Author Topic: Hydrovac modification update  (Read 585 times)

8683jb

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Hydrovac modification update
« on: February 26, 2021, 10:23:55 PM »
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
« Last Edit: February 27, 2021, 09:00:32 PM by 8683jb »
'42 Autocar M2A1

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p3aflyer

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Re: Hydrovac modification update
« Reply #1 on: February 26, 2021, 11:21:02 PM »
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

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Re: Hydrovac modification update
« Reply #2 on: February 26, 2021, 11:52:59 PM »
You bet John. Holler if I can help.
'42 Autocar M2A1

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Jesse

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Re: Hydrovac modification update
« Reply #3 on: March 18, 2021, 07:02:05 AM »
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.