Geo Metro Mastercylinder Replacement

[blancojp]

Howdy all!

As promised, this thread is dedicated to the installation of a Geo Metro mastercylinder onto the existing Opel booster. We will cover the installation in 4-steps and should answer all of the questions out there on this subject.

But before we go into details, let us look at a brief comparison of several mastercylinders and what we can learn from each.

1) The first three pictures show the Geo mastercylinder as you might have seen on different books or web pages. The Geo unit has a 13/16" piston bore, has three M10x1 ports and a low fluid sensor inside the container.

2) The fourth picture is a Honda Civic mastercylinder which also has a 13/16" piston bore and two M10x1 ports. This mastercylinder is from an ABS equiped vehicle and would require a "T" for the front circuit if installed on the Opel booster.

3) The fifth picture is a BMW 325i mastercylinder. This is a multi-bore mastercylinder since the front circuit has a 17mm bore and the rear circuit has a 22mm bore. Also note this unit has one front circuit and two rear circuits which is opposite from the Opel unit.

The first thing we need to look at is the distance from the fluid inlet to the fluid outlet on both front and rear ports:

1) The Geo unit has a long front circuit stroke and a short stroke on the rear.
2) The Honda unit has a short front circuit stroke and a long stroke on the rear.
3) The BMW unit has a long front circuit stroke and a short stroke on the rear.

You might be thinking at this time "why is this important" and "what do I care about this". Well, if you talk to any hydraulics expert about brakes and/or if you visit the Wilwood technical site, you will see that they recomend you use the longer stroke for the circuit that has the largest caliper piston diameter to limit pedal travel. The second point is that this difference in volume helps you balance the braking system without the need of metering and/or balancing adapters most of the time. The Geo mastercylinder is biased at 60/40 which is a good ratio for our application.

There are no mysteries in a brake circuit, they are all based on the same principles and they all work the same way. You press the pedal, fluid goes through the lines and expands a few pistons, clamps or expands liners to a rotating surface and the car stops. I know I left out a few things but the object is for you all to get the general idea.

You are more than welcome to post any comments on this thread as long as you stay within the scope of the work so far posted.

And now, on to the installation....

[opelnut10]

Is the Geo and the Suzuki Swift sharing the same master cyl. or is the Geo master cyl. different. Some of us might find it easier to locate the Suzuki than the Geo.

[blancojp]

Originally Posted by opelnut10 Is the Geo and the Suzuki Swift sharing the same master cyl. or is the Geo master cyl. different. Some of us might find it easier to locate the Suzuki than the Geo.

The Geo Metro and the Suzuki Swift are the same vehicle with minor differences due to Chevy requirements. They share all of the mechanicals and all parts are interchangeable with the exception of a minor few.

Most local shops and used part places will sell you a master/booster combination for less than a rebuilt master cylinder. They are made by Aisin and of great quality so should last good time.

[tomking]

Thanks for giving us some new options BlancoJP! As we are learning our master cylinders are getting harder to find and this is good information to have.

[scrapman]

I echo Tom's Praises. Alternative solutions are always of huge help. I do have one question. Does the GEO/Suzuki MS require any type of adaptation to fit and work or will it just mount and work as is? Thanks

Rick

[blancojp]

Originally Posted by scrapman I echo Tom's Praises. Alternative solutions are always of huge help. I do have one question. Does the GEO/Suzuki MS require any type of adaptation to fit and work or will it just mount and work as is? Thanks Rick

Both you and every one ready this are welcome to the info and I thank all of you for the support. In a car everything is possible as long as you have a plan and do your research.

The installation procedure will be posted in detail in the next few days.

[Hiro]

thanks for the infos BlancoJP,
very usefull for race application & upgrades.
do you know of a bigger bore master cylinder that could fit our brake servo?
something about 23-24mm would be great to fill calipers with 55-56mm pistons.
thanks,
Hiro

[blancojp]

Originally Posted by Hiro thanks for the infos BlancoJP, very usefull for race application & upgrades. do you know of a bigger bore master cylinder that could fit our brake servo? something about 23-24mm would be great to fill calipers with 55-56mm pistons. thanks, Hiro

The biggest mastercylinder you can install on the Opel booster that will actually stop the car, without you standing on the brake pedal is made by Aisin for a Honda Prelude with a 15/16" bore. Anything bigger would be a waste of time with a 7" booster.

