Porportional valve adjustment no pressure to rear cyl

[Furno68C20]

Just converted to a front disc on my 68 C20. New brake booster, master, and proportional valve (POL). I still have the rear drums and my problem is bleeding the rear breaks. I'm getting the fronts to bleed but not fluid to the rear. I see there is a adjustment at the for the rear brakes on the proportional valve, where is this adjustment supposed to be?

When I tighten the rear adjustment all the way the brake pedal hardens and no fluid comes of the rear. So I loosened the rear brake adjustment a couple of turns and the petal gets softer which is good. I also have the break line to the rear removed so i'm just looking at how much fluid is coming directly out of the rear of the proportional valve.


I have that plastic bleeder tool in place of the break light sensor. How is this tool used and what does it do?

Any help would be great! What is the proper bleed procedure for a proportional valve front disc/rear drums?

[AussieinNC]

Remove that tool and leave the port open....

Place a shop towel over the port....

Press sharply several times on the brake pedal....

Now replace the tool....but check the end of the tool first...it may have snapped off...

I dont use the old pump the pedal up to pressure then have someone crack a nipple...I use a pressurised container of brake fluid with an adapter that fits the master cylinder...pressurise the container...start bleeding in this order....Drivers rear, passenger rear, passenger front, drivers front....

Do not pump the pedal during this process...

Finally, did you check the booster pushrod clearance into the back of the master cylinder...it must have clearance...if it doesnt have clearance the rear circuit port will not open and you will not get any fluid to the rear wheels....

If you dont get any fluid from rear lines, loosen the master cylinder off of the booster about 3/8"....and try again...if it now flows fluid, you have the wrong master cylinder or pushrod for that booster...

[Furno68C20]

thank you for some direction on this...

Where would you pressurize the master without depressing the petal? are you suggesting a "t" fitting between the outlet of the master and the inlet to the proportioning valve? or.. tap into the cap? hopefully not the later.


What clearance should I have on the booster push rod? the push rod sticks out about 2.5" from the edge of the booster. The booster has an adjustment behind the housing against the firewall.

How loose or tight should the rear adjustment nut be on the rear of the valve?

Thank you

[RustyPile]

Originally Posted by Furno68C20

thank you for some direction on this...

Where would you pressurize the master without depressing the petal? are you suggesting a "t" fitting between the outlet of the master and the inlet to the proportioning valve? or.. tap into the cap? hopefully not the later......................

There are several pressure bleeder designs.. This is one...

https://www.autozone.com/test-scan-a.../brake-bleeder

[RichardJ]

What actually makes you think there is an adjustment for the proportioning valve?

On the POL website, I found this.
> All valves contain a low pressure shut off valve and warning light switch to ensure driver and vehicle safety should your brake system lose hydraulic line pressure. Aftermarket imported master cylinders with built in adjustable proportioning valves DO NOT feature this and could put you and your vehicle at risk. <

"Aftermarket imported masters". I have no idea what they are trying to say there, but they must be referring to someone else's product.

"low pressure shut off valve" is actually in all Combination Valves such as the one you have. They must think theirs is a little better. They are referring to the green piston in the pressure differential switch, which is in the center of the Combination valve - that consists of three separate valves, each with their own, independent job.

The pressure from the front and the rear of the MC should be equal. If the pressure is not equal, the Green piston is pushed to the side that has the lower pressure. The Red pin normally sits in a groove on the piston. If the piston is moved away from its normal center position, the Red pin is pushed up into the switch, closing the switch contacts and turning the instrument, Brake Light ON.

That plastic thingy you have is supossed to keep the piston from moving while you bleed the brakes'

You should not be trying to bleed the brakes with any brake lines removed.

Even if you had an adjustable proportioning valve, you should not be trying to adjust it until after the brakes are bled.

Manual brake bleeding with a helper has been the tried and true method of bleeding for 100 years. POL offers that method on their web page.
https://www.performanceonline.com/bl...ster-cylinder/

This is POL's drawing from their web page. I colored the piston.

[Furno68C20]

I know the proper way to bleed brakes is to have all lines connected. I only disconnected the rear to see if I was getting fluid out of the back of the combination valve.

If you look at the ends of the body the front spring has a threaded plug with a little nipple sticking out and the rear spring is held in by another fitting like a 7/8" brass nut. Are these fittings supposed to be snug to the combination body or back out a few turns to adjust spring pressure? Are these plugs/fittings there to change out the front and rear valves?

I think I have a push rod clearance issue so I'm going to take off the master and see what kind of clearance I have from the master piston and booster rod. Hopefully there is not pre-load from the rod to the MC piston.

Ill keep you posted, we'll get this done right. Appreciate everyone's help

[AussieinNC]

The pressure bleeder unit I use has a variety of caps that fit tightly over the master cylinder reservoir fill hole. I use it because I work alone 99.999% of the time...even if there is someone around, I stick to the pressure bottle based system...especially if the system has ABS in the system. It simply works...

You should not be altering any of the fittings in the proportional valve...the "cap thingy" ends must be tight.

You need all lines connected before any bleeding process begins...

Make sure the master cylinder piston is not being depressed down by an excessive length booster rod....the should be 1/32" to 1/16" clearance....

Keep us in the loop...thanks

[Furno68C20]

Update! After hours of diagnosis and nothing making sense I’ve finally have a solution to this brake issue. Recap, i orders a front disc conversion kit from Brothers Truck. In addition to the conversion I updated the master cylinder and booster. After everything was installed I had trouble bleeding the new disc (front) original drums. The first issue was in the MC. Apparently the master cylinder I received was from a BAD casting batch from brother’s supplier. The lines connecting the MC to the porportioning valve leaked. Not a NPT Tap on the master. Contacted Brother’s about problem and they sent me a new MC. Swapped new MC and no leaks but now still not getting presssure to the rear. OMG. I then took apart every brake fitting from the front to rear. After hours of testing lines with compressed air I narrowed down to the single brake hose to the rear being clogged. Of course the last section of brake to check is the problem. Replaced rear brake hose with a 23” line from a suburban. Bleed system and now getting rear circuit to work. Hope this helps the next guy.

Thanks for all your help guys.
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