Join Date: Mar 2020
Location: Stillwater, Minnesota
1967 disc/drum hold-off valve operation
I have read many interesting andinformative threads about early disc brake Chevelles with hold-off valves, but Iam missing one critical piece of information. Curiosity made me wonder how thenew valve that I bought worked. So I blew air into it and found out it wouldpass air in the side and out the front, but would not pass air at all in fromthe front (check valve it appears). I have a factory ’67 disc brake car and tookmany pictures on my phone before pulling anything apart. I put in a newbooster, master, hold-off valve, and all new lines. I did reuse myproportioning valve, but thoroughly cleaned it and checked the brake warningstud with an ohm meter to the proportioning valve body, to make sure that ithad not shifted inside to turn on the brake warning light. After hooking everythingup exactly as GM had it, and thoroughly bench bleeding the master, I hookedeverything up and got fluid to all four corners with a vacuum bleeder. A friend pushed on the pedal and the frontbrakes stopped the rotors but would not release to the point of having aserious drag. Then, I checked the rear brakes, rotated the drums by hand withhim stepping on the brake pedal. I then brought out the maintenance manual todouble check my routing along with my pictures in a little diagram I’d drawnfor myself. It was all plumbed correctly. But then I got thinking, how couldthe front port on the master be feeding directly into a hold-off valve thatwouldn’t even pass air through it to the front. It appears I either have afaulty hold-off valve or it should be routed from my masters front port to theside of the hold-off valve which passes air freely. Of course then, it wouldn’tbe holding anything off unless it was metered down internally. Makes me wish I’dhave just gone with Wilwood’s. I knowthis is plumbed correctly, so is that valve backwards in its direction? Any helpregarding the operation internally of this hold-off, or direction of fluid flow would be greatly appreciated.