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Hydrovac repair

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  • Hydrovac repair

    Years ago I bought a used Hydrovac to put on my Hawk. The seals (rubber) were shot and allowed engine vacuum to draw brake fluid into the engine. Talk about smoke! I never pursued repairing it but I have now. It must be a pre-59 model becuase the internal seals and piston are slightly diferent than what my manual shows. A seal kit is impossible to find and I didn't feel like paying mega bucks to one of the rare rebuilders for the privilige of refurbishing it. The biggest obstacle was the seal on the slave cylinder, 1" OD and 5/8" ID which it seems is impossible to acquire. I ended up using the seal out of a 70's-early 80's Chrysler product car master cylinder. The OD was right but the ID was 11/16" which was entirely too loose to seal. After another night thinking when I should have been sleeping, I ended up applying soldering to the groove on the spool and turned it down to the proper diameter with a makeshift lathe (drill press, flanged bearing, c-clamp, wood chisel, yeah I know, kind of crude!). That done, I was able to get Napa to special order a seal for the piston shaft ($11). Again kind of an oddball size. I don't have the seal # at present but the hole is something like .313 shaft size and .990 OD. To replace the little square rubber seal that seals the vacuum chamber I found a $3.26, 6-3/4" diameter, 1/8" thick o-ring from Ace Hardware that worked great. I reassembled it, squirted on a little paint and saved myself several hundred dollars in the process. It's unlikely you will be able to get seals only for the Chrysler master cylinder, but buying an entire rebuilt unit will still be cheaper than sending your Hydrovac to a rebuilder. Don't overlook a junkyard, I had several master cylinders at home from the mid 70's and all of the seals were still in good shape. If anyone tries this, I wouldn't use anything but solder to build up the spool. Brazing requires heating the base metal to a dull red and I'm afraid doing so could possibally distort or deform the piston. In addition, you'd have to remove the springs to prevent their loss of temper. If anyone has questions, holler.