Besides, you should not be running a brake booster on racing applications. Race engines are not much on vacuum at low rpm's so imagine at or close to WOT what you have.

[Hiro]

Originally Posted by blancojp The biggest mastercylinder you can install on the Opel booster that will actually stop the car, without you standing on the brake pedal is made by Aisin for a Honda Prelude with a 15/16" bore. Anything bigger would be a waste of time with a 7" booster. Besides, you should not be running a brake booster on racing applications. Race engines are not much on vacuum at low rpm's so imagine at or close to WOT what you have.

a 15/16" bore would be great for my application,
very surprisingly my two race cars both have a brake servo & enough residual vacuum to actuate it,
the cams are not very big (310° duration @ 0.3mm) & I never brake at WOT maybe that's the reason why it still works properly?
Hiro

[blancojp]

Howdy all!
I have returned home and once again I am on mandatory vacation. Hopefully this time I might be able to take more than one day. Before we go onto the installation of the master cylinder, let talk about the Opel brake booster and see what we have.

The Opel booster is a 7", single diaphragm unit which can produce approx. 19.19psi of assist for every inch of vacuum applied at 100% efficiency. Though this might sound funny per say, the formula is sound since we only want results from the high vacuum end. With 16" of vacuum which should be the low end on a 1.9L, you will have approx. 307psi of assistance added to your pedal ratio. If you increase your booster size, you decrease the effort but increase the sensitivity of your pedal. So what rodders do when they install large brake calipers is to select the master cylinder according to the volume needed and then get a brake booster which compensates the required brake pedal force.

This is the formula to figure your output booster pressure.

Force in pounds = (Diaphragm area in square inches) x (manifold vacuum in inches Hg) x 0.5

Example: 7” single diaphragm booster with 16 inches of vacuum. 3.5” x 3.5” x 3.14 = 38.465 square inches x 16 inches of vacuum x 0.5 = 307 psi

Now on to the master cylinder installation.

[blancojp]

Well it is time to install the master cylinder to the booster.

I do not have a Geo unit but I will show how to install a Honda unit instead. The process is the same for both so there should not be any problems installing it. I will also show the firewall mount version when I install brakes on Stealth latter this month.

1) The first thing to do is to cut the nipple on the brake booster internal rod. Secure the rod with a needle nose pliers and using a dremel tool, cut the tip off which should shorten the internal rod about 1/4".

2) Measure and drill two mounting holes on the master cylinder as shown. The Geo and Honda units have the mounting points are centered 1/2" on the outside so new inner holes need to be drilled.

3) Install the master cylinder on the booster, tighten both mounting nuts and re-install your brake lines. You are now ready to bleed the brakes and fine tune the system.

The Geo unit has dual front circuits and the Honda requires a "T" for the front circuit since it has a single outlet. Also, with the Geo unit you will have 1/16" play on your brake pedal which can be adjusted out with the brake rod. Make sure your rear brakes are well adjusted otherwise pedal travel will vary with use.

You will notice the difference between the stock unit and the Geo master cylinder. Better firmer pedal, more reactive and less effort will be the first noticeable improvement. The other advantage of the Geo unit is that it is 2" shorter in length and also is 1/2 the weight of the stock unit. The other advantage over the stock unit is an additional 100psi being delivered to the front/rear circuits without any additional pedal travel.

So to recap, for $35 and 10 minutes of work you can replace your old master with an aluminum unit at 1/2 the weight and slight increase in pressure.

Sounds good to me......

[jmbinjax]

Great information - thanks!!!

A few questsions:

1. Does the fluid reservoir work from the Geo MC or do you need to fabricate a reservoir to fit the GT?
2. Does the low pressure switch work with existing wiring (assuming you splice in new connectors)?
3. Based on what I've read previously, when doing this conversion there is no longer a need for a proportioning valve when using 3/4" rear-wheel cyclinders due to the brake bias of the Geo MC, correct?

