to dress the repair.

The average DIYer might do better to look for a good used casting.

I should add that it's not unsual to find pitting damage in the casting, which requires attention/repair, but I've never seen a chip like that in a BTV before.

As noted the seal pushes out against the wall, which should be smooth. Don't much like the look of yours. And no, the chip should not be there. Have lots of better ones in the basement. PM if interested.

I'll agree with everyone else that the chip should not be there. I'm holding one in my hand rite now and it has no such chip.

Just out of curiosity, drag your thumb nail or a finger nail around and thru the chip. DOes it feel ruff or smooth????