Disc front, stock drums rear. New MC, front hold off valve and rear hold off valve. This is a 1970 and is the only year it has that combination of valves. Have a Motive power bleeder on it at 25 psi. Fronts bleed great and I can purge a 1/2 quart through them in about a 30 seconds (once I push the little button on the front hold off valve) The rears, nothing, just a tiny dribble and the pressure does not drop on my power bleeder. I narrowed the problem down to the new rear hold off valve (mounted on the rear frame). I took it back off and I can only blow a tiny bit of air through it with my compressor. Anybody know how these valve are supposed to work? I never had a problem bleeding my rear brakes before until i replace my 40 year old original valve with the new one. Not sure what the internals of this valve are. Any '70 owners have experience with this little contraption?
This thing, here is my old one...
http://i240.photobucket.com/albums/ff160/Epistuff/100_2568.jpg

Make sure there is enough free play in the brake pedal. The pushrod may be too tight and not allowing the M/C piston to fully retract which will prevent the rear brake fluid port from recharging.
Had this exact issue with my '70.

Checked it Bill. Bore in the new one is the exact depth as the old one. Stroke is the same as well. I just took it back off again and measured everything. If I crack the fitting before that rear valve, I can shoot a stream of fluid from it pretty good. Hook it back up and nothing comes out of the rear bleeders. Even if I stand on the brake pdeal, only a drip comes out of the rear bleeder.

:thumbsup:
I would think a simple fix would be to eliminate that valve with a brass union temporarily and see what results you get.
If you are then able to bleed the rears and have no adverse negative braking effect...leave it.
I've done numerous front disc conversions and never had to add any valving to the drum rears.

I had this problem when I install new stainless lines on my 68' SS.Traced everything down to the flex hose over the differential,It was cloged so bad I could not even run a coathanger wire through it.Replaced it and solved the problem to the rears.

I have a new flex line above the rear as well, stainless braided flex. I may stick the old rear hold-off back there and see what it does. I got the new one from Inline Tube. Called them and they said to send it back to them for an exchange. Not really sure how that thing is supposed to work though. I did plenty of reading and it shows that 1970 is the only year Chevelle had that valve. Different front valve, different distrubution block on the frame and the additional rear valve. I would bypass it, but wonder why GM stuck it there in the first place.

The one on the frame is a proportioniong valve that progressively limits pressure to the rear drum brakes based on line pressure at the inlet.

The function of the round metering valve (or "hold-off" valve) under the master is to allow the rear system to see 30-40 psi (which moves the rear drum shoes out into contact with the drums) before the front discs see any pressure, to avoid disconcerting "front brakes first" on pedal application, which causes "nose-dive" at low speed.

The proportioning valve contains a spring, o-rings, seal cups, washer and a piston. Usually you can take them apart clean them and reinstall. The hold off valve contains more of the same.

If your rear flex line is old and collapsed internally you will have no rears either. A commonly overlooked source of rear brake failure.

This is a pre '70 set up on Camaros.

Scott,
The block on the frame on a '70 is not a proportioning valve it is only a distribution block and the brake light pressure switch. This is a '70 only power disc configuration. You are correct though that this was also used on pre-70 Camaros.

Front hold off valve
distribution block
rear valve (correction valve, hold-off valve, proportion valve...everybody seems to have a different name for it)


My flex line going to the rear housing is new also, stainless braided. I did some searching on this site and found this post...
Can't get fluid past rear hold off valve (http://www.chevelles.com/forums/showthread.php?t=226085)
Looks like a couple of people have had issues with the new valve. Makes me wonder if the replacement they send me will do the same thing.:( Although the original poster (Dave, aka danhalt replied that his second new valve worked)
I can blow air easily through the old one in both directions, but could not with the new one. Sure, I can just bypass the thing, but I wanted to retain the stock configuration of valving on the car.

ITraced everything down to the flex hose over the differential,It was cloged so bad I could not even run a coathanger wire through it.Replaced it and solved the problem to the rears.

Exact same problem on my 69...rear hose was completely clogged. NAPA carries the replacement hose at a reasonable price.

Exact same problem on my 69...rear hose was completely clogged. NAPA carries the replacement hose at a reasonable price.

Flex line over my differential is a brand new stainless lne. No clogs there. Have pressure/flow before the rear hold off valve, not after it. This valve is on the frame before the rear diff flex line.

Disc front, stock drums rear. New MC, front hold off valve and rear hold off valve. This is a 1970 and is the only year it has that combination of valves. Have a Motive power bleeder on it at 25 psi. Fronts bleed great and I can purge a 1/2 quart through them in about a 30 seconds (once I push the little button on the front hold off valve) The rears, nothing, just a tiny dribble and the pressure does not drop on my power bleeder. I narrowed the problem down to the new rear hold off valve (mounted on the rear frame). I took it back off and I can only blow a tiny bit of air through it with my compressor. Anybody know how these valve are supposed to work? I never had a problem bleeding my rear brakes before until i replace my 40 year old original valve with the new one. Not sure what the internals of this valve are. Any '70 owners have experience with this little contraption?
This thing, here is my old one...
http://i240.photobucket.com/albums/ff160/Epistuff/100_2568.jpg

I have replyed to your e-mail. My hold off from In-Line was bad. The second one was also bad. I ended up using a NOS one. It took me awhile to find it and used Bill's idea off using a union with no ill effects until I found it.

