A few years ago I ran into problems bleeding the back brakes on my mustang.
I talked to my friend who owns the local repair shop and I trust him. He
said he often has trouble bleed the rear brakes on cars and it is the
proportioning valve. I have also experienced problems removing the
driveshaft on another mustang. The brakes regardless of how hard pressed
would not stop the driveshaft from turning. I also have a CanCar friend who
has raced forever. They all said get rid of the proportioning valve. If the
front is responsible for 70% or the brakeing and the back 30% what if both
ends got 100%. I removed the valve from both the vet and mustang and the
increase in braking power was very noticable. The problem bleeding the rear
brakes on the mustang was gone and peddle feel or firmness is much improved.
If I wanted a quick improvement in braking I would remove this brass
proportioning valve. Plumb the front lines to the front of the master
cylinder directly and another line from the back of the master to the rear
brakes. If you loose one end due to a failure the other end is totally
unaffected. Try this with the proportioning valve installed. Open a bleeder
and try hitting the brakes . The pedal goes to the floor. Now with the
proportioning valve removed do the same thing and the brake pedal will feel
almost normal. The line pressure to the good end goes up and the brakes will
work fine.
I am a big fan of the hydraboost but without it removal of the proportioning
valve is the next best step for improved brake feel.
I run sticky rear tires so why shouldn't they do their share of the braking? I also never run in the rain or snow. This could cause a problem with rear lockup but for a fairweather car like mine it is a big improvement.

 

Great info Norval, I wonder why I never thought of that :smash: :lol: .

I bet there is lot of guys very thankful to you after they see this. Great tip, thanks for sharing! :cheers:

 

Hi Norval,
Great information.
A question, is the proportioning valve on a Corvette really a proportioning valve, or just a spring loaded piston, that when seeing an imbalance in front/rear pressure will turn on the "brake" light on the dash ?

Barry

 

norval
in 1990 my 67 ford - pedal went to floor, no brakes. 1 leaking wheel cyl.
What good is a dual braking system???? :confused:
so i threw away that valve :smash:
Proportioning should be with wheel cyl/piston size.
What are those car engineers smoking? :confused: :rolleyes:

 

Barry
i think you are right. it just triggersthe red light

 

I am considering this as well with the main difference being the addition of a cockpit adjustable manual proportioning valve. That way I can adjust the rear wheel braking myself, depending on the conditions.

Hans

 

Barry. I agree it is a switch to turn the dash light on but fluid still has trouble going through it. If you loose one end like I said open a bleeder and push the pedal, it will go to the floor. Without this little light switch it will not. The pedal will still feel firm. Why? Because the system's are isolated from each other and if one fails the other doesn't know it so it continues to act normal. The pedal will still activate one end fully.
If you only get 30% braking on the rear what happens to the rest of the pressure?
I ran big stickly MT ET streets on the rear and they did little braking. After removing the proportioning? valve they felt like someone grabbed the back bumper and was hanging on. I could get them to lock if I tried after the valve was gone. The important thing was they were giving 100% in trying to stop the car not just riding along. The mustang is the same , it had power brakes but the back was very difficult to bleed and with the bleeder open the fluid did not come squirting out, just trickle.
I believe my system is safer then stock. I lost a brake line on my ford truck and I almost went through the light. I had to work at stopping, slowly pump the brake to get alittle pressure and downshift. The master cylinder has a very shallow dividing wall that allows nearly all fluid to drain out the leak. Our vets have full internal walls seperating the front and rear, so no pressure loss in the remaining working side.
If you ever had a brake failure you know what I am talking about. Remove the light proportioning valve and have this happen and you will hardly feel the difference in the pedal. Try it . You might find out I am right.

 

drives61. Barry is right it triggers the light but it also reduces or totally destroys the pressure to the rear brakes. I have experience this 3 different times with different vihicles. Like I told Barry my ford truck lost a line and it was almost totally without brakes. I bet anyone without a valve, 2 seperate systems running from the master cylinder you can open the bleeder on one end and still go out and lock the other end without the peddle going to the floor. Try this with a light switch connected in the system.

