The Stovebolt Site Forums The Shop Area General Truck Talk Master cylinder single to dual upgrade: '56 GMC

Hi, I am upgrading the single reservoir/cylinder master cylinder to a dual reservoir one in my 1956 GMC model 150 3/4 ton 2wd pickup. My question is this- do I have one cylinder do the front brakes and one the rear, or do I have one do front and rear diagonally opposed wheels, and the other the other two wheels?

Dave

That is not a tuffy. 2 front 2 back. It would be interesting ifyou had a falure or bad adjustment if they were side and side

Was this a real question????

I'll agree with gazim, 2 front and 2 back. But it is a valid question, I had a Saab once that came from the factory with diagonal plumed brakes. So it isn't that unusual.

I asked this exact question way back when I was doing this. I believe most new cars are set up diagonally so at least one front wheel will be helping stop the vehicle since the rear brakes are only responsible for 10% of the stopping power. The diagonal set up seems logical to me. But in the end, I did the 2 front and 2 rear for simplicity sake. Hopefully my front brakes don't go out!

VWs used the diagonal split systems too.
I would imagine a front brake failure on a front/rear split would be 'interesting' in the wet!

Most cars with the diagonal split are also front wheel drive. Rear drivers and trucks still use the traditional front/rear split. Keep in mind, the port closest to the pushrod is for the front brakes, the other is the rear.

How crucial is having the port closest to the push rod going to the front? I replaced the master on my 47 1/2 ton with a dual from a 67 full size Chev car. I ran the front line from the port away from the push rod (for ease of bending the line to run along the cross member and I could use one standard length of tubing and not have to make it 2 lengths joined together). It is a drum/drum system. I know that for disc/drum cylinders the chamber for the discs is the large one. On my master, both chambers are the same size. The system seems to stop the truck well.

Rusty...AFAIK The front brakes should be the ones that receive pressure first.
That is, the first circuit to operate,bearing in mind that the fronts do most of the work.
I'm not sure what would happen the other way round but I imagine in an emergency stop you wouldn't want the rears coming on too hard first.
Especially on a truck which has a lighter back end anyway.
Therefor, I would reckon it is pretty important you plumb the master correctly....

Anyone else have the same opinion?

I've searched about this on the 'net and have seen the primary and secondary chambers slated as front and rear, respectively. But some of the diagrams show that the primary is the chamber further away from the push rod, in others it's the closer one. It appears that the push rod pushes pistons in both chambers at the same time. Any more thoughts on this? Thanks.

Rusty, I think it depends on what your sytem is based on.
Mine uses a Corvette style unit and bearing in mind it is reversed from the way it would be in the car.
Master Power Brakes state that the front port, furthest from the pushrod is the front circuit.
As you will see from my pic,I had to get 'creative' with the brake pipes.
It is important they are oriented correctly.
This is obviously something to do with the way the piston operates within the cylinder itself.
Here's a link showing some of the more common Master cylinders.
I hope this helps.
Jock.