I have deleted the some messages in my inbox. Not sure what happened there. I just upgraded my membership to lifetime and it seems that it does not allow as much message storage???????

Glenn,
Yea, your PM box said it was full. Thanks for the info. If the second valve comes in bad also, I'll either clean up the old original and use it or bypass it. Most like use the original one though as long as it does not leak.

Flex line over my differential is a brand new stainless lne. No clogs there. Have pressure/flow before the rear hold off valve, not after it. This valve is on the frame before the rear diff flex line.

hi jim; a friend of mine bought one of those rear hold off valves from "the right stuff" brake line company and he couldnt get fluid to go past that valve. he ended up buying an original gm one off of ebay and it worked fine and he was able to complete the line bleeding.

thanks, dave

Scott,


The block on the frame on a '70 is not a proportioning valve it is only a distribution block and the brake light pressure switch. This is a '70 only power disc configuration. You are correct though that this was also used on pre-70 Camaros.

Front hold off valve
distribution block
rear valve (correction valve, hold-off valve, proportion valve...everybody seems to have a different name for it)
My flex line going to the rear housing is new also, stainless braided. I did some searching on this site and found this post...
Can't get fluid past rear hold off valve (http://www.chevelles.com/forums/showthread.php?t=226085)
Looks like a couple of people have had issues with the new valve. Makes me wonder if the replacement they send me will do the same thing.:( Although the original poster (Dave, aka danhalt replied that his second new valve worked)
I can blow air easily through the old one in both directions, but could not with the new one. Sure, I can just bypass the thing, but I wanted to retain the stock configuration of valving on the car.

My post is a general statement for the most part and not directed toward anyone in particular.

I'm confused by your statement Jim. You say the one circled in red is not a proportioning valve? But it is indeed. It's not a distribution block and brake light pressure switch. If it is where is the wire on it? There are multiple posts at CRG clearing up this misinformation. Very few people call these components by the right name or even know what they do, especially the vendors. It's not uncommon for reman units to be bad. There are numerous pictures on CRG showing the internal components of these valves. Sometimes when disassembled and cleaned the proportioning valves and hold-off valves will work again. They are elementary in design. Get a rebuild kit or send them out to be refurbished if date codes are important. Others have taken the guts out of a new one and put it in theirs to save the date coded housings. Perhaps a quick disassembly will determine what's wrong with yours. Good luck in your endeavor.

.

I'm confused by your statement Jim. You say the one circled in red is not a proportioning valve? But it is indeed. It's not a distribution block and brake light pressure switch. If it is where is the wire on it?

Sorry if I have confused things. Must be all of the brake fluid that has been dripping on my head while laying under the car. :D I meant the one on the frame up front is not a prop valve/combo valve that is used in 71-later Chevelles. I have a factory 3-component system. A hold off valve mounted to the MC, a distribution block/pressure switch on the front frame and this goofy valve of some sort on the rear frame above the driver side lower control arm. I agree with you now that is must be some sort of prop valve for the rears. I did take the old one and the new one apart. Both looked identical but I could not get any of the internals out of either so I am not sure what was wrong with the new one. Hopefully the replacement works. But doing some web searches and seeing a few comments on this site, it looks like hit/miss with the repros. I tried registering on CRG so I can get some info there and can not get the registration process to work. Can you point me to a diagram showing the internals of this valve? Thanks for your help. Much appreciated. :thumbsup: This is what I have;
Front on MC, plumbed to front brakes:
http://i240.photobucket.com/albums/ff160/Epistuff/valve1.jpg

Front frame:
http://i240.photobucket.com/albums/ff160/Epistuff/valve2.jpg

Rear frame:
http://i240.photobucket.com/albums/ff160/Epistuff/valve3.jpg

I can e-mail pics to you, this photo hosting is for the birds. Easier to post from my files. Today is quite hectic however. Try registering on CRG again, recently someone else had problems also and they were able to do it. Because that site is independent (no vendors) and not saturated with chit-chat, retrieval of information is relatively easy using their search function.

I'll try the CRG registration again today.
If you have the time to send a pic though, my e-mail is JVR70@MSN.COM
Thanks

EDIT>>>>> Just tried CRG again and get the same error when trying to get the activation e-mail.

http://i240.photobucket.com/albums/ff160/Epistuff/CRG.jpg

Jim, I just went thru what seems to be the same problem on my 68. I installed a completely new brake system and could not get fluid to the back. The problem on mine was in that during the bleeding process the pressure differential switch was tripped and isolated the rear brakes. You can prove this by attaching a test light to the pin where the tan wire connects to the distribution block and the other end grounded to the distribution block. If the light turns on, that is the problem. You need to open the front bleeders and smack the pedal really hard to try to re-center the switch. I was told that that may work??? I did not on mine. What I ended up doing removing the rear line from the distribution block and using a piece of heavy coat hanger to tap the pressure differential switch back close to center. Then I had the wife just touch the pedal and I had plenty of fluid squirting from the rear of the distribution block. Then I reconnected everything and successfully finished the normal bleeding procedure. BTW the light is still on but the pressure differential switch should re-center itself completely with some drive time. One thing I learned was that If I ever do an entirely new brake system again, I will gravity bleed the system prior to touching the pedal. I hope this helps and good luck.