Here is a little background first.

The car is a 71 convertible, 454 BB, manual, Right hand drive (Australia). Been in the family since 76.

The calipers have all been re-sleeved with stainless steel & have o-ring pistons installed.

Back in the late 80s Dad had someone install a VH40 remote booster to the brake system. As you may be aware, this booster assists the braking of the front calipers only. It works extremely well.

There is no room for a vacuum booster that attaches to the master cylinder directly.

The problem lies in the “proportioning valve”.
Under normal braking, the car appears to slow down using front & back brakes, but under heavy braking the valve “senses” the stronger boosted pressure going to the front & it then isolates the rear thinking that there is a problem. When this happens, the nose dives & even on gravel the back brakes won’t lock up.

Normally the brake warning light would illuminate on the center console to notify you of the problem, but on investigation, the mechanic cut the wire at the valve. I guess because he couldn’t get it to stop doing it. I have run a test wire from the valve into the car & attached it to a buzzer to listen when the valve sends a warning signal. It happens every time I brake hard, but not light braking.

If I disconnect the vacuum line from the remote booster & test the brakes, obviously it takes an enormous amount of effort to stop, but I can actually lock up all 4 wheels on gravel. This tells me that the valve is not isolating either front or rear brakes but doing its job. The buzzer stays silent under this heavy braking example.

I hope I have explained the problem well enough for someone to suggest a way of getting the brakes working the way that they should.

Any help is appreciated. Thanks in advance.


Andy

 

Hi Andy. I'm in Queensland, Sunshine Coast.
Here's the thing. that safety switch isn't really a proportioning valve. It's designed to swing to one side if a line blows and illuminate the brake warning light.
You would do well with this system to purchase a adjustable proportioning valve, available from places like Summit Racing. And replace the stock safety valve with an actual proportioning valve.
I have used these on custom trike conversions I have built for handicapped individuals. they can be dialed in to get your balance spot on. But I don't think you will get the warning light to work.
P.M. me if you wish.
Best of luck.

 

I reckon the adjustable valve is the way to go. I have been told that I can also use a Residual Pressure valve like they use for front disc / rear drum. But all it does is let the rear brakes activate before the fronts engage. This would not remedy the pressure difference between the front boosted & rear non boosted.
Cheers.
Andy

 

another possibility is hydroboost. uses power steering pressure instead of vacuum. way smaller than a vac booster. can be found on late 90's early 2000 gm trucks and vans.

 

I will check that out.
Cheers.

Andy

 

A hydroboost system would be the ultimate answer. getting all of your Calipers back onto power brakes. But not knowing much about the system you have now that only assists the front Calipers, it could be a bit of, " reinventing the wheel ".
However, if your budget allows and it can be set up compatible with your master on the wrong side of the car.
It would be the best answer.
The adjustable proportioning valve would cure the problem at hand for a fraction of the cost of a hydroboost system.

 

You could do away with the valve/switch and just use a t piece for the two front lines and a joiner to connect the rear. But the nature of what you are doing will always provide a braking system that is unbalanced to the front. The hydroboost would be better if your car has power steering and you must have the power brakes. Manual brakes can work very well if setup correctly with good pads, the right master size and pedal ratio (I converted from boosted to manual brakes and prefer it personally).
An adjustable propertioning valve probably won't help much as it can only reduce rear braking, not increase it.

 

Metalhead is correct. An adjustable Prop V. is a pressure reducer, usually to reduce the rear brake set-up.
More common on short wheel base vehicles such as street rods where full braking pressure would tend to bring the back of the car around to the front. Not fun.

Also known as bias valves to some. But basically it tries to get that front / rear around 70 / 30 or even 80 / 20 bias brake pressures.

 

Is the booster mounted before or after the valve? I has to be plumbed in downstream of the valve or there will be a difference in input pressure on both sides of the valve. When that happens, the warning light will come on, as it should.

 

I thought that at first too. But because the valve also splits the front brake lines, I don't think that it can be plumbed that way, at least not easily.

 

I believe that the master cylinder is the original. In fact, I think the whole system is still factory supplied ( except for the fact it & is located on the other side).
To install an adjustable Prop valve on the front brakes would almost have the same effect as removing the booster.
The booster is located between the master & the distribution block. If I was to relocate the booster to between the block & the front calipers, this would definitely eliminate the warning light problem, but there would still be a massive difference in front to rear brake balance.

It looks like the hydroboost system would supply pressure to the master cylinder without interfering with front & rear brake balance.
It has also been suggested to add a second remote booster to rear part of the brake system.
Any thoughts on this??


Andy

 

Well Andy, I've been a mechanic for almost 50 years now and I have never seen a power brake system that only supply's half the car. Yes if the budget allows, lose your current system. Put in a hydroboost system. And have proper power brakes. For safety's sake I would find a way. Or if money is tight. set it up properly with good manual brakes. Keep in mind the difference in master cylinders between manual and power brakes. Which do you have?

 

I have seen a vh44 and a vh40 set up do do what you want two boosters on the one vehicle , however it’s a compromise and when it comes to brakes, there is no good reason to compromise.
i assume your vehicle is RHD .
you could consider a 7”’ double diaphragm booster Depending 0n how you conversion was done you may not have room for that.
the best option is the hydroboost as suggested by others here ,
They are available here in Australia on a number vehicles ,
You will have to make sure there is adequate structural strength in the firewall to support the hydroboost.
Bfit

Sent from my iPad

 

Doc,
I suspect the master cylinder is the original manual type that came from the factory. As the booster is mounted away from the master, I dont think the mechanic that did this was too worried about changing it out.
I will have to remove it & take measurements to know for sure. Unless you know of an easier way. I am all ears.

 

You should post pictures of your RHD engine bay, it would be interesting to see.

 

I will just throw this in, hope it helps. I had about 10 Chevy Astro vans for my company. All had hydro-boost systems. Some of the custom guys would get them at a bone yard for next to nothing. I don't know if the Astro was available down under. It may be worth a shot. Jerry

 

I will try to get some posted soon.

I don't think that I would find any in Aus. Thanks for the info, though.

 

Keep in mind the Hydra Boost system is also available kit form from corvette vendors. The only thing I could think of that would be that the Hydraulic hoses would need to be longer.

 

Here is a pick of the RHD engine bay. Please don't be too harsh, It is still a work in progress. As you can see on the right, there is not much room for the hydroboost.


I am much happier to show the RHD interior.





Andy