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Hi Batman,

Drum brakes have spring tension and shoe to drum clearance to overcome so they're a little slower acting than disc brakes. The metering valve adds a slight delay to the front discs until the rear drums have started to work.

Bleeding the brakes by pumping the pedal should build enough pressure to open the metering valve on its own. The little button is used to manually open the metering valve when you're bleeding the brakes with a pressure bleeder since they usually don't operate at high enough pressure to open the valve.
 

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Hi Batman,

Drum brakes have spring tension and shoe to drum clearance to overcome so they're a little slower acting than disc brakes. The metering valve adds a slight delay to the front discs until the rear drums have started to work.

Bleeding the brakes by pumping the pedal should build enough pressure to open the metering valve on its own. The little button is used to manually open the metering valve when you're bleeding the brakes with a pressure bleeder since they usually don't operate at high enough pressure to open the valve.
thanks Ray!!
 

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I don't mean to "hi-jack" this thread but I have a question about the stock "metering valve" on my '69 Nova SS. Long story short, I had the master cylinder rebuilt (didn't even need it!!) and in the process of bleeding the brakes, the rears went fine but the fronts aren't getting any fluid. I'm using a vacuum type bleeding tool and, as you've said Ray, you need to push the plunger on the back of the "metering valve" when using one of those. My question is, how hard do you have to push to get that plunger to go down!! I've removed it from the system to see if there's any blockage in it and it won't budge!! Also, does that plunger go in and out every time you push on the brake pedal?? Thanks in advance for your help!!
 

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Hey Longroofguy, thanks for your reply. I just tried to press the plunger in on my stock metering valve in a vise (!!) and it still won't budge!! How hard is it to depress that plunger?? I'm thinking this must be what's causing brake fluid from reaching the front brakes??
 

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There are a few different designs of metering valves. Some require pulling out (instead of pushing in) on the button to open the valve. It should move freely (in one direction or the other) without excessive force, if not it might be stuck due to internal rust / corrosion.
 

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Thanks for your response Ray. Do you know if the plunger goes in and out when you press on the brake pedal?? Just wondering. I also think the plunger might just go in because it has a rubber boot that fits over it and there's not any room for it to "pop out". Unless it doesn't move very far?? After all of this, I'm pretty sure I have a faulty "metering valve".
 

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I think the button probably moves when the brake pedal is pushed, but I've never watched one while someone was pushing the pedal to say for sure. With yours having the rubber cap completely covering the plunger/pin, it's likely the style you have to push to open. The pull to open style usually has a nail head style button that protrudes through a small hole in the center of the rubber cap/seal.
 

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I've always had success with the CPP combination valve just gravity bleeding the front calipers. Maybe my metering valve isn't functioning, which allows me to do this? I kind of doubt that.
 

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From what I have been able to learn;
I do not remember the exact pressures, but disc brakes operate between 2psi and 1000psi. Drum brakes operate between 10psi (due to springs holding the pads back) and 500psi (lockup due to the shoes binding inside the drum).
Three different items are needed to balance the combination of discs and drums. Residual pressure valves, metering valves, and proportioning valves. Residual pressure valves hold slightly less than 2psi and 10psi respectively in the lines, so the brakes are "ready to go". If a 10psi residual valve is on the discs, the brakes are always on. These also keep fluid in the lines in hot rods when the master cyl is lower than the calipers. The metering valve stops the front brakes from engaging until just after the rear brakes engage. Basically doesn't let the front brakes start until the system reaches 10psi. The proportioning valve stops the rear brakes from locking up before the front brakes. Basically holds 500psi (or whatever the real number is) to the rear brakes while letting 1000psi to the front.
A combination valve specifically designed for disc/drum cars will have 2 or 3 of these built into it.
 

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Here are 2 tools that I use when bleeding drum/disc systems, especially when I'm alone. The tool for holding in the button is homemade, the other for keeping the proportioning valve centered is available commercially. View attachment 402412 j View attachment 402414 View attachment 402413
Hey Longroofguy, regarding the screw-in plug that you have pictured. Is that for the stock junction block, where all the brake lines feed into? How do you know if the "system" or "block" has moved to not allow fluid to flow to either the front brakes or rear?? I'm assuming the plug is to keep it centered while bleeding the brakes??
 

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Hey Longroofguy, regarding the screw-in plug that you have pictured. Is that for the stock junction block, where all the brake lines feed into? How do you know if the "system" or "block" has moved to not allow fluid to flow to either the front brakes or rear?? I'm assuming the plug is to keep it centered while bleeding the brakes??
I'm 'not 'Longroofguy' ( that's Ralph) . . . . . . .. and , I don't really post too older post (from Feb, 2012) .

BUT, I can answer the "BLACK KNOB" that's screwed into that 'combination valve' - to keep that 'spool centered' . . . . . . . very common tool = = " Brake Valve Centering TOOL " ..
YOU DO - NOT need that centering tool = = = = = USE Your test - light . . does the same thing .
Unplug the wire going to the "brake warning light-switch" . . . . . hook up your test light 'clip' onto that brake warning switch ; then stick your test light point onto your + (positive) batt term. (use a longer wire , or jumper wire) .

IF Your TEST LIGHT IS OFF - - - "spool IS NOT tripped" . keep bleeding .

IF Your TEST LIGHT IS ON - - - "spool HAS tripped" . ( ( doesn't matter which way it tripped : light - on , it's tripped) = = = keep it simple .

When / if . . . the spool " trips" remove "test light clip" ; remove "warning light switch" ; use (something like) an
'ice-pick' , and re-center the spool . Reinstall "switch & test light clip" . . . . back to bleeding.

(note : when (if you need too) you remove the warning switch = = = removing that switch WILL NOT (if working correctly) , leak brake fluid ; or ; let air into your system . - - - - - - - - - "key word = "working correctly"
Note # 2 : I bought that "Black Knob - centering tool - - - and, still used my test light. If you have another
person - pumping your brake pedal - - - - - they , also watch THAT Test light . . . . . . . . KISS .
- - - - - don't make it any harder than you need ……………...



or ------------------------------------------- I should be in the 'Test Light Business' ----------------- lol , jim
 

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Thanks to everyone for their input and suggestions. I ended up buying a new metering valve (because I thought it was causing the blockage), pressed the plunger (it didn't seem to move much, if at all) and then was able to bleed the front brakes. The pedal is back up to the normal position and is rock hard!! Guess I got all of the air out of the system? And no leaks, so I'm a happy camper!!
 
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