Brake conversion, not mine

[NC-car-guy]

Just browsing eBay and saw this: https://www.ebay.com/itm/1954-Buick-Power-Brake-Conversion-Master-Cylinder-Booster-Special-Century-54/132326183992?hash=item1ecf415438:g:I6gAAOSwK31Zt0X3

 

So the builder states it will not fit any other year or series, only 54 special and century.  I DO NOT plan on buying this, but I was curious, why would it not fit a 55 small body? same car...  

why wouldn't it fit the larger series?  Do the larger series have different air plenums?

Just curious, I don't have $700 to lay out on this..

[Beemon]

The air plenum and pedal box assembly changes each year from 54 to 56. That seller also lists a 56 factory look alike A.C. system for 54-56. I couldn't stand to loose the driver side plenum with the washer jar (and my brakes work fine with the "ultra premium" shoes), but removing the passenger side plenum box wouldn't be an issue for a look alike system. 

[Beemon]

Quote

A modern GM booster and master cylinder give a significantly improved braking feel and power while adding safety through the dual circuit system. 

 

No extra safety involved. I'm not sure where this myth came from. The front and rear circuits of the master cylinder rely on each other to function correctly.

As you can see from the diagram, you need fluid in both sections of the master cylinder for it to function. Air is compressible, so guess what happens if you have a leak in either line? You loose braking, whether you have a single stage or dual stage. The only way a true dual brake system works is if you have two different master cylinders working in tandem - one for the front circuit and one for the rear circuit. The dual stage master cylinders, because both the front and rear circuit work together, is also a proportioning valve where it is dictated by spring force and pressure differences between the front and rear circuit. On most OEM applications without a proportioning block, look to see if one tube is larger than the other.

 

Quote

The other brake line goes to the factory junction block where the brake light switch is located and threads into the now removed rear brake feed (again no cutting or flaring needed).

 

This line tells me the seller has no idea how the brake system works in these cars. The junction block, at least in 56, has a 10lb residual pressure valve to keep the wheel cylinders primed and keeps the fluid from back feeding back into the master cylinder reservoir. On disc brake cars, you want about 2 lbs of residual pressure. So unless you take apart that junction block and remove the residual valve and add a 2lb residual valve in-line (if you go with discs), the disc brakes will drag heavily on the rotor because they will be incapable of releasing the pressure in the line.

 

Lots of stuff to look out for here. Just FYI, if anyone was thinking about purchasing one of these kits. Don't get me wrong, the extended brake lever is a plus over the stock (at least 56) system, and is the only benefit to a system like this.

Edited December 8, 2017 by Beemon (see edit history)

[Bloo]

Actually those will work with one half of the system dead. That was the whole justification for their existence, but I would also question how much of a safety improvement it is.

 

IIRC these were introduced on Ramblers about 1963 or so, and expanded to everything in the USA (almost) by federal mandate in 1967.

 

As long as you have enough pedal travel, you can indeed operate half the system. In theory, that is safer because you can use 2 wheels to stop, where otherwise you would have nothing. In practice there are a couple of issues. Here is a master cylinder kit for the dual master cylinder I am working on right now:

 

 

You may notice there are 5 seals that run on the bore. Now here is a typical master cylinder kit for a single system:

 

 

Thats right , 2 seals, making the dual cylinder over twice as likely to fail due to seal leakage. On rear-wheel drive cars, the systems are split front-to-back. You might lose the back brakes, or you might lose the front. The most likely reason is an exploding brake hose, or if you live in a salt state, maybe a rusted-out steel brake line.

 

When the seal failure is on the bottom piston, it will bottom out on the cylinder, and the other half will still work. If the bad seal is in the upper half, the spring (on the upper piston in the first picture above) will compress until the steel sleeve inside of it bottoms out, allowing you to operate the bottom piston with the pedal.

 

In either case the pedal will be extremely low. There is also no guarantee that a pedal and/or booster designed for a single cylinder will have enough pedal travel to make this happen. It would have to be checked. It takes less pedal travel to bottom out a single master cylinder.

 

The best possible case is you lose the rear brakes. The fronts will stop the car handily, with only a low pedal. There is a 50% chance this will happen.

 

Don't forget having a failure in the first place is over 50% more likely because you have over twice as many seals in the master cylinder.

 

There is also a 50% chance you will lose the front brakes. it will be very difficult and slow to stop the car, particularly if you are carrying any speed. This is why rear-only brakes were abandoned in the 1920s. It is better than nothing at all, but even then, in most cases you have these same rear brakes available mechanically via the parking brake. The only real advantage you have in this situation is that your foot is already on the brake pedal, improving your reaction time somewhat over using the parking brake.

 

For the sort of people who never change their brake fluid, or the rubber lines, maybe a dual cylinder is safer. For the rest of us, I suspect not. It certainly isn't as cut and dry as some people make it out to be.

