[Shiny]

How should the pin on a brake combo valve behave? Mine is stuck "in" and I tried bleeding a front line as suggested on other threads, but it did not pop back out .

The valve (iron body used original 71 or 72) was taken apart, cleaned up, and put back together before installing, and I made sure the internals were free. I have a soft pedal, and have bled every wheel repeatedly, with a vacuum pump, with pressure-then-crack, etc., but still not firm. I expanded the back drums, thinking they may have been too loose, but still not right. I pulled the master and tipped it "down" and repeatedly pushed the piston to try and burp it, but no improvement. The vacuum pump had no trouble pulling fluid through all 4 bleeder valves.

How do I diagnose the combo valve? Is there a preferred sequence or method to get it centered?


thanks

[GStage1]

The relief plunger should come out when the master applies pressure to the combo valve...you can simulate this using compressed air....if the air works the relief plunger...you may have a weak/bad master....buy a new master...never a rebuilt one.

[Shiny]

GStage1 - thank you for the suggestion

which port would I pressurize?

[GStage1]

Use the front port from the master cyl...then place your finger over the two front ports...when the compressed air enters the master cyl port it should "pop" out the plunger....if not, you have to disassemble the combo valve and clean it...most likely you have some contamination in it.

[Shiny]

The central piston was "stuck", and I may have had the proportioning valve assembled incorrectly.

Are there any detailed diagrams of these valves available to guide a rebuild?

Can I get replacement cups and o-rings... basically a rebuild kit for these combo valves?

[GStage1]

There is a cut-away in the factory shop manual....no rebuild kit that I know of for them.
You can easily get another combo valve in case yours is not repairable.

[Old Goat 67]

Hooray for you guy!

[Shiny]

Yoohoo!!! that's gotta feel great! probably explains that big grin !

I'm happy for you and glad to hear it went well. You must have covered a lot of details to have it go so well. Nicely done!

And, yes, I'm really glad to hear you have brakes.. way to go !!

[PontGuy]

Thanks guys. After all the trouble with the brakes I was actually more worried about them than anything else, but they turned out to be a non-issue. I still have plenty of debugging and tuning to do before putting the hood back on and declaring the car fully functional though.

If I ever figure out how to re-center the switch I will post on it. I have some ideas...

Walt

[Shiny]

Walt - After all I have been through on my brakes, it seems as if replacing the rear cylinders is the first thing I should have tried. Who knew? I'll be home for Christmas and will try to squeeze in some car time. When I left, the car was stopping pretty well with the residual pressure valve, but I am definitely swapping the rears now too.

I don't know if the switch interacts with the metering or proportioning function in the valve, but you may want to fix it before you find out the wrong way. The techs at Inline Tube told me it can interfere with the bleeding, so who knows? Let us know when you get that figured out.

[Brad_bb]

Shiney, did you ever get more done? I'm waiting for that Hooray post, where you tell us problems solved. Don't give up man. "Knaw on it, son" -from Cool Hand Luke

Gosh, reading this post makes me think twice about changing over my 64 gto to disc/disc! I've been trying to learn about what parts are available for my changeover, and it's not easy with varying info from a number of sources. I like the fact that these retro fit kits use off the shelf parts from later vehicles, but unfortunately many companies show a pic of their kit and don't tell what caliper, or rotor they are using(the application like a 99-02 Astro Van AWD caliper). I'm all confused about what combo valve, MC and booster, and possible adjustable prop valve, to use. Oh well, I'll do my research, see if I can get a successful install, and then write about all my findings on these forums.

[PontGuy]

Brad, I wouldn't get discouraged- having good brakes is well worth it. And I really don't think that most people have near the problems that Mike and I did. But sorting out brake issues can be a real pain and hopefully this thread has helped others out.

By the way my brakes are still working great, though I am still waiting for the switch to magically center itself. It is on my to-do list but I am concentrating on other areas right now.

Walt

[Shiny]

Brad,

Thanks for the encouragement. I won't give up... I might not eat 50 eggs, but I will fix my brakes !

You should not hesitate to do the swap on yours. The odds of it being as painful as my experience are about 2 (Walt and I) out of 1000 (other people on this forum that never suffered this problem). You have a lot of satisfaction to gain, and it really is straightforward.

I posted Walt's photo again to summarize the knowledge we gained - rear cylinder style on the left is good, the style on the right is pure evil.

As far as my hooray, I am close, as I do have brakes now, but I did it by adding a residual pressure valve bandaid and will whoop louder when I finally exorcise the rear cylinder demons. It was too cold when I was back home at Christmas, but I will get it done some day.

[Shiny]

I am opening this up again (like an old wound) because I am finally motivated to replace the rear cylinders in my car that were letting air suck back in.

I had cobbled up mine by installing a residual pressure valve, which worked, but I never trusted it so was on a do-over. Now I'm not so sure.

If you read the saga below, PontGuy had solved the same problem with a simple cylinder exchange.

I went out and bought a pair of the 37024 cylinders from NAPA about a year ago, same as PontGuy.

I was convinced they would have cup expanders behind the rubber cups, but before I put them on, I opened them and found they do not.

^#&*! I said.

So then I looked closely at PontGuy's pics, and his say "Assembled in ???", mine say "Made in China". His casting has a number 29452-S and "USA". Mine have nothing in the casting.

Before I think these are better than mine, can anyone tell me:

1. Should these rear cylinders have metal cup expanders or spring expanders like in a seal?

2. If so, where do I get them?

I looked at Inline Tube's catalog and they had a picture of inner springs with cup expanders on each end, so I thought "eureka" and ordered them. They showed up today and the picture is still there, but the expanders are not...

^#&*! I said.

3. Should I expect these NAPA cylinders to be less prone to sucking air than the ones I have? How can I tell? Is there a way to find out? Pull a vacuum on them?

Thanks for any info

[PontGuy]

Wow, this is like opening up a very old wound!

My "made in USA" Napa cylinders have worked great, and its been 3+ years now.

I swapped the original rear end out with a 12 bolt about 1-1/2 years ago and re-installed the same cylinders. A few minutes of bleeding and back to perfect brakes again. Other than that I have not done anything to the system. I even have enough faith in the brakes to run the car down the 1/4 mile now.

I never did get the distribution block switch centered though...