Some time back, I rebuilt my sitting-for-decades hydrovac using an NOS kit I got from Dave Yamulla. It worked great for a short while but then about 3 or 4 days later the master cylinder was empty, and the pedal was on the floor. No external leaks anywhere; the fluid had transferred into the vacuum cannister.
Hmm… I tore it all down and disassembled the slave cylinder again to make sure everybody was installed correctly and tried it again. Same thing happened – good for a few days, then fails. Evidently this is not uncommon with these particular units and there is a fix for the slave cylinder that involves machining something and installing an improved seal somewhere. None of the rebuilders would give any details about the repair though; they would only include it in a complete overhaul for 650 to 700 bucks - plus ridiculous shipping costs there & back. Well I didn’t want the whole unit overhauled; I only wanted to send the slave cylinder to get the seal modification done – the rest had just been overhauled and was working fine.
Then I tried the other option, which was to buy a later hydrovac and build mounts for it and be done. No one has any. Apparently, they’re core-reliant and there are no cores coming in at this time. I had paid for one back in November and after two months with still nothing in sight, I finally cancelled the order and thought about finding out why mine leaks.
I called an outfit called Karps Power Brake in Upland, CA. (
http://www.karpspb.com/) and spoke to a fellow named Jeff. What a great help!! He spent several emails and phone calls with me trying to get to the bottom of it. After sending him some pictures and some scans of the Tech Manual, he suspected that the trouble was with the atmospheric valve piston. After studying the schematic of the system, it made perfect sense. When the engine is running, vacuum is always present on the control valve side of the piston. Brake fluid is always present on the slave cylinder side of it. There is no seal on the piston, and with the wear on the piston and its bore over the years, there becomes enough clearance to allow brake fluid to pass into the control valve body and then into the vacuum canister.
I thought I’d try simply installing two o-rings on the piston to see if that would cure it. I cut two grooves in the piston to accommodate two ½” O.D. x 3/8” I.D. EPDM o-rings.
I reassembled the slave cylinder and after disassembling and cleaning the vacuum cannister and all the vacuum components, mounted it back on the half-track and bled the system. It’s been two weeks since the unit was reinstalled and there’s still a firm pedal (I’m delighted there’s ANY pedal) and the half-track stops perfectly.
I hope I haven’t spoken too soon, but this seems to have fixed mine and it all cost less than $16.00 (I only needed two o-rings but had to buy a bag of 100 from McMaster-Carr).
Many, many thanks to Jeff at Karp’s Power Brake for all the time & trouble! It’s so refreshing to find a company who’s willing to help a guy and not just try to generate business.
Jon
