Hydrovac modification update

[8683jb]

I've been meaning to post an update on the o-ring modification I made to the hydrovac a year ago, http://halftrackinfo.com/index.php?topic=2378.0, and thought this latest event would be a good reason. It's been working well, but I noticed lately the pedal was getting a little soft. I checked the master cylinder yesterday and it was very low. No external leaks, so I pulled the hydrovac and when I opened the vacuum control valve and atmospheric valve chambers there was brake fluid present so I knew the piston was letting fluid past it again.







I removed the piston and found a nick in the o-ring.








Looking for the cause, I noticed a sharp edge on the bleed hole in the piston's bore and realized I had to have shaved the o-ring on that during installation.






I replaced the o-ring and then temporarily slipped an 1/8" brazing rod into the bleed hole and lifted it flush with the sharp edge so the o-ring could slide into the bore undamaged. I disassembled the rest of the hydrovac and cleaned all the brake fluid out of the vacuum cylinders and it's back on the half-track now. After bleeding the brakes tomorrow, it should be good to go again. I'll do another update after I get more time & miles on the modification if there's any interest.

Jon

[p3aflyer]

Thank you Jon ,  good catch, appreciate the update.  I’ll have to review your technique on the rebuild once I get closer to it for my halftrack. 

[8683jb]

You bet John. Holler if I can help.

[Jesse]

The small cup in the rebuild kits is espcially prone to rot over the years. I used a teflon cup of the same dimensions with very good results.