Brake Bleed Problem
Thread Starter
Heel & Toe
Joined: Dec 2013
Posts: 20
Likes: 1
Brake Bleed Problem
So I did a frame off "rebuild". While I had the body off I replaced the entire brake line system with stainless steel lines and a new proportional valve. Also replaced the Master cylinder and booster. I bench bleed the master cylinder before install. I'm not a mechanic but I'm pretty handy with this sort of stuff. The problem is that I can't get any fluid to the back brakes. The proportional valve is reset/centered and I blew air through each line to make sure they weren't clogged up with anything. I tried to bleed the front just to see if I could and it bleed fine. I triple checked the proportional valve to be sure it was fully opened and, yes, I'm using that little proportional valve tool to keep the valve in place. I'M AT A LOSS !!! What am I missing??? ANY help would be very much appreciated.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
No brake-fluid at the rears so you have to backtrack the system. There is a fitting at the rear of the Prop Valve. Just barely crack that open.
Place a towel under the junction and barely apply brake pedal pressure. If no fluid comes out, then the Prop Valve is not being supplied by the master.
If you do get a squirt, then a blockage is between Prop V. & calipers.
Because you have the centering tool installed in the Prop V. it should be a straight shot for the fluid to pass through. And the Prop V. will not "trip" closed.
I assume new rubber hoses were installed at the rear?
Place a towel under the junction and barely apply brake pedal pressure. If no fluid comes out, then the Prop Valve is not being supplied by the master.
If you do get a squirt, then a blockage is between Prop V. & calipers.
Because you have the centering tool installed in the Prop V. it should be a straight shot for the fluid to pass through. And the Prop V. will not "trip" closed.
I assume new rubber hoses were installed at the rear?
Thread Starter
Heel & Toe
Joined: Dec 2013
Posts: 20
Likes: 1
Yes, EVERYTHING new on brake system. By "Fitting" at the rear of valve I assume you mean the brake line that's attached at the rear of the proportional valve correct?? There is no "reset button" on this valve. That's a great idea. I'm so baffled I didn't even think to do that !! When I pumped air through the line I disconnected that line and bypassed the valve so if the valve is the problem I wouldn't have known. Oh well, I'll check that out. Thanks
GM Cert Corvette Tech
Joined: Apr 2008
Posts: 2,559
Likes: 585
From: Anaheim Hills California
Go out and buy a mighty vac or you can go to Horror freight and buy they're off brand vacuum and brake bleeder. Fill up your master with brake fluid an attach the vacuum line from the mighty back to the zerk fitting of the rear calipers (Also remember on some rear calipers there are multiple zirk fittings.) and create vacuum then open up the zerk fitting as if you were bleeding the brakes and draw the brake fluid through the lines into the caliper. It works like a charm and take your time and when you're done vacuum bleeding your brakes you will have a very nice high brake pedal. I think there are YouTube videos that show the procedure so get yourself familiarized with it. Below is the brake bleeding sequence. Also remember to keep on checking the level in the master cylinder of the brake fluid. Keep on adding fluid as needed.
According to the service manual,
left rear inner, left rear outer
right rear inner, right rear outer
left front
right front
According to the service manual,
left rear inner, left rear outer
right rear inner, right rear outer
left front
right front
Thread Starter
Heel & Toe
Joined: Dec 2013
Posts: 20
Likes: 1
Hey D.M. Thanks for the info. I happen to have a Horror Freight vac pump LOL. Also the rear brakes have THREE zirk fittings. One is on the bottom end of the caliper. Not sure what that's supposed to do but I think you just leave it closed. Thanks again.
Intermediate
Joined: Oct 2023
Posts: 32
Likes: 14
From: Thousand Oaks, Ca
Just my 2 cents but when I did my 70s brake caliper replacements I used a Motive Products Power Bleeder kit # 0105 from Summit. If you read most of the brake articles they recommend "pushing" the new fluid through and not "pulling" with a vacuum unit as this can damage the seals. I did not use the included J hooks and instead used 2 C-Clamps to seal the plate to my master cylinder. Worked great and I then added the optional caps to use on my other cars. Oh and you can do it all alone, I did mine without any help and you will not run out of brake fluid, just buy the large bottle to fill the hand pump and I recommend cleaning it with denatured alcohol after use.
Good Luck
Gary
https://www.summitracing.com/parts/mvp-0105?rrec=true
Good Luck
Gary
https://www.summitracing.com/parts/mvp-0105?rrec=true
Last edited by GGsL46; Dec 4, 2023 at 04:47 PM. Reason: Additional info
Instructor
Joined: Jun 2019
Posts: 174
Likes: 91
From: Colorado Springs
I attempted the vaccuum brake bleed on 78 with all new components installed. It works well getting fluid moved throughout but not so well as a bleeder because it will suck the caliper pistons. I used air powered vac bleeder not the hand pump, the hand vac pump may work well for simple jobs.
Ended with two man to get the job done
Ended with two man to get the job done
Last edited by VelocityGs; Dec 4, 2023 at 04:03 PM.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Chlarock 1,
The rear calipers do have numerous ports for bleeder screws or brake lines. The ones not used should actually have a permanent plug, but a bleeder screw will work.
Your main concern now is good flow to all four upper bleeders at the rear calipers.
The Proportioning Valve is designed as a safety feature in the event of a brake failure, either in the front system or rear. The little piston inside is spring loaded. If it detects a loss of pressure (leak) it is to close off that section of brakes so as not to empty the master and still provide "some" braking.
So, my question to you: The rear system has no flow. You installed the tool to hold the P.V. piston.
Do you think the piston is locked into position to close the rear line? In other words, the Prop Valve was tripped, sensing a leak. You then locked the Prop Valve in and now the rear line is closed off. The spring can not return to center.
To reset a Prop Valve, all bleeder screws must be closed. Then three to four quick jabs of the brake pedal will usually "reset": the valve. But, you will need to remove that tool first.
To see the inner working of the P.V. click on my avatar, look for photo albums, brake bleeding.
The rear calipers do have numerous ports for bleeder screws or brake lines. The ones not used should actually have a permanent plug, but a bleeder screw will work.
Your main concern now is good flow to all four upper bleeders at the rear calipers.
The Proportioning Valve is designed as a safety feature in the event of a brake failure, either in the front system or rear. The little piston inside is spring loaded. If it detects a loss of pressure (leak) it is to close off that section of brakes so as not to empty the master and still provide "some" braking.
So, my question to you: The rear system has no flow. You installed the tool to hold the P.V. piston.
Do you think the piston is locked into position to close the rear line? In other words, the Prop Valve was tripped, sensing a leak. You then locked the Prop Valve in and now the rear line is closed off. The spring can not return to center.
To reset a Prop Valve, all bleeder screws must be closed. Then three to four quick jabs of the brake pedal will usually "reset": the valve. But, you will need to remove that tool first.
To see the inner working of the P.V. click on my avatar, look for photo albums, brake bleeding.
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Safety Car
Joined: Sep 2005
Posts: 4,226
Likes: 643
From: Orange County Ca
From my experience, all the currently available "proportioning blocks" are all made in the same chinese factory.
I replaced mine in my '69 and it mostly worked. I eventually took the car to "the brake guy" and he found that the internal machineing did not really match up with the ports.
If you still have your original one, I'd suggest you have it rebuilt.
I replaced mine in my '69 and it mostly worked. I eventually took the car to "the brake guy" and he found that the internal machineing did not really match up with the ports.
If you still have your original one, I'd suggest you have it rebuilt.