Proportioning valve

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83slimer

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Apr 9, 2015
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Location
Nova scotia Canada
My Car
1973 sportsroof mustang


Hi All,

Having a hard time with my brakes,

Almost like over boosted braking,  

New re manufactured booster , used proportioning valve , all new lines etc. New master cylinder.

Ive bled the lines several times and the pedal feels ok when not running, as soon as I start the engine the pedal gets soft and is very grabby lol  when the brakes engage when rolling.

Ive played around with the Push rod length which seems to just engage the brakes sooner when i lengthen the rod. 

Master cylinder,  I didnt bench bleed it at first, then took it out and bench bled it and didnt notice mutch of a difference before and after.

I have the small bowl of the MC going to the rear brakes part of the proportioning valve. And Large part of the MC to front brakes.   

When I push the brakes lightly with MC cover off only the small bowl of the MC squirts fluid up. 

The top 3 ports of the pro porting valve are for the front brakes, I have the MC line going into the end port , the one with the brass washer , then left and right ports going to the front calipers, I'm pretty sure thats how it was before I rebuilt my car. 

I see with the new proportioning valve from scott drake the diagram shows the MC for the front brakes plummed to the top port the next to the brake fail connector. 

Ive read that all 3 ports on the front side share a connection, SO Does it matter where the MC line goes?

I havent tried the new Proportioning valve yet as I have to re-route some lines if I do. I'm not convinced that its the problem but bough a new one just in case. 

The brakes seem to bleed fine I get flow out of rear and front when bleeding. 

The MC is a Dorman M36445 , Pretty sure its the right one for Disc Drum.

Pic is the way its plumbed now.

Any suggestions?

 
Last edited by a moderator:
It looks like you have it plumbed correctly. When you stop does it pull or stop in a straight line? How long has it sat since the last time you drove it?

The most common grabby brake problem is from pads or shoes contaminated with brake fluid or oil, but this is usually just on one brake, resulting in pulling. Another problem is from rusted drums and rotors, but will usually clear up unless there is too much pitting.

However, because you said you have a reman booster I suspect it was not assembled correctly or a part is missing. There's supposed to be a rubber disk that the brake pedal rod pushes against, called the reaction disk, installed in the booster between the push rod and the diaphragm that will cause both a low brake pedal and grabbing brakes when it is missing.

 
It looks like you have it plumbed correctly. When you stop does it pull or stop in a straight line? How long has it sat since the last time you drove it?

The most common grabby brake problem is from pads or shoes contaminated with brake fluid or oil, but this is usually just on one brake, resulting in pulling. Another problem is from rusted drums and rotors, but will usually clear up unless there is too much pitting.

However, because you said you have a reman booster I suspect it was not assembled correctly or a part is missing. There's supposed to be a rubber disk that the brake pedal rod pushes against, called the reaction disk, installed in the booster between the push rod and the diaphragm that will cause both a low brake pedal and grabbing brakes when it is missing.
The rubber disk is there, but i did notice the my old MC and booster were siliconed together? Is there supposed to be a seal there as well? My front discs are surface rusty, since I havent put the new discs on yet.  Guess I should put the new discs on and see how it feels, right now its grabby but feels like its even from left to right.  I havent had it up to speed yet to try the brakes, was waiting on lower rad supports as right now its only being propped in place.  I just got the brackets today so I'll install them and take it for a little test ride. 

Thanks for the input.!!

 
It looks like you have it plumbed correctly. When you stop does it pull or stop in a straight line? How long has it sat since the last time you drove it?

The most common grabby brake problem is from pads or shoes contaminated with brake fluid or oil, but this is usually just on one brake, resulting in pulling. Another problem is from rusted drums and rotors, but will usually clear up unless there is too much pitting.

However, because you said you have a reman booster I suspect it was not assembled correctly or a part is missing. There's supposed to be a rubber disk that the brake pedal rod pushes against, called the reaction disk, installed in the booster between the push rod and the diaphragm that will cause both a low brake pedal and grabbing brakes when it is missing.
The rubber disk is there, but i did notice the my old MC and booster were siliconed together? Is there supposed to be a seal there as well? My front discs are surface rusty, since I havent put the new discs on yet.  Guess I should put the new discs on and see how it feels, right now its grabby but feels like its even from left to right.  I havent had it up to speed yet to try the brakes, was waiting on lower rad supports as right now its only being propped in place.  I just got the brackets today so I'll install them and take it for a little test ride. 

Thanks for the input.!!

Wait a minute.... I just looked it up , reaction disc,  I dont have the rubber part that is the same size as the plunger ,.,,,,, shit I hope its still in the old booster, anyone have a Pic of these parts better than a random internet searchplunger01.jpg

 
It's between the booster and the MC. It fits over the end of the plunger (push rod) flange and into a recess on the diaphragm. They normally are not glued to the push rod flange and if the push rod is pulled out some it can fall down into the booster housing.

boost.jpg

 
Last edited by a moderator:
It's between the booster and the MC. It fits over the end of the plunger (push rod) flange and into a recess on the diaphragm. They normally are not glued to the push rod flange and if the push rod is pulled out some it can fall down into the booster housing.
Is it possible to check this rod without unplugging the brake lines?

Sent from my SAMSUNG-SGH-I337 using Tapatalk

 
Possibly, if there is enough length, bends or coils in the brake lines between the master cylinder and proportioning valve to allow the master cylinder to be pulled forward and to the side without kinking them. I believe I could in my '71, but I haven't tried it.

 
I'm wondering if that is the reason that my brakes feel too firm after doing the original reman booster conversion. I did (unknowingly) remove the M/C push rod to adjust the clearance between the tip and the M/C plunger to .015". Crap, hope it doesn't matter too much as I don't feel like pulling it all apart again!

What do you think Don? But wait, if the reaction disc was not there, wouldn't the push rod be really far back from the M/C plunger? Don't remember it being like that, so I may have dodged a bullet.

 
I know mine is not there, where would you get one?

I asked at my local napa and they never heard of such a thing.
You can use the old one or get one at a wrecking yard. Otherwise, you would have to find someplace that rebuilds them and will sell you one. I could never find a place that sells repair kits, for our boosters, to private consumers.

Your's may also be in the bottom of the booster housing.

 
Last edited by a moderator:
I'm wondering if that is the reason that my brakes feel too firm after doing the original reman booster conversion. I did (unknowingly) remove the M/C push rod to adjust the clearance between the tip and the M/C plunger to .015". Crap, hope it doesn't matter too much as I don't feel like pulling it all apart again!

What do you think Don? But wait, if the reaction disc was not there, wouldn't the push rod be really far back from the M/C plunger? Don't remember it being like that, so I may have dodged a bullet.
Yeah, you must have lucked out, and your's stayed in the recess in the diaphragm.

 
You are right, pulling it out to check it may result in it falling off and into the bottom of the housing.
Any suggestions? In my case i changed the MC but never checked the rod location. It feels like my car should be braking better but i have had no time to troubleshoot yet. I can at least lock the rear brakes but i have a feeling that the front is lacking. I have an adjustable proportioning valve and is set just above the rear brakes lock out.

Sent from my SAMSUNG-SGH-I337 using Tapatalk

 
I don't think that the reaction disk is your problem. I would try adjusting the proportioning valve, try to get more front brake bias.

Did you check the push rod/master cylinder clearance when you put the new one in?

 
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