[fabandfirewood]

both brake lines from the master cylinder go to this and a line goes to the back brakes and the front brakes. There is also a wire which comes into the wiring harness. I have no clue what it is do y’all think it could be why we’re getting no flow to the back brakes?

 

[712edf]

That is usually called a proportioning valve (it has other names) and yes it could be where your issue is. Is has a sliding "piston/valve" type thing inside that is normally centered. If you develop a leak in the front, it will slide to one end & block off fluid to the front, allowing you to still have rear brakes without loosing ALL the fluid. It just stops sending fluid to the half of the system that is leaking, uses only the non-leaking portion of the braking system at a reduced efficiency. This is know as a dual (front & rear) braking system, the M/C has two reservoirs. This system has been required on vehicles since 1966.

What you have to do, assuming all leaks are fixed, is to re-center that valve. This is accomplished by bleeding the brakes, in your case you need to start with the fronts. When you crack open a front bleeder the system should sense a "leak" in the front & the valve should head towards the end of the bore to block flow to the front, this should open up flow to the rear.

Bucky

 

[712edf]

The wire should activate the red "BRAKE" light in the instrument cluster when the valve is off center.

Bucky

 

[fabandfirewood]

thanks y’all. can i bypass it or will that have negative affects on the braking?

 

[712edf]

thanks y’all. can i bypass it or will that have negative affects on the braking?

Well to bypass it you would either have to remove it & do some plumbing modification, or take the guts out of it. And I will admit to having done that once before on an old Plymouth RR I used to race around in.

When you do that then the master cylinder sends fluid to all four brakes at the same time & pressure. This may cause front to rear balance issues if you have discs on front & drums on rear as most of the 60's-80's vehicles came with. One axle may lock up before the other.

And you would lose the safety of the dual system, any leak would subject the whole hydraulic system to lose pressure, thus you'd have no brakes. Plus it may cause a liability/legality issue on a street driven rig.

Bucky

 

[KurtfromLaQuinta]

Read these pages for some more explanation... Proportioning Valve Replacement?

 

[DODGEBOYS]

STOMP with both feet on the brake pedal as hard as you can till the light go's out / then gingerly bleed the brakes till the pedal comes up and your done [ you need to recenter the combi valve/proportioning valve ] other wise the brakes will never work

Note / you DO NOT need to replace anything

 

[SuperBurban]

Technically it is called a combination valve.
It contains a warning switch, that turns on the brake light if there is a leak in one side. (does not block any fluid pressure to any brakes, just prevents fluid passing from front to rear, or rear to front in case of a leak.). And a hold off valve, that holds off the front brakes for the first few PSI of braking pressure, so the rear brakes can over come the springs, and start to apply.
On 1/2 ton and RC's, (Not 3/4 and 1 tons trucks), it also has a proportioning valve built in.

Here is a small video of the different functions.

Also a cutaway view of the two different valves.

PTDC0099.JPG

• SuperBurban
• Jun 26, 2021

1/2 ton combination valve

PTDC0097.JPG

• SuperBurban
• Jun 26, 2021

3/4 and 1 ton combination valve

Sometimes it does work better to carefully open a bleeder on the front, ot back while someone is applying moderate brakes, until he light goes out. But it is supposed to recenter itself once the two sides are back to having equal pressure.

 

[DODGEBOYS]

to get the rear brakes working you hafta "recenter" the valve , and you can't remove it without installing another one [ you already have one ] and don't get me wrong sometimes it can be very frustrating to get the valve to kick , even after pumping several qts of brake fluid thru the system , but like l said above JAB the brake pedal HARD as many times as it takes to center the valve [ you,ll know when it centers as the pedal will drop to the floor ] once centered very easy [ light pedal pressure ] bleed the rear brakes till the pedal comes back up and your done

 

[BlazeR/T]

STOMP with both feet on the brake pedal as hard as you can till the light go's out / then gingerly bleed the brakes till the pedal comes up and your done [ you need to recenter the combi valve/proportioning valve ] other wise the brakes will never work

Note / you DO NOT need to replace anything

to get the rear brakes working you hafta "recenter" the valve , and you can't remove it without installing another one [ you already have one ] and don't get me wrong sometimes it can be very frustrating to get the valve to kick , even after pumping several qts of brake fluid thru the system , but like l said above JAB the brake pedal HARD as many times as it takes to center the valve [ you,ll know when it centers as the pedal will drop to the floor ] once centered very easy [ light pedal pressure ] bleed the rear brakes till the pedal comes back up and your done

SO I assume you don't want to try this while driving huh?

 

[DODGEBOYS]

SO I assume you don't want to try this while driving huh?

probly not

 

[fabandfirewood]

thanks y’all. i have no light that come on so what should i do to tell me when it’s centered?

 

[DODGEBOYS]

IF the valve is centered then you would have Equal pressure front and rear when you bleed it and the pedal would be perfect

 

[DODGEBOYS]

your dash light might not work , push down the the E-brake pedal the light WILL come on if it works

 

[fabandfirewood]

oops sorry i meant that there isn’t any light bulb to turn on.

 

[DODGEBOYS]

so you have NO clue as to weather the light works , but just the fact that you cant get the rear brakes to bleed says the valve is kicked

 

[fabandfirewood]

yep, so tomorrow ill try to recenter the valve like y’all said too