Edit done to list actual parts available for this repair updated June 15, 2021.

If your Atwood Brake actuator needs a rebuild here are parts available today to get that done.   I have not found a source for a new shock absorber for this coupler but my boat did not need a new brake actuator shock.  If one of you can come up with a new replacement shock for this coupler that would be great.

Some of the parts listed in the original 2018 post have now failed so I am updating to help give an accurate list of good repair parts available today in 2021.  

I am going to edit out my original posts to reflect these changes.

Dorman sells a Rebuild Kit for the 1" hydraulic Master Cylinder, parts in the Dorman kit allow you to rebuild your master.  The new piston cup and parts fit and work, there are slight differences in manufacture style and even length but these parts work and fit like new.  

Dorman Master Cylinder Rebuild Kit is Part # TM19356, this kit comes with a boot for the master cylinder that is too large and will not work.  You will toss this dorman boot.  The important internal Dorman rebuild parts work perfectly and this kit is available many places, Amazon was around $16 O'Reilly AP $32. so you can get these needed parts.

For the Rubber Master Cylinder Boot I found a part sold by EMPI, a old VW Air Cooled supplier.  The old VW rectangular shaped master cylinder has the right sizes to work with the ATWOOD coupler.  EMPI part # EMP-16-2162-0 also listed elsewhere as 16-2162 is a little longer, about 3/8" than the OEM ATWOOD part but fits and functions perfectly.  Any old Air Cooled VW house can get the EMPI part or AMAZON lists it for around $16.  It does fit the end of the Master but I thought it popped off a little easy so I used a zip tie and tightened it around the boot end and the master cylinder.  With the Zip tie it is secure, fits well and I have no leak concerns.

I repaired my ATWOOD two years ago because the master had rust in it, a quick hone, (master cylinder hone tools are available at most parts stores) and the master cylinder cleaned right up, with new parts it worked.   The boot I installed in 2018 failed, it split and water got in so my master rusted again and would not work this year.  The VW EMPI boot is an OEM quality part for a passenger car, it will last many years, very good quality.

Fel Pro sells gasket stock on small rolls which you can use to make a new gasket for the master cylinder seal to the atwood coupler.  I cut my new gasket from Fel Pro Gasket Stock with scissors and used a drill to make my bolt holes, since no factory gasket is available for the Atwood coupler.  I did not record the Fel Pro part # for the gasket roll but if needed I can go back and get it..

My Atwood coupler is welded on and parts are discontinued. Prices for those parts if available have gone up by five fold if you can find them.

In 2018 I tried a Tilton Tilton Master Cylinder Rebuild Kit part #74-1000RK, and the boot lasted only 2 years which allowed another failure.   Use the EMPI rubber boot and the Dorman Kit, they are a better repair for the ATWOOD.

I tried to add additional photos showing the parts purchased but only one photo would load.  The picture shows my Atwood coupler on our 1995 DHM boat trailer.  The receiver hitch is installed and I use a 1/2" bar about 14" in length with 100 pounds of weight slipped on to apply the brakes for adjustment and for bleeding the trailer brakes.  I wish I had an adapter to connect my Motive Brake Bleeder to this trailer, that would make it easy to bleed the brakes but with this set up I can bleed the system in about 30 minutes, not too bad.  With the set up attached with 100 pounds of weight like in the photo the weight will either be spongy or hard when applied.  If they are spongy you have air in the system, keep bleeding and get the air out.  Working proper with no air in your brake lines you will feel the brake actuator move when you apply the brakes and there is a firm feeling to the application, it goes hard and stops, you can lean on it and add more pressure but it should be hard at that point, no spongy feeling.  With 100 pounds of weight on the brake the tires on mine will rotate backwards with firm hand pressure but not at all forwards.  To get the left and right tires adjusted equally I use just 50 pounds of weight on the coupler and then adjust the wheel brakes manually with the weight on the coupler, I adjust till both wheels can be turned with a strong hand on them spinning them forward.  A couple clicks on the star wheel adjuster in each brake allows you to adjust each tire to apply equally.

Hope this helps.  I saw one of these Master Cylinders for sale on EBAY, just the Master cylinder, not the coupler, they wanted $400.   With the listed parts most of these old cylinders could be rebuilt so you avoid needing a new Brake Actuator which are available from companies like Titan for around $250 but if yours is like mine it is welded on to the trailer so not an easy swap.

Edited in 2021, the repair below did not work.   I listed parts that do work in the opening post.

Further Searching found a Dorman Part # TM19356 available on Amazon for $16.  There is a testimonial from a user that installed this kit in the ATWOOD actuator and say's it works perfectly.  I ordered and will install Monday.  These Master Cylinder parts look just like the ATWOOD repair Master Cylinder parts in Kit Part # 85852 which has been discontinued for many years but there is one for sale on Ebay for $40.

I also found the original Atwood part # for the boot is 84258 shown as part #8 in the Atwood parts picture.

I will report back if there are any problems with the Dorman kit.  These Atwoods use a 1" master cylinder bore so parts are pretty standard.

Hope this helps the next guy with an old Atwood Actuator for the Trailer Brakes.  

Empi makes a replacement boot, sold for old air cooled VW master cylinders part # 16-2162, this boot shows it's size in the advertising and appears it will work.  ID front and back are correct and the length looks good.  Hard to get, I had to order and wait.  Should have parts by this weekend but I can't get to this until later.  Will report back on the fit.  If this is a correct part between the two items an Atwood could be rebuild for under $30.   Better than cutting it off the trailer and installing a new actuator for $225.  

EMPI BOOT and Dorman Kit shown

JQ

I like them to work when needed.   If not so frugal I would have upgraded like you to disc brakes and a new coupler.   If the titan kit had a better rubber boot it would have worked.

After all the practice the Atwood can be rebuilt and working again in about 2 hours.  Bleeding the system takes a chunk of that time.

The last two times I've used it the trailer it has been super difficult to release from the hitch (without any boat on the trailer).  Specifically, when I pull up on the tongue release mechanism, the trailer coupler will not release from the ball of the hitch.  I've had to resort to pulling the hitch off the tow vehicle and then releasing the ball of the hitch from the trailer coupler. 

When the hitch is separate from the trailer coupler, I can feel the the part of coupler mechanism(not sure that's the right description) that slips under the ball of the hitch;  when I pull up on the release that part of the coupler does not release.  I have to actually push it out of the way in order to get it to release.

I wasn't sure if anyone on this thread has experience can provide advice.  Is there something that needs lubricated?  Repaired?

Any thoughts on how to proceed would be much appreciated!

Thanks.

Dave