Combination valve service/replacement?

[jaredts]

Found the wire to my combination valve cut, so I hooked it up.  Of course brake light is on.  Couldn't get air out of the rear brakes after hours of bleeding.  Finally found the tiniest, slow leak at the combination valve where a large reducer fitting connects the rear brake line to it.  I can't get it to tighten in place, and also can't push the pin in after removing the rubber cap.  I don't know if that means the spool inside is frozen or just under pressure.  Either way it looks like I'm in for a fun time taking this off and don't know that the best replacement is or rebuild kit or what.  I see universal units out there but not stock replacements.  Who knows, maybe I tighten the fitting and everything is peachy.  Any help out there?

[ralphhemi]

What i do is to bleed the opposite end first to center the valve , by starting the engine if it has power brakes, pump the pedal fairly fast than wedge something between the seat and the brake pedal to hold pressure on the pedal ,shut the  engine off and open the bleeder.repeat 3 times  for each wheel. If that does not make the light go out than start the engine and let it idle than kick the brake brake pedal like a mad  man. hard rapid stomps about 10 times

[jaredts]

My problem isn't bleeding brakes.  My problem is a leak at the combination valve sucking in air when I try to bleed brakes.  I just got the valve off and oh boy what a chore.  I'll tear into this tomorrow, but I'm not sure I want to just try and tighten the fitting and put it back on, risking more trouble.  I wouldn't hesitate if it was easy on and off.  I think I want to replace it while its off.  btw, I used a new shock that I haven't installed yet between the seat and pedal.  The spring action made it a perfect tool for one person brake bleeding.

[jaredts]

Here's one place that has them:  http://www.carolinaclassictrucks.com/81-87-GM-FS-Truck-Prop-Valve.html
Didn't want to wait for shipping or pay that much, but I will if I have to.  Are these still available from GM dealers?

[bd]

My problem isn't bleeding brakes...

Rapidly hammering the brake pedal with one's foot is a common and often effective method to recenter a tripped combination/proportioning valve, hence, turn the dash light off.  Take a look at this complete thread, as well as the linked tech page:

http://forum.73-87chevytrucks.com/smforum/index.php/topic,22940.msg190476.html#msg190476

I haven't torn a combination valve apart to service it, but I wouldn't hesitate to do so.  I would only use a universal valve as a last resort, if the correct valve was not available (only because of the expense and lack of experience with adapting universal valves - perfer to avoid a PITA when possible).

Check with Chris.  He may have them or be able to help.  You can also try the dealer and Delco, as well as aftermarket sources.  If the part is obsolete, ask the dealer to look on their national part locator system.  I recently procured an obsolete crossmember that way.

[bake74]

     What truck model and year are you working on ?  (sorry I read it somewhere but can not remember everyone's specs).  Maybe someone can find you a replacement/rebuild kit.

[jaredts]

'83 c10.  The little pin on the end of the valve seems to be seized up--at least I can't get it to move.  I tried taking a punch and a hammer and tapping it, but nothing.  Who is using an aftermarket valve and likes it?

[jaredts]

What looked to me like a reducer fitting is a little more than that.  It has a couple of rubber seals on it, and it was already pretty tight.  Since I wouldn't know where to find replacement seals, I'm definitely replacing this thing.  Should I get something adjustable?  The link I posted?  No parts stores carry one, so I'm just looking for the best setup, or best bargain if I go with an oem style.

[bd]

...Since I wouldn't know where to find replacement seals, I'm definitely replacing this thing.

If you're going to replace it anyway, tear it apart.  The o-rings may not be that difficult to find - Napa (et al), local hardware store, Harbor Freight....  At the very least, it'll be educational, and you might surprise yourself.

Here's one place that has them:  http://www.carolinaclassictrucks.com/81-87-GM-FS-Truck-Prop-Valve.html

If you decide to go this route, it's really not horribly priced - doubt that you will do much better for a new spec valve.  It's just the down time, waiting for it to arrive.

Whatever you decide, Best of Luck!

[78 Chevyrado]

Only Big problem with a universal Combo valve is you will have to redo some or all of the tubing connecting to it...

[bake74]

If you're going to replace it anyway, tear it apart.  The o-rings may not be that difficult to find - Napa (et al), local hardware store, Harbor Freight....  At the very least, it'll be educational, and you might surprise yourself.

     I agree with this statement 110 %.  I know that not everyone is wanting to learn something new everyday.  But when the opportunity presents itself and you are not going to use such item, why not tear it apart and find out how/why it works. 
     The next time you run across the problem or something similar you will have the knowledge and maybe you can pass that knowledge onto someone else.
     Good luck with whatever route you decide to go.

[jaredts]

Thanks.  I already took enough apart to see the seals that were leaking, and they are not orings.

[VileZambonie]

I used one of these for my disc disc conversion. $45 bucks and chromey to match my booster, master and brake lines which are all chrome. I figured I'd post that in here in case anyone has done a conversion. If you search ebay for GM disc drum combination valve you should find them easily as well http://www.ebay.com/sch/i.html?_nkw=gm+combination+valve+drum&_sacat=0&_odkw=gm+combination+valve&_osacat=0&_from=R40

These are not worth rebuilding, I can tell you that for sure.

[jaredts]

Thanks, Vile.  I did search ebay, summit, jegs.  There are gm style combination valves for disc/drum cheaper than the one I posted, but summit doesn't show it as working for my truck.  Is there a reason that a combo valve would be different for a truck than a car?  Is the rear end pressure reduced further for a truck's light rear end?

[VileZambonie]

I don't know if and where you can find the calibrations by application but they are pretty generic and your foot has the majority of the control. The hold off pressure on the average full size car or truck is around 600 psi. With a disc disc set up you don't want the metering valve just the proportioning valve. The differential pressure switch is only helpful if you have the warning lamp hooked up.