 

Norval,

Just curious if you have braked hard while in a turn and it was ok?

I have a willwood adjustable valve somewhere around here that I haven't put in yet.

 

Page 9-2 of the Haynes Corvette Manual #24040 says the valve will prevent loss of pressure in one side if the other fails. On the other hand, your statement about opening a bleeder is correct. I wonder if anyone can explain how this works.

 

I have never had to brake hard in a turn. I have never while driving the vet had to do a panic stop. I have in fun done repeated 100 mph panic stops and locked up the brakes at lower speeds just for fun. But no I have never had an emergency stop in the vet. I know all 4 wheels would lock up solid. Do you ever watch the Grand national stock cars come into the pits and just stop, no front end dive, just stop. My car feels like that. It feels like the rear is doing alot of the stopping, nice feeling, feeling the rear end pulling back while the front is doing it's thing. Once again I am a fair weather driver so in a real emergency I would probably panic and lock everything up but in just shear brakiing power from 100 mph I will stop as well as anyone. The cockpit does get a little smokey. :) :)

 

Just be carefull without the proportioning valve, it may cause the rear brakes lock up before the front wheels. You'll be spinning around quick if this happens.

 

the rear pistons are smaller.
the valve is just a warning light, that tells you the brakes won't work beacuse it wasted them :rolleyes: :crazy:

 

from my 1972 Chilton:
PRESSURE DIFFERENTIAL WARNING VALVE..."the signal consists of warning light activated by a differential pressure switch located below the master cyl.....warns the driver that a failure has occurred"
:rolleyes:

 

Oh i get it :eek: :confused:
This is how it works.
With a direct connection as norval suggests, you could drive around for years not knowing there was a partial failure.
The government mandated safety switch causes the car to lose all brakes, so you have to get it fixed right away. :rolleyes:

 

Oh i get it
This is how it works.
With a direct connection as norval suggests, you could drive around for years not knowing there was a partial failure.
The government mandated safety switch causes the car to lose all brakes, so you have to get it fixed right away.


Good analyises drives61

 

OK, guys, from personal experience, I guess I will have to speak up here....
some years ago I bought a drum brake version of a Pontiac Lemans...4 drum brakes, damn thing barely stopped, had no proportioning valve, as all the brakes were sized and of the same type for even enough stopping...
drum brakes are self-actuating to a degree...hold that thought now....
so I go convert the front to more modern disc brakes off a 70's firebird...no sweat..just do the obvious...but no proportioning valve....I say screw it and set it up...go for a drive...well obviously I needed something,...junkyard was bare at the moment...so I put in a Chrysler adjustable pressure valve in the rear brake line...acts like a proportioning valve sort of....
well that evened out the braking just fine....worked like a charm...
but in a 4 wheel disc setup I agree with Norval there...can't see the need, and may at some point just change my system over to just two brass distributiion blocks and have over with it....frankly never had any trouble in MY car traceable to the proportioning valve....but seen a few other cars that did...
and think it's better to get rid of it, trouble is...on their cars there is a liability issue...on MY car....well I gotta remember how to drive....

GENE

 

My brakes are OK right now but it looks like this winter I will put in a hydro boost system and remove the prop. valve. My rotors are at the limit also so I will rework the whole system this winter. Providing I don't break anything else that ties up money for the Vette.

 

First of all i am not a mechanic. I just bought a new preportioning valve from GM when i had trouble with mine. Second I have a friend who installed an adjustable preportioning valve on his 454 heavily modified Vett. His valvle has an adjustment **** to put the braking power where you want it front or rear. I don't know the brand of valve he used. Hope this helps. :conehead

 

For C5 vettes, I believe Doug Rippie Motorsports used to sell a similar setup but it adjusts the front/rear to be more agressive. I just went over their website but couldn't find the valve by itself anymore; it looks like they are selling it in a package to make more $$. I think the newer vettes have larger brakes in the front than in the rear, so maybe that's why they are selling them in a package now?

Wow, this is a must-do mod for a C3!! This and the hydraboost is definitely on my list of to-do mods. :cheers: