I'm studying up on bleeding my brakes and search the forums for tips, I don't see anyone mentioning the need to hold the pin down on the metering portion of the the combination valve mounted on the frame in front of the master cylinder.  The Chevy shop manual says to use special tool J-23709 to hold the pin down while bleeding brakes.  Is this necessary?  Thanks!

I have not used one and everything has been fine.....so far!

I think the tool is to center the valve and keep it there. If its the valve I'm thinking of it is designed to shut off the front or rear circuit when no fluid flows through one of those circuits. Its a safety feature. If your replacing your MC or have one that is completely empty, just bench bleach it first and if you still have old fluid in your brake lines connecting to the MC (small amount of air won't trip the valve) your ok. The tool that ships with the MC kits my company sells is just a small plastic pin on a little nub about the size of a grape. That's about all you need to hold the valve. You know you tripped it when either your front or rear brakes don't move when you push the very hard pedal. It can be reset. Dave

Thanks Dave - I have rebuilt all 4 calipers with o ring kits, replaced all brake hoses and all the lines, (except the front to rear becuase I didnt take the body off - some say it can be replaced without taking the body off, but I'm not related to Houdidni so I dont see how!) and the MC, so... I will watch out for the symptoms of a tripped valve.  I did know I need to bench bleed the MC, so good there, was just wondering if I needed to keep that switch pressed.

If you are going to be using the brake pedal, and a helper to bleed the brakes, as long as you do NOT push the pedal all the way to the floor, the switch will not trip, and you'll be able to bleed them with no problems....

I can't remember ever needing some "special tool" to keep the p-valve centered while bleeding...

Thanks Joel.  I am a one man operation so I am going to attemp to build a power bleeder that I read about in other posts.  Question- they say to use a piece of mild steel as the master cover for building the bleeder, I have an old spare master cylinder cover, can that ne used if I fit it with an air fitting?

The lid should work as long as their is an opening between the front and rear reservoirs. Otherwise you will only pressurize the front or rear circuits depending on where you tapped in the air connector. I think those lids have a 1 piece gasket that seals on the divider between the reservoirs....might need to cut a hole in that area of the gasket.
Dave

And Joel, yes exactly, I've bled many cars with full system replacements but have never tripped the valve or need the tool but I have had customers call in who have tripped it and wanted to know how to reset it. I guess it happens.

It depends on the cover. IF it has the "tunnel" formed into it to connect the two chambers, then yes, you can use it. If not, you would need to cut a section of the rubber out so the two chambers will be connected.




oh...and btw....when using a pressure bleeder, you don't need much pressure at all....10 psi is enough. Get too much pressure and the cap will start leaking/spraying brake fluid everywhere....doesn't take much pressure to actually bleed the brakes, so be careful.



|UPDATED|5/7/2014 10:04:38 AM (AZT)|/UPDATED|

Normally if you do manage to trip the switch plunger you just open a nipple at the other end and bleed until the warning light goes out, or what I've done in the past is unscrew the switch from the body and you can poke the shuttle back to the centre with a small screwdriver or similar.