Edited December 8, 2017 by Bloo (see edit history)

[Beemon]

This is not true, and I speak from experience. You cannot compress the brake fluid in a dual stage master cylinder if one side has blown because air is compressible and you won't be able to compress either side. It needs fluid in both sides to compress. I blew a front line on my 02 Jeep Liberty and had nothing (no pedal) but the parking brake. And it's even worse with anti-lock brake systems. It's just sales pitch nonsense. If you loose the rear wheels, you need to bottom out the rear piston before you can compress the front side and vice versa for if you loose the front. If you get anything at all, it will at the absolute end of the pedal travel. Having been in this situation myself, they simply do not work as they were "intended".

 

[Bloo]

While I don't doubt what you experienced, and I'm not going to comment on ABS for fear of muddying up the conversation, a typical post-67 American rear wheel drive car loses half the brakes, unless the owner keeps driving until the OTHER two go away as well.

 

I fixed a gazillion of these before 1988 or so. They roll into the shop with the customer complaining of a low pedal, and sometimes a "BRAKE" warning light on. Those "BRAKE" warning lights didn't always work as designed even when present. They indicated a pressure differential between one circuit and the other. A pressure differential indicates one circuit has failed.

 

 

 

 

In the bottom of each reservoir there is a tiny pinhole, at the bottom of the cone shaped area in the bottom of each reservoir. Notice how close they are to a seal on both halves.

 

As soon as that seal covers up the pinhole, the fluid cannot squirt back into the reservoir. The brakes start to work.

 

Lets call the seals #1 through #5, left to right.

 

Lets say the circuit at the right fails. Maybe #5 seal failed, and the reservoir drained out on the ground or got sucked into the brake booster. Maybe #4 failed and leaks fluid to the right where it can go up the large port back into the reservoir. No pressure is created in the right circuit. Furthermore, the "fluid piston" normally formed between #4 and #3 (#3 is a reverse cup) does not exist. What happens? The piston (the one with seal #5 and #4 on it) moves left compressing the spring to the immediate left. It bottoms out on the steel sleeve thing to the left, where it pushes on the other piston for the other circuit. The circuit to the left operates as normal.

 

Now lets say the circuit at the left fails. #1 seal has failed. It covers the pinhole, but fluid just blows past back toward seal #2. From here I suspect it blows back up into the reservoir via a large port they might have forgot to draw. In any event, no pressure is created. When the driver steps on the pedal, the circuit to the right covers it's pinhole, and attempts to work as normal, but since there is no pressure in the back, it cannot do anything. The "fluid piston" moves to the left, moving the left piston and compressing the leftmost spring until the leftmost piston physically bottoms in it's bore. Then the right circuit starts to work as normal.

 

I suspect the only purpose of seal #3 is to keep a left circuit failure from unseating reverse seal #4 and draining the "fluid piston"

 

Anything like a blown brake line or caliper or cylinder that lets air into one circuit (and drains the fluid) has the same effect.

 

 With a failure of either circuit, you get a low pedal and 2 wheel braking.

Edited December 9, 2017 by Bloo (see edit history)

[Bloo]

By the way, I completely agree with you that you only get action right at the end of pedal travel, it is extremely hard to stop on the rears only, and that this is a lot of marketing nonsense. This is one of those things people think they need because someone told them they did, kind of like 12v conversions, etc...

 

On the other hand, if converting to front disc, a dual cylinder could simplify things.

Edited December 9, 2017 by Bloo (see edit history)

[NC-car-guy]

3 hours ago, Beemon said:

The air plenum and pedal box assembly changes each year from 54 to 56. That seller also lists a 56 factory look alike A.C. system for 54-56. I couldn't stand to loose the driver side plenum with the washer jar (and my brakes work fine with the "ultra premium" shoes), but removing the passenger side plenum box wouldn't be an issue for a look alike system. 

Really?  I'm going to have to hold my 55 boxes up to the 54...

 

[lancemb]

This discussion does highlight a couple important points on which there is widespread misconception regarding the efficacy of dual-circuit brake systems.  1) When you lose a circuit you don't wind up with "half" your braking ability; you either wind up with substantially more than half or substantially less and 2) If your brakes are not adjusted and bled properly to begin with and your reservoir full you may get nothing or nearly nothing.

[60FlatTop]

I won't be converting any of my cars to dual braking systems. Maintenance and quality parts have never been a problem with me. However, I do see a lot of information and recommendations for the conversion. And I looked at this setup.

 

Here the builder has 3/16" lines running from the master cylinder to adapter/splitter blocks that connect to the original 1/4" factory designed lines. I am pretty sure both 3/16 and 1/4" tubing was available in 1954. I know GM purchased a lot of material based on economies of scale. So why did GM and other manufacturers choose 1/4" tubing for their brake systems. It must have cost a bit more.

I have seen other instances where the tubing reduction is at other locations in the system.

 

All I would like to see is some budding young engineer show a proof that the reduction in line size, in random locations, has no effect or is equal to the original hydraulic design calculations. I am picturing a group of GM brake engineers on a conference call with the counter guy at Pep Boys checking on adapter availability.

 

If left on my own I would tell people that brake tubing is planted from seed and they didn't come up with a hybrid seed for 3/16's tubing until the mid '60's. Now, you know that is a cynical and sarcastic remark, so I must be missing the obvious answer. Cough it up, my bumper and grille could depend on it.

Bernie

Edited December 9, 2017 by 60FlatTop (see edit history)