Lou Posted April 25, 2022 Share Posted April 25, 2022 Started bleeding brakes working farthest away to closest. I'm using my Mighty Vac and developing good pressure but it doesn't seem like I'm pulling enough fluid through the lines, any of them. I'm obviously pulling some as I have been keeping the master topped off but at 15-20 psi that fluid should be flowing. The only things I can think of is a blockage in the combination or proportioning valve. ?? Quote Link to comment Share on other sites More sharing options...
MC1of80 Posted April 25, 2022 Share Posted April 25, 2022 When using the mighty vac with that much vacuum that quickly, you are in essence tricking the proportioning valve into thinking there is a massive leak and the valve shuts off the fluid flow. 1 Quote Link to comment Share on other sites More sharing options...
Lou Posted April 25, 2022 Author Share Posted April 25, 2022 I got to 20 psi gradually when I didn't get any fluid at lower psi. I tried depressing the 'button' under the rubber boot of the prop. valve but it feels solid so I didn't force it. Could it be stuck? Quote Link to comment Share on other sites More sharing options...
Lou Posted April 25, 2022 Author Share Posted April 25, 2022 Apparently, this is a 'hold off' valve? That's the valve im talking about. Quote Link to comment Share on other sites More sharing options...
Lou Posted April 25, 2022 Author Share Posted April 25, 2022 No button on combination valve. Quote Link to comment Share on other sites More sharing options...
Scott S. Posted April 26, 2022 Share Posted April 26, 2022 Aren't you supposed to start at the closest and work your way out to the farthest ??? Quote Link to comment Share on other sites More sharing options...
MC1of80 Posted April 26, 2022 Share Posted April 26, 2022 2 hours ago, Scott S. said: Aren't you supposed to start at the closest and work your way out to the farthest ??? No, just the opposite. Start at the bleeder furthest from the master cylinder and work your way to the closest. 1 Quote Link to comment Share on other sites More sharing options...
Dtret Posted April 26, 2022 Share Posted April 26, 2022 I have seen that valve before but neither my 70 or 71 have it. I believe it’s mounts to the drivers side master cylinder bolt but what is it’s purpose if you have the proportioning valve on the frame. Does this predate the frame valve? I’m asking cuz I don’t know. Quote Link to comment Share on other sites More sharing options...
MC1of80 Posted April 26, 2022 Share Posted April 26, 2022 The valve basically delays the application of the rear brakes as to prevent the rear brakes from locking up in a heavy brake application and possibly causing the car to spin out. 71s and 72s do not have this valve. Quote Link to comment Share on other sites More sharing options...
Dtret Posted April 26, 2022 Share Posted April 26, 2022 41 minutes ago, MC1of80 said: The valve basically delays the application of the rear brakes as to prevent the rear brakes from locking up in a heavy brake application and possibly causing the car to spin out. 71s and 72s do not have this valve. Thanks for the info. So if I do the 70 I’ll need to aquire one of them. Quote Link to comment Share on other sites More sharing options...
MC1of80 Posted April 26, 2022 Share Posted April 26, 2022 Depends on the build date. I have had a few 70s, built at the Baltimore plant, that did not have the extra valve. My 70 SS, Big Red has the valve and was built at the Flint plant Sept 69. Quote Link to comment Share on other sites More sharing options...
jft69z Posted April 26, 2022 Share Posted April 26, 2022 My 69 Camaro had that extra valve near the brake booster. Always had trouble bleeding the brakes with the Vacula (vacuum bleeder). That car would only bleed properly with a pressure bleeder. Not sure how it will be once I get the car back together, but all that stuff was eliminated with the 4 wheel disc setup that's going on it now. Quote Link to comment Share on other sites More sharing options...
Lou Posted April 26, 2022 Author Share Posted April 26, 2022 34 minutes ago, MC1of80 said: Depends on the build date. I have had a few 70s, built at the Baltimore plant, that did not have the extra valve. My 70 SS, Big Red has the valve and was built at the Flint plant Sept 69. My 70 was built in April at Flint. How do you bleed your brakes or could the valve be the problem? Quote Link to comment Share on other sites More sharing options...
