Because I do not have the knowledge, I used a mechanic in a commercial garage to help me with my conversion from a manual single reservoir drum brake system to a manual dual reservoir drum brake system. I ordered my new MC from SuperCars Unlimited in Portland. It is for a '67 Cutlass. I used the same pushrod as I had in the old system.

The brake line system is as follows: The front port on the MC is connected to the front brakes through a new 3 way distribution block. The rear port is connected to the rear brakes through an adjustable proportioning valve to the new stock distribution block on the differential. The brake lines are new and the fluid is silicone.

My brakes are worse now than they were with the single reservoir system.

I would appreciate suggestions from you.

Thanks

Pat Mills

 

Can you be more specific? In what way? Pulling to one side or the other? Excessive pedal travel? Spongy?

If it's pulling to one side or the other or there is excessive pedal travel, that suggests that one or more wheels need to be adjusted. Sponginess would suggest air in the line and bleeding is needed.

If it's none of these and you have brakes that just don't seem as effective as they should (have to really push hard on the pedal to get the car to stop at all, even though it's firm and the brakes engage immediately), then you probably have an issue with distribution of braking force between the front systems. Perhaps only the front or only the rear brakes are engaging at all. Someone with more knowledge than me (which is most people!) will have to provide advice on this one.

 

Step one, find a mechanic who has a clue about four wheel drum systems. There is NO proportioning valve used in a drum/drum brake system.

Step two, are you sure the pushrod is correct? The single circuit master cylinders have a significantly deeper hole in the piston than do the dual circuit M/Cs. The pushrods are usually different.

Step three, did the pushrod get put in the correct hole on the brake pedal? There are two holes, one for manual brakes and one for power. The hole closer to the pedal pivot pin is for manual brakes as it provides greater leverage to make up for the lack of a power booster.

Step four, is the system actually bled properly?

 

Thanks for your responses.

The car does not pull one way or the other. The brakes do not feel spongy but I have to really stand on the pedal and even that does not have much effect.

The pushrod appears to work, Joe. - i.e. it engages the piston and the brake pedal is in the right position. How else would you know if it was correct?

I also wondered from some of the other discussions if the ports on the MC were hooked up properly. For example - does the rear port actually go the rear wheels and the front to the front wheels?

Thanks again.

 

It sounds like the pushrod may not be in the correct hole on the pedal.

The only correct way to check for pushrod length is to compare the dimension from the yoke to the M/C mounting flange on both the old and new M/Cs.

The lines should be connected front-to-front and back-to-back, but in your case it really doesn't matter since both pistons in the M/C are the same diameter. The M/C should have a 1" bore for this car. You may want to verify what was actually in the M/C you got.

 

Thanks for your help Joe. I will check these things you have mentioned.

 

The 67 master cylinder has the pushrod seat .425" inset, and the 66 has .76" inset so the rod goes in deeper in 66. To use the 66 rod you can cut down the OD of the 66 retainer plate to 1.0" to hold the flange so it can't come through the slot in the 67 cup. The retainer cup MUST be used to prevent the rod from falling out of the master cylinder. In 66, a retainer plate and snap ring were used instead. The cup can be hard to find, but 67-72 manual brake A bodies used them. In 67 the rod tip to clevis length is about 4.125".

Silicone fluid requires that all the old fluid be purged completely beforehand. Many have found silicone to leak easier at fittings and it is water intolerant so in humid climates the water will puddle in the system and cause problems. That is why conventional fluids (DOT 4 is excellent) can absorb water and don't last forever.

I have converted two 66s to dual master cylinders and both work fine, front brakes to forward port on the cylinder, rear to rear.

 

Thanks for all your help. I have discovered that my push rod is in the wrong hole in the brake pedal.

Since the proportioning valve is already installed in the rear brake line, is there a problem leaving it there and adjusting it so that the front brakes are stronger than the rear brakes? I do intend to upgrade to front disc brakes in the near future.

 

Since the stock drum brake wheel cylinders are sized to NOT have a proportioning valve in place, using the valve will reduce the effectiveness of the rear brakes, overloading the fronts and further hurting braking performance, which is the whole reason why you wrote the original post. I'd recommend removing the valve for now and simply using couplers and a section of brake line to replace it. When you do the disk conversion, put it back in.

 

That's what I wanted to know Joe, thanks for your advice.

 

 

How can I put a 67 master cyl on a 65 power booster. Does anyone have the rod lengths so I can cut them. I tried to use a 67 booster but it is 11" and hits the fender well. 65 is 9" Anyone done this before? Thanks for your help