So a week or so ago I went to move my recent craigs list find, a '72 4 door that runs & drives great. I hit the breaks & the pedal went to the floor & the dash "brake" light came on. A minute or so later I find the brake line has rusted & is leaking along the frame right under the drivers seat. So a day or so later I spliced a new section in & refilled the master cylinder. The light stayed on of course so I gently wacked the prop/combo valve with a small hammer to get the switch to reset & of course it won't budge. I tried slamming on the brakes both while stopped & while cruising at a slow speed & still no luck.
So I broke out the service manual to see what I can find. According to this, the valve should reset itself after the leak is fixed. Apparently my valve doesn't know this info.
So.... I jacked the car up & removed all 5 brake lines from the valve & luckily all of them cooperated. Then I cleaned & disassembled the valve & found 42 years worth of partially congealed brake fluid?? in the valve assembly. So I cleaned everything internally & reassembled everything & the valve is not stuck anymore, yay me.
Then I bled all 4 corners & now everything is good again.
I wasn't really sure how the system worked but it's really pretty simple.
IMG_20140922_223450_305_zps8c949154.jpg

After a good cleaning.
IMG_20140922_223532_184_zpsec2ce7f6.jpg

Crud from inside the valve.
IMG_20140922_223556_996_zps5d630974.jpg

 

Is the 71/72valve the same as the rear drive cars through 1988? If they are,wouldn't it be a good idea to use the newest valve possible. Not knocking what Rat did just trying to find out if they are indeed the same.

 

I had a similar experience with my '70 Supreme when a front brake hose blew. I barely made it home, driving slowly in first gear (for engine braking) and using the park brake.

I have read many times that these valves are supposed to keep one half of the system operational if the other half loses pressure, but that wasn't my experience at all. What was your experience - did you have any braking ability at all after the failure?

 

you have to bleed the whole line from the brake cyclinders theres air in the line. now that u undone all of them they all have to be bleed .start with the farthest one away and hope u don't snap the cyclinder bleeders or yoll be replaceing them too.

 

Mike,
Not sure how long they used this valve? I think since I cleaned it it should work for many years to come or until the car gets so rusty I have to part it.

Fun,
I did have some stopping ability, but barely enough. Nothing I would feel safe about driving over 5 mph or so.

Johnny,
Already bled the entire system & am back on the road.

 

Oh yeah,I completely understand. I was thinking the later valves are the same and would be a good source for someone needing one.

 

Does anyone know of a rebuild kit for these valves? Thanks for posting picture of valve apart. I have been wondering how hard they would be to take apart.

Don W

 

I lost the master cylinder on my 72 Chevy a few years back.

Light did not come on because 0 on the front and 0 on the back means equal pressure on both sides still. Light came on as I limped it into the garage, having made it home with the e brake and downshifting (which is NOT hard, if no traffic)

 

Kit for cast iron valve
https://www.musclecarresearch.com/gm-pd-valve-kit-1

 

Thanks ROCKETVAPOR.

Don W

 

My 70 did exactly the same thing. I broke a front hose last Saturday and had no brakes at all. Like you it was low gear and parking brake, luckily I was only 2 miles from home

 

Glad they keep these post from 2014....I think I have some of the same issues with my distribution box... not able to bleed rear, just didn't find a line leak, assumed master cylinder... might try cleaning valve and see if thay does it