What rear brakes did you have before?

I would try a vacuum or pressure bleeder

 

It had 4 wheel drums.

Just got back from Harbor Freight with a Mity-Vac hand vacuum pump. :laugh:

 

Did you upgrade to a Disc/Disc MC?

 

Did you bench bleed the master before install? If not it will take some time to get fluid to the rear. Try to at least get fluid to the front by leaving both bleeders open and let it gravity feed. Although it is always best to bleed the farthest brake first try it anyway. If after that still nothing to the back i would say you have an air leak

 

Yes I did bench bleed the MC.

Did you upgrade to a Disc/Disc MC?

I bought a disc/drum kit from the Right Stuff about 2 years ago. It had a booster/MC and proportioning valve combo.
I installed a disc/disc PV for the disc/drum PV.

I'm assuming the MC didn't do any pressure changes since it came with an external PV. But it still doesn't seem to be pumping.

I disconnected the rear brake line just before the flex hose that connects to the rear end. Connected the vacuum pump there ; wasn't able to pull much vacuum but I did get small amounts of brake fluid after pumping the brakes and applying suction.

But it's still not right. I am using Dot 5 silicone fluid if that makes a difference.

I did buy a kit for bench bleeding the MC. It didn't appear to have any air in it. And the MC was pumping ok on the bench. Took it over and mounted it immediately and started bleeding - no dice.

I also blew out the front-to-rear line with air in both directions. This has me puzzled.

 

How did you make out with brakes?

 

CIf rear brakes are not bleeding, and front does, check switch assembly under MC.
You can remove brown wire and check resistance to ground.
if switch has moved to rear system, nearly zero ohms will be indicated, or brake light stays on.
This switch started in 66 Corvette and became std equipment in 67.
If a leak happens, the rod inside slides to the leaky side blocking off fluid and maintaining braking for the other axle and BRAKE light stays on.
There is a 1/8 inch(?) diameter rod sticks out on the front indicating its position.
If short, say 1/8 inch, then rod has moved to the rear system blocking it.
Normal is about 3/8 inch(?) sticking out.
For centering rod, open opposite non-leaking caliper, and slam pedal to floor. Close bleeder and retract pedal. Repeat as necessary for centering rod, makes the BRAKE light go out.
When rod centers and light out, you should be able to bleed. There is a tool for keeping the rod center when bleeding.

 

Stock 1967 GM Factory brake warning switch does not shut off fluid flow. If it did, then it would not be possible to manual bleed the rear brakes without a special tool. Some aftermarket switches may. Ford factory valves in the early 70's did shut off fluid flow.

 

If you went LS1 brakes on all corners just get rid of that distribution block and use this:

https://www.summitracing.com/parts/wil-260-11179

Don

 

I just finally figured out what was wrong with the rear brakes on mine. I had the worst time getting them to bleed a couple years ago when I changed the rear end. Then recently was having some issues and put in an adjustable proportioning valve. Could not get the backs to bleed at all. Not gravity bleed, no brake pedal bleed, no vacuum pump bleed....nothing.

At the end of the day the 50 year old rubber line at the rear end was swollen shut. Replaced that line and it fixed everything. If you have never replaced the factory rubber brake line that goes from the body to the T at the rear end, change it out.

Cost about $15 at NAPA.

 

At the end of the day the 50 year old rubber line at the rear end was swollen shut. Replaced that line and it fixed everything. If you have never replaced the factory rubber brake line that goes from the body to the T at the rear end, change it out.

Just had the same blockage on my right front flex line. I put in an NOS master Cylinder and could not get that line to bleed. I decided to replace all three 51 year-old original flex lines. That led to a new front to back line because the connection to the flex line in back was a rusty fluid-soaked seized mess. At that point, I also replaced a bunch of the other hydraulics system parts.

 

Thanks for all the suggestions. I had to leave for 4 days for work; will be back at it on Thursday.

I got rid of the factory distribution block that was on the frame below the MC.

I bought a disc-disc proportioning valve and installed it in place of a disc-drum valve that came from "Right Stuff."

https://www.jegs.com/i/JEGS/555/631292/10002/-1

I found you don't need an adjustable PV with the LS1 brakes as they are designed with a 75-25 front-to-rear bias in braking.

The brake lines and hoses are new; the 1/4" front-to-rear feed line is original and appears in good condition. I also blew it out with air in both directions.

I'll try again thursday and keep everyone posted ! thanks

 

Thanks for all the suggestions. I had to leave for 4 days for work; will be back at it on Thursday.

I got rid of the factory distribution block that was on the frame below the MC.

I bought a disc-disc proportioning valve and installed it in place of a disc-drum valve that came from "Right Stuff."

https://www.jegs.com/i/JEGS/555/631292/10002/-1

I found you don't need an adjustable PV with the LS1 brakes as they are designed with a 75-25 front-to-rear bias in braking.

The brake lines and hoses are new; the 1/4" front-to-rear feed line is original and appears in good condition. I also blew it out with air in both directions.

I'll try again thursday and keep everyone posted ! thanks

I prefer an adjustable proportioning valve to dial in the ideal amount of rear braking. But I autocross and know how the rear brakes should feel. It takes some skill and experience to properly set rear brake bias and not everyone is good at it, so the non-adjust proportioning is good for them.
Rear drum brakes tend to lock up near the point where the tires skid because they are self-engergizing. The front shoe presses on the rear shoe. Rear disc brakes are much more linear & less likely to lock up near the limit so I prefer rear disc brakes for competition. The difference isn't huge but there are times when we use more rear brake to alter the handling at corner entry.

 

make sure that your proportioning valve hasn't tripped to only allow fluid to the front brakes. There is a tool that you are supposed to install when bleeding to keep that from happening. use a test light or an ohm meter on the switch on the valve. if it shows continuity then that valve has tripped and will not allow fluid to the rear. I just had that same exact problem this week. the fix is to remove the back of the valve and push the piston forward. I was told it will stay in the neutral position but mine did not. I ordered the tool from Summit along with a new valve. (mine was old and not sure it was functioning correctly. hope this helps.

 

here is a link to the Proportioning valve tool. https://www.summitracing.com/oh/parts/clp-pv-tool

 

summit Proportioning valve tool
https://www.summitracing.com/oh/parts/clp-pv-tool

 

Thanks John,
I will definitely give the tool a try.

I am getting fluid at the rear calipers using an air-pressure driven vacuum bleeder , but it's not enough to fill the bleeder line.
Seems like the flow is restricted, maybe by the proportioning valve.