If a machine utilizes a sponge bar, it's essential equipment. The function of the sponge bar is to press the needles down. Inside the bed, where we can't see, there is a comb spring pressing the needles up. the tension between these two forces keeps the needles positioned to perform optimally.
Some European made machines such as Passaps, Superbas and Orions use a spring to do more or less the same job. While the spring is a wear item, it lasts a lot longer than a sponge bar. A sponge bar may wear gradually or may seem to fail very suddenly.

What happens when a sponge bar stops working? The short answer is "mayhem." In greater detail, absolutely everything may start to go wrong, including but not limited to: occasional mis-patterning, failure of a cast on that should work fine, disastrous mis-patterning, failure to knit off some stitches randomly, total failure of fairisle and eventually terrible carriage jams. So, clearly, keeping the sponge bar in good shape is something every knitter wants to do. Sometimes, the machine continues to work fairly well in stockinette or work with extra weight but misbehave when under any challenge such as tuck stitch or fuzzy yarn.

A sponge bar that looks okay may or may not be. It should be puffy and bouncy with the sponge part extending well above the metal part. I'll call the metal part a tray because that's what it really is. It's a long skinny tray that holds the sponge. I have had "new" sponge bars that looked absolutely fine fail to function when inserted in the machine. Close examination showed that these had been stored for a long time. Though they were un-used, the foam had lost its bounce. This isn't a really common problem but is something to look for if your troubles continue after changing a bar. They don't last forever in storage. My old/new bars were part of an estate find. I'm sure that's how un-used bars got so old.

Some experienced knitters recommend removing the sponge bar from the machine when the machine is not in use. The reason for this is that the sponge remains compressed while in the machine but relaxes when it is removed. Allowing it to relax may extend its life. I don't personally do this but I knit constantly so the machines are hardly ever not in use. I do see the sense of the advice.

Original equipment manufactured sponge bars have become both expensive and sometimes hard to find. Some brands are simply unavailable. Recently, we have been hearing from a lot of customers who had trouble with brand new sponge bars. I've done a good deal of research and bought some after market bars myself and here is what I think is happening. Inexpensive copies that are a pretty good fit for the machines are being made in China. Some of these my be fine but the ones I have bought were not good performers. Some of them just didn't do the job well and others rusted almost immediately. Customers have told us of bars that didn't fit properly, too. My sense of the matter is that those manufacturing the bars don't have enough knowledge of their function to produce exactly what we need. So at this point, we are recommending spending the extra money for original equipment from a dealer or re-building your old bar.

Re-building is actually quite simple. It involves a careful cleaning of the tray and replacing the old, worn sponge with fresh sponge. I have a video on the subject. But re-building has its hazards. The first thing to beware of is the sponge itself. I personally use only sponge from spongebar.com It has been satisfactory every time. I use the self-adhesive kind and the adhesive is good. But this product is not cheap and similar looking sponge is available at much lower prices from eBay sellers. I don't recommend it unless you are very experienced. The reason is that an experienced knitter will know right away if the sponge is or is not working well. She can afford to risk a few dollars on a potential bargain and discard it if worst comes to worst. A newer knitter won't be certain and may suffer endlessly and even damage her machine.

The link to my video is https://www.youtube.com/watch?v=06MmooHAkZ8
Easier than copying the link is to visit theanswerlady.com From the home page, click on the video index. It's on the right side of the page. All of our videos are indexed there with direct links.

Another popular way of re-building is to find weather stripping or some kind of craft foam that fills the bill. The process is the same and this can be a real bargain. World wide, the available options vary considerably and this makes things tricky. I know several knitters who have perfect records re-building using hardware or craft store materials. I know of quite a few epic fails, too. I think the issue is that what is sold in my neighborhood under a certain description may not be the same product as sold in your neighborhood under a similar label. The process for re-building with this sort of sponge is the same as with purpose-built sponge. Again, I recommend this approach only for experienced knitters, not newbies. If you find ideal weather stripping and it is self-sticking, don't trust the adhesive. Add your own.

All re-builders must be extra careful that the ends of the sponge are firmly fastened to the tray. Otherwise, when the bar is inserted into the bed, the sponge will ripple up into a nasty wad. It requires a total dismantling of the machine to undo this quick, easy mistake. I have known machines to get discarded over this issue! So be careful. Removing and replacing the end clips or taping the new sponge securely are both acceptable options but security is a must.

A sponge bar is inserted above the needles with the metal side up and squishy sponge side down. When it is in correctly and in good shape, the needles should be flat on the bed. Some models will lay flat even without the sponge bar in position. But extend a few needles and push up from below them with your fingertip. It should take some pressure. They should not rattle and should snap back down. If all that is true, very likely you are ready to knit.