Before & After's Of Metering Valve

Hey Ya'all
Brakes were " Grabbing " : 1st Pic is the original GM Metering Valve. ( Most brake experts say that
' Proportioner is in incorrect term ? ) . GM of Canada apparently recalled this part , back around 1990 (Product revision # 0R11) . You can see that the Valve stem is ' stuck ' , & when I took it apart , I found that is was pretty well seized from rust? Any way , 2nd Pic is the new one. about 60 bucks thru Pirate 4X4. After I flush & bleed I'll post results. I think Rosie will be good to stop now ( I Don't feel like using any of our Trees to do it , this Decade ) --- Lockman

 

Jeep calls them a "Combination Valve." On a CJ with front drum brakes, it has two components (warning switch and proportioning valve); on any of the front disk models it has three (adding a hold-off valve for the front disks). These valves are covered thoroughly in the '76 TSM http://oljeep.com/gw/76_tsm/Section9.pdf starting at 9-17. Pretty sure everybody is selling the same part; take a look at Inline Tube and you'll see a Disk-Disk combo valve which I believe is the same as what you would use on a drum-drum system. Product List – Inline Tube

 

Proportioning valve and metering valve are two different things. Proportioning valve controls the pressure to the rear brakes to minimize lockup. Metering valve holds off the pressure in the front brakes until sufficient pressure is built up in the rear brakes then allows the front to apply (normally only used on front disc/rear drum applications). This is why front disc/rear drum systems can have a delay in braking if the rear are out of adjustment. On some applications the front can lock up before the rear applies. Especially on front wheel drive cars in slick conditions.
There is also a pressure differential switch. Simply a shuttle valve that moves one way or the other in case of loss of line pressure and activates a simple switch, similar to how the reverse light switch works in many transmissions.
A combination valve is simply a combination of two or more of these devices. Typically with a pressure differential switch as the common denominator.

 

They are on disc/disc systems too. They're used to apply rear braking before front braking to assist in straight-line braking, especially in turns.

 

Nickmil's summary is pretty good. I will add that it is preferable to have the fronts lock before the rears, because in a turn locked rears can send you spinning. You also have weight transfer to the front which reduces the vertical load on the rears. That's the main function of the proportioning valve - limiting rear line pressure relative to the front. Drum brakes have a bit more mechanical latency than discs, so the metering valve allows the drum circuit to work before the discs actuate. The safety valve (center section) works off balanced master cylinder pressure to the combination valve's 2 circuits - f/r. If there is a downstream failure in either one, the next brake application will cause the spool to move and block off the failed circuit and ground the brake warning light switch.

Ever notice that master cylinders are usually tipped on an angle f/r? Also that the rear (lower) reservoir feeds the front brake circuit? That's so your fronts will typically be the last to lose fluid pressure from a downstream failure.

 

Thanks Gent's.... BTW, regarding Bleeding . The TSM says to start @ the Front Left ? This old boy always was taught to start with the Brake / wheel cylinder furthest away from the Master Cylinder, IE : the Right rear ? What's the correct procedure / order ? My Rosie has Qaudra-trac .
------Lockman

 

You are Correct! Start from the furthest and work to the closest.

 

So... Front Left & Follow the TSM ? Question : Is that procedure because of 4WD ????

 

They can be but much less common. It's much more common for manufacturers to use caliper piston diameter to control the front rear bias which allows them to do away with a metering valve altogether.

 

Unless a misprint follow the TSM. Many vehicles have an odd bleeding pattern due to ABS systems (doesn't apply here obviously) or due to routing of the brake lines/hoses. Normally on a rear wheel drive/non-abs vehicle you would start from the furthest master cylinder and work closer. Not always though. Front wheel drive much of the time you start at the closest to the master cylinder and work toward the furthest away. Again, depends on the vehicle.