1967 disc/drum hold-off valve operation - Chevelle Tech
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post #1 of 10 (permalink) Old Mar 28th, 20, 3:48 PM Thread Starter
Pat
 
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Location: Stillwater, Minnesota
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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.
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post #2 of 10 (permalink) Old Mar 28th, 20, 5:25 PM
 
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Re: 1967 disc/drum hold-off valve operation

Pat it looks like you have answered your own question (maybe). To double check the two front facing ports go to the front wheels the next port is from MC Large chamber (since disc brakes take more fluid than a wheel cyl). Next is your switch etc. if this is a standard GM "prop" valve. This hold-off valve is similar to a 2-5 lb residual valve?? used on disc/drum I don't think so since MC is on firewall.
For fluid flow direction schematic AND technical questions look up/ call Master Power Brakes (they use GM parts and have ENGINEERS to answer any questions).1-888-351`-8781 and www.mpbrakes.com .I have no tie end with them except for buying 2 of their disc brake conversion kits which are made very very well (50 years auto repair). I have called them re. engineering questions-I've taken fluid dynamics-but they were the ones who found the correct answer ! Hope this helps Rick
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post #3 of 10 (permalink) Old Mar 28th, 20, 6:15 PM
 
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Re: 1967 disc/drum hold-off valve operation

Pat,
I don't know if the man upstairs did this, but a copy of the factory assembly line guide installation schematic fell out of my papers ! I can't fax etc. to you=sorry but here is source. http://i912.photobucket.com/albums/a...Brakelines.jpg Yes it's for Chrysler corp. cars/trucks BUT shows excellent placement of rear brake antilock valve in relation to combination (prop) valve. Next page shows LF front brake tube,RF brake tube,primary brake tube from MC,secondary brake tube from MC,RR wheel cyl tube,Lr wheel brake tube in relation to MC and combination valve. SORRY=second page DIFFERENT ADDRESS http:www.justanswers.com/uploads/molurch_164808_a... GL Rick
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post #4 of 10 (permalink) Old Mar 28th, 20, 8:21 PM
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Re: 1967 disc/drum hold-off valve operation

I have a 68 with factory 4 piston disc brakes and they work awesome. There are many ways to get the valve to recenter so the fluid flows as it should. I had a similar problem and my son fix it I believe by just hammering on the brakes. Google it and you will see some procedures to resolve. It is probably not a bad valve.
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post #5 of 10 (permalink) Old Mar 29th, 20, 12:35 AM Thread Starter
Pat
 
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Re: 1967 disc/drum hold-off valve operation

Bill thanks for the info. I'm pretty sure this issue is involving the hold off valve and not the proportioning valve because if you blow a line a sleeve in that valve shifts and goes to ground which turns your brake light on. My fear is if I buy another hold off valve and have the same trouble I've gotten nowhere. Good to hear yours work good for you, I'm sure I will get to the bottom eventually. Another member told me to get a hold of Master Power Brakes which I will do on Monday. Thanks guys!!
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post #6 of 10 (permalink) Old Mar 29th, 20, 12:42 AM Thread Starter
Pat
 
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Re: 1967 disc/drum hold-off valve operation

Thanks Rick , I will call on Monday. It just makes no sense to me if it won't pass air from that front port on the hold off how can the front port on the MC go to the exit of the hold off. Read on another tread that someone bypassed that valve and their brakes work fine. GM must have had their reason.
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post #7 of 10 (permalink) Old Mar 29th, 20, 3:41 PM
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Bill Wente
 
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Re: 1967 disc/drum hold-off valve operation

First off, you seemed unsure of proper brake lines routing, see attached pic. The rear side of the master feeds the rear brakes and the front master feeds the front.

The hold-off valve is in the front circuit, it's purpose is the give a momentary "hold-off" of the front brakes to prevent rear brake first lock-up resulting in a spin in panic stop or slippery conditions. I would be surprised if the hold-off valve is faulty, it is a very simple devise.

Sounds like your distribution valve is properly centered or the "BRAKE" light would be on. If it IS on at any point you can usually center it by "Jabbing" on the brake pedal.

I suspect you may have a bleeding issue. 4 piston systems can be difficult to fully bleed, especially if you are trying to "Force" the process. My best success is with a total gravity bleeding, patiently starting at the right rear, one at a time, ending at LF, closest to the master. Re-bleed if necessary. You can also help bleeding by firmly depressing the hold-off valve pin on the rear side (under the rubber cap). Lightly stroking the pedal at the very top can also bleed out any residual air. The last one I did took 45 minutes total with a hard pedal the first time.

Good Luck!!
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A few '67s and a '66
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post #8 of 10 (permalink) Old Mar 29th, 20, 3:52 PM
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Re: 1967 disc/drum hold-off valve operation

Attached is an exploded view of a hold-off valve for you reference.
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post #9 of 10 (permalink) Old Mar 29th, 20, 5:22 PM
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Bill Wente
 
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Re: 1967 disc/drum hold-off valve operation

Another thought, there are two lengths of the booster to master pin. Did you compare pin length to match the new booster and master?

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post #10 of 10 (permalink) Old Mar 29th, 20, 11:33 PM Thread Starter
Pat
 
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Re: 1967 disc/drum hold-off valve operation

Hi Bill, Thanks for your response. As I said in my starting post I had taken pictures of the original routing plus drew a little map for my self and also checked the manual. I know it is hooked up correctly but out of curiosity not knowing anything about that valve I blew air through it before installing it to see what happened and the air only passed from the side port out the front but would not pass air from the front opening where the front port of the master feeds it. Was acting like a check valve. If it won't pass air how is it going to pass brake fluid. I read tons of posts on this site hoping to learn more about the direction of flow and the inner workings. After seeing your picture of the guts with the spring maybe the fluid coming in the front overpowers that. Still can't figure out why the front disc's are dragging. They spun freely before this whole bleeding procedure. Thanks very much for the info and I will go through the bleeding again. Oh and my booster pin is the correct length. I had to match that with the sleeve supplied with the master.
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