'68 master cyl. question

[David Greenburg]

Can anyone with a '68 tell me which (front or rear) brake lines attach to the front of the master cylinder?  I had a dual master cyl. installed by a shop a while back on my '59 60S, using the original brake booster and an unknown master cyl., but the brake performance was never as good as the original set up.  Based on advice from this board, I recently replaced the master cyl./booster assembly with a rebuilt '68 assembly.  The brakes work better, but still don't have the stopping power of the original single cylinder.  So I'm wondering if the brake lines are connected to the correct ports.  The '68 MC on my car has one brake line port sticking off the front end, and one on the side (facing the left fender)

[The Tassie Devil(le)]

Basically, in the case of a Master Cylinder where the Reservoirs are the same size, it shouldn't matter which end is used, as the actual Cylinder and Piston is the same Diameter, and therefore the pressure applied at each fitting will be the same.

If the Reservoirs are of a different capacity, then the larger one is for the front.

The other thing to consider is what difference is there if any in the Diameter of the Cylinder between 1960 and 1968.

The later model Set-up should be virtually the same as the Single type, but you will have the added safety margin of knowing that if one line fractures, you won't loose all brakes, just 2 of the 4.

Bruce.

[Tom Magdaleno]

Usually they both run down to a proportioning valve on the frame and then go to the front and rear wheels.  Sounds like you will have to reroute your brake lines to run a dual cylinder. 
Are you able to lock up the wheels?  You can buy an adjustable proportioning valve from Wilwood that will let you reduce the pressure to the rear brakes if needed.

[David Greenburg]

I have not looked inside to see if one compartment is larger than the other; if so that would be determinative.  I just assumed that it is set up with one chamber dedicated to one set of lines.  The brakes are pretty hard to lock up, and it feels like the rear lock first, which is what made me wonder about whether the lines are reversed.  As for proportioning valves, it is my understanding that they are not used with 4 wheel drums.  There is a junction block where the line splits into two to go to each wheel, but there is no valving.

[Glen]

The size of the reservoir has nothing to do with the pressure out of the master cylinder.  I think Bruce meant to say the size of the cylinder.  IIRC the front and rear cylinders in a 68 master cylinder are the same size.  If so then the pressure out of each port will be the same.  Since the original system ran on a single line then it would be safe to say it was designed to operate with the same pressure front and back. 

If you are not using disk brakes on the front and drum brakes on the rear then I don’t think you need a proportioning valve.  Braking effort front to rear is proportioned by the wheel cylinder size and the swept area ratios front to back. 

Glen

[The Tassie Devil(le)]

I did mean the Reservoir, and not the Diameter of the Piston/Cylinder.

Plus, yes, these old cars with a single circuit use different size wheel cylinders to obtain their proportioning.

Bruce.

[Dave Shepherd]

Didn't the 68 have ft disc brakes for the first year?  If so the 68 master would have different size reservoirs.

[The Tassie Devil(le)]

If that is the case, and you are using one of the M/C's designed for Disc/Drum, then you will have to install a Residual Pressure Valve in the Front Port otherwise you WILL have poor braking performance.

Disc Brakes don't need the Residual Pressure Valve as the Pads don't have Return Springs, and rely on the movement of the Disc Rotor to push the Pads back when the Brake Pedal is released.   Drum Brakes have springs to do this.

Bruce.

[Dave Shepherd]

Bruce, most disc- drum m/c's that I have installed have a built in residual valve, he should be using a drum/drum master for his application.  Excellent point here and one that gets past most people.

[dadscad]

Dave,

Cadillac first installed a dual master in either 61 or 62, you might have better luck using a master cylinder - booster that is closer to your cars age. My 63 has a Bendix dual master cylinder with 4 wheel drum brakes and I've seen pictures of 62's with a similar Bendix master cylinder. Cadillac also used a dual Delco Moraine master cylinder during the same period, approximately years 61 - 64. A Booster - Master unit from those years may perform better especially if the wheel cylinders are similar in size from those years to your cars cylinders. I don't have my manual handy to be sure, but I think there are cylinder size listings in it. I'll see if I can find the cylinder size information for you unless someone else has information available sooner for you to compare.

For what it's worth, on my 63 the front line at the end of the master, is the rear circuit and the line next to the booster is the front circuit.

[David Greenburg]

Thanks for the input, everyone.  '68s still had 4 wheel drums, which is one of the reasons that it was recommended for this particular swap (I forget who the posters were), and I believe the wheel cylinders are compatible, although I'll need to check this.  I know the '59 rear cylinders are quite a bit smaller.  If the hookup of the lines didn't change between '63 and '68, my lines are hooked up correctly- the rear lines are connected to the front reservoir.   

[dadscad]

Dave,

I looked in my literature to find the bore diameters of the wheel cylinders and the Master Cylinder. The 59 had a 1" Master, 1-1/8" front cylinders and 1" rear cylinders. The 63 has a 1" master, 1-3/16" front cylinders and 1" rear cylinders. The 68 has a 1" master for disk and drum brake applications, 1-3/16" front drum cylinders and 15/16" rear cylinders with drum fronts.

When using a 1" bore dual circuit master cylinder, I don't know if your car's smaller diameter 1-1/8" front wheel cylinders would make the difference in braking power you have experienced verses the 1-3/16" front wheel cylinders used with the dual circuit master cylinder on the later model cars. Your car came with a 1" master cylinder and I would think a 1" dual master cylinder would operate the brakes the same as the original single master cylinder did. The extra 1/16" diameter surface area of the later wheel cylinder would create more force on the brake lining with equal pressure. Installing the 1-3/16" wheel cylinder is something you might consider if the later cylinder would fit in your backing plate the same as your original.

HTH,
David

[David Greenburg]

David:

Thx for digging up those wheel cyl. specs. My reference library gets pretty sparse by the mid '60s.   I think I need to go find a vacant school parking lot or some place where I can do some serious hard stops to evaluate what's going on.  Frankly, I'm inclined to go back to a  single MC before I start experimenting with wheels cylinders.  It's far more likely that I'm going to face a situation where I need ALL my braking power than a situation where I lose brakes due to a fluid leak.

Dave

[Steve W]

68's also had the power front disc brake option!

[dadscad]

Dave,

Another thought. If you had new shoes installed at the same time the master cylinder was installed, it may be the shoes are not fully seated to the drum. If the shoes were not arched to the drum prior to installation, then it will take many miles for them to fully seat in and provide complete brake action. You might pull the drums off and take a look at the shoes to see if they are contacting the drum from one end of the shoe to the other.

David

[The Tassie Devil(le)]

Yes, people forget that the Shoes need "Radiising" to fit the Drums.

Should be done every time Shoes are replaced.

Bruce.

[David Greenburg]

the shoes have actually been on for some time, since the original dual circuit setup was installed (probably 800-1000 miles ago) so I think they should be ok.  I was planning to go out this past weekend and find a place to do some good hard, controlled stops, but the rest of my life caught up with me and the Caddy spent the weekend in the garage.  Unfortunate, Halloween seems like a good time for a black 59 Fleetwood to be out!