Dtret Posted April 27, 2022 Share Posted April 27, 2022 2 hours ago, MC1of80 said: Depends on the build date. I have had a few 70s, built at the Baltimore plant, that did not have the extra valve. My 70 SS, Big Red has the valve and was built at the Flint plant Sept 69. This pos 70 is 4/70. Flint. All the brake line were cut off when the master was taken off( found it in the trunk, not sure it’s even correct) so I don’t know it’s supposed to have it. Quote Link to comment Share on other sites More sharing options...
Scott S. Posted April 27, 2022 Share Posted April 27, 2022 23 hours ago, MC1of80 said: No, just the opposite. Start at the bleeder furthest from the master cylinder and work your way to the closest. I still used the hose in a jar method to bleed the brakes.... Does that make a difference ??? Quote Link to comment Share on other sites More sharing options...
Scott S. Posted April 27, 2022 Share Posted April 27, 2022 17 hours ago, Dtret said: This pos 70 is 4/70. Flint. All the brake line were cut off when the master was taken off( found it in the trunk, not sure it’s even correct) so I don’t know it’s supposed to have it. Mine came from the flint plant as well, (3rd week Jan '70) and doesn't have the bleeders at the master cylinder. SS's only ?? Quote Link to comment Share on other sites More sharing options...
GatorDog72 Posted April 27, 2022 Share Posted April 27, 2022 I had a heck of a time trying to get the air out of the lines on my car. We finally resorted to the 2 man method. Still couldn't get a solid pedal until we put about 2 quarts of brake fluid through it. It seemed to help to pump the pedal a few times before opening the bleeder screw. 2 Quote Link to comment Share on other sites More sharing options...
Jason72 Posted April 29, 2022 Share Posted April 29, 2022 A trick I found to keep these prop valves from closing while bleeding brakes: gravity bleed all 4 wheels first. Fill master, leave lid off, open all bleeders and wait. As fluid starts to flow from each wheel close that bleeder. Then bleed the brakes as usual. This ensures there is fluid in the lines before you press the pedal and trip the valve. Also use low pressure on the brake pedal when bleeding the rear. A jolt of fluid will trip the valve. also, that valve pictured is an old style proportioning valve. That is what will close off fluid flow to the rear brakes. The newer style we all recognize that is under the drivers floor is technically called a combination valve. Does the job of a proportioning valve, distribution block and pressure switch. 1 Quote Link to comment Share on other sites More sharing options...
70MC402 Posted April 29, 2022 Share Posted April 29, 2022 The one mounted to the frame is simply a distribution block, in ‘70. There is a rear hold off mounted to the rear frame crossmember a few inches from where the line goes through to the brake hose. The one mounted to the driver bolt of the master is used in conjunction with certain masters. From what I’ve read some masters in 70 were different than others due to mass numbers of parts manufacturers in ‘70. So these hood offs were used to standardize brake function. Ask me how I know lol. 1 Quote Link to comment Share on other sites More sharing options...
Lou Posted April 30, 2022 Author Share Posted April 30, 2022 Yes. I have both. Are you saying there's a third one back by the rearend? Quote Link to comment Share on other sites More sharing options...
70MC402 Posted April 30, 2022 Share Posted April 30, 2022 Yep, the front corrects the pressure from the various master cylinders used. The rear line on the master goes down to the distribution block where the brake pressure sensor attaches and heads back along the frame rail, over the lower lower arm front mount point, goes about a foot then climbs up and over towards the passenger side. Quickly it meets the rear brake hold off valve, then heads towards the passenger side even more to the hose that goes through the crossmember to the rear end. Quote Link to comment Share on other sites More sharing options...
Lou Posted April 30, 2022 Author Share Posted April 30, 2022 I probably saw that but thought it was just a junction block. I replaced the rear driver side line. Quote Link to comment Share on other sites More sharing options...
70 MONTE Posted September 17, 2023 Share Posted September 17, 2023 I have a 70 Monte with that hold off valve at the back. I have converted to disks in the back and got a new master and proportioning valve. Removed the valve on the master bolt. Do I need to remove the valve in the back too? Quote Link to comment Share on other sites More sharing options...
riverdogs00 Posted September 27, 2023 Share Posted September 27, 2023 On 9/16/2023 at 9:05 PM, 70 MONTE said: I have a 70 Monte with that hold off valve at the back. I have converted to disks in the back and got a new master and proportioning valve. Removed the valve on the master bolt. Do I need to remove the valve in the back too? I did on mine. converted to 4 wheel, manual discs and removed that valve. I was plumbing new lines anyway. Quote Link to comment Share on other sites More sharing options...
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