You can read upon on seal dynamics and sealing solutions on the Trelleborg site.

http://www.tss.trelleborg.com/com/www/e ... _range.jsp

Excessive pressure will not cause an o-ring to last longer. It will do the opposite.

You might want to see if you can find the right seal for the piston. Optimal performance and longevity require the right type of seal(s) and wear-rings, exact dimensions, and a suitable duro for the elastomer, plus lubrication.

The tolerances are narrow: if the seal fits sloppily in the groove, and/or has too wide a cross-section so that it fits too tightly against the wall of the piston chamber, or if the piston itself is not kept parallel to the walls by a suitable wear-ring, then excessive or uneven wear of the seals can result, shortening their life and/or compromising performance.

P.S. I know firsthand that sourcing the perfect seal can be very time-consuming and expensive. I've had more luck dealing with European companies than with American ones, for sizing.

peacecup wrote: ...
Incidentally, I have a VAM that has the heavy-duty piston like the one on the far left, but with a hole in the piston face. That makes at least six different piston designs!

Peacecup, having a hard time following you here. The piston pictured at the far left does have a hole in the piston face. Did you mean the far right?

I think he must have done

well, after a few days, I still have intermittent leaking. I can fill the kettle up to any given point, and see a few drips, or sometimes, there is no dripping at all, everything seems to be good, but I keep a cup on the tray and in the morning the cup will be full of water.

Is there a chance that anything else could be causing this leak, or is it just an instance where finding a suitable replacement part for a 50 year old machine is at best a "close-enough" job?

noah wrote:Just replaced it again, this time with some DOW 111. When I refilled the kettle, I noticed something I had forgotten previously - that the leak is connected with how full the kettle is. If the kettle is only about 1/4 full, then no leak, but beyond that, there is leaking, even with the new ring.

Also, there is a water line, a discoloration if you will, at this point beyond which there will be leaking - I wonder whether or not this has always been a problem with this machine - or whether there is more than the small o-ring at play here.

The seal could have too wide an I.D. so that water is leaking behind the seal.

Get a pair of digital calipers at PepBoys ($14) and take some measurements. The I.D. of the seal should be several mm smaller than the O.D. of the piston at the groove. The cross-sectional diameter of the seal should both allow it to fit tightly against the wall of the piston cylinder and yet be not so large that the ring is too fat for the groove and gets extruded during use. It could be than an o-ring won't do (i.e. won't be able to meet both requirements) if the original design called for a v-type or u-type seal or even a straight-face/rectangular-cross-section seal, where it is possible to increase the width/thickness without increasing the height. In an o-ring, the two dimensions are inseparable.

I have a tendency to "put away" the caravel at the end of the day, so I have never left it standing with the reservoir full or half-full overnight, but I wouldn't be surprised if mine also let some water past the o-ring in the same way, especially after they've cooled down with no pressure on the lever. It's then just the weight of the handle resting on the o-ring and that's not much.

And the fuller the reservoir the more the likelihood of the water managing to push past the o-ring, which fits with what you have observed.

I suspect it's nothing to worry about.

I think mine even drips a little when left on during the day, though I've never really thought about it.

Anyway, I'll leave mine out like that and see what happens.... and then let you know. I'm sure others will chime in.

Cheers

Mike

michaelbenis wrote:
I suspect [leakage is] nothing to worry about.

Except that it can translate to diminished brew pressure and to seals failing to seal. If the pressure of standing water is enough to let water past the seal (direction: kettle to basket) then when pulling the shot, you could get water moving in the opposite direction, basket to kettle.

Hi Timo,

You always get some grounds in the Caravel reservoir if you do a Fellini move or more than one pull. A little always gets past the o-ring, which is of course supposed to "open" when you pull the handle up. This does not stop everything from sealing properly to apply the requisite pressure to the puck when pulling the handle down, when the pressure on the o-ring is obviously more than when the handle is at rest.

So when you insert leakage in your quote from my post you are, I think, misinterpreting me.

I am not talking about leakage from the main seals on the outside of the piston, I am talking about whether the simple weight of the handle applied to the O-ring inside the piston will be enough to prevent some water seepage overnight, especially when you have a full reservoir. My argument is that it may not be sufficient.

And I am suggesting that if this is the case, it may simply be inherent in the design and not a flaw either in the OE-sourced o-rings or a performance problem in terms of the espresso one can produce with Caravel (because the o-ring will "seal" when pressure is applied during a shot). To that extent, on a Caravel this may both be "normal" and something not to worry about. I am not saying that a leaky piston is not in other circumstances (e.g. leaking main seals) and on other machines something to worry about.

I hope that's clearer.

Hi Noah,

I hope you're getting along well with your Caravel now.

I can report that I filled up both my Caravels and left them for a day with water in and without using them, so the situation is a bit like yours overnight. No water leaked past into the drip tray from any of the seals, so it looks like Timo was on the ball there and I got it wrong.

Cheers

Mike

Hi,

I have the same problem with mid-century yellow Caravel.
On the outside of the piston it has the U shaped seal on the lower part and a thin seal on the upper part.

In the past couple of days I left it with little water in the tank.
Most of the water was leaking out, almost spilling out of the tray.

I don't understand if this is an expected behavior and how this affects the shot quality.
I feel that this is not right and that I do have a sealing problem. I checked the seals and they look OK to me.

What can I do to solve this?