Thanks again!

[Hiro]

Originally Posted by blancojp Well it is time to install the master cylinder to the booster. I do not have a Geo unit but I will show how to install a Honda unit instead. The process is the same for both so there should not be any problems installing it. I will also show the firewall mount version when I install brakes on Stealth latter this month. 1) The first thing to do is to cut the nipple on the brake booster internal rod. Secure the rod with a needle nose pliers and using a dremel tool, cut the tip off which should shorten the internal rod about 1/4". 2) Measure and drill two mounting holes on the master cylinder as shown. The Geo and Honda units have the mounting points are centered 1/2" on the outside so new inner holes need to be drilled. 3) Install the master cylinder on the booster, tighten both mounting nuts and re-install your brake lines. You are now ready to bleed the brakes and fine tune the system. The Geo unit has dual front circuits and the Honda requires a "T" for the front circuit since it has a single outlet. Also, with the Geo unit you will have 1/16" play on your brake pedal which can be adjusted out with the brake rod. Make sure your rear brakes are well adjusted otherwise pedal travel will vary with use. You will notice the difference between the stock unit and the Geo master cylinder. Better firmer pedal, more reactive and less effort will be the first noticeable improvement. The other advantage of the Geo unit is that it is 2" shorter in length and also is 1/2 the weight of the stock unit. The other advantage over the stock unit is an additional 100psi being delivered to the front/rear circuits without any additional pedal travel. So to recap, for $35 and 10 minutes of work you can replace your old master with an aluminum unit at 1/2 the weight and slight increase in pressure. Sounds good to me......

sounds very good to me too,
thanks!
Hiro

[blancojp]

Originally Posted by jmbinjax Great information - thanks!!! A few questsions: 1. Does the fluid reservoir work from the Geo MC or do you need to fabricate a reservoir to fit the GT? 2. Does the low pressure switch work with existing wiring (assuming you splice in new connectors)? 3. Based on what I've read previously, when doing this conversion there is no longer a need for a proportioning valve when using 3/4" rear-wheel cyclinders due to the brake bias of the Geo MC, correct? Thanks again!

The reservoir is small and fits perfectly. The low pressure switch is now called the low fluid level switch but works the same way. The prop valve is not needed if you have the rear brakes adjusted properly.

This is one of the neatest conversions I have ever made, it is low cost and far better than the original. Even with 4-piston calipers up front it works like a champ and chances are you will never replace it again.

Not bad for just drilling a couple of holes!!

[jmbinjax]

Is there any benefit to also using the Geo booster other than availability?

[sawdust]

Any one thought of using the Miata MC/Booster?

[blancojp]

Originally Posted by jmbinjax Is there any benefit to also using the Geo booster other than availability?

An additional 100psi of pressure and the ability to purchase anywhere!!!

[Phoenix]

The Geo Metro MC that I aquired has a plactic nipple on it.

Also, the front most brake line on a GT comes in from the bottom while this one comes in from the top. Do you alter the stock line to come in from the top or do you get an adapter to make it. If an adapter, where can one be obtained from?

[blancojp]

Originally Posted by Phoenix The Geo Metro MC that I aquired has a plactic nipple on it. Also, the front most brake line on a GT comes in from the bottom while this one comes in from the top. Do you alter the stock line to come in from the top or do you get an adapter to make it. If an adapter, where can one be obtained from?

Don't know what you mean by "nipple", post a pic please.

The front brake lines can be done either way. You can use a banjo adapter or bend the lines carefully to suit. The easy way would be to use a banjo with a 10mm banjo bolt and 2 crush washers. I have plenty of them in stock so when you are ready, let me know you can pick up or I can send them to you.

[Phoenix]

It's an imput rod.

[blancojp]

Originally Posted by Phoenix It's an imput rod.

The actuator is made out of aluminum. You might have the brake booster rod stuck in it.

Check it against the picture at the top of this thread. I do however have several of them so I can trade it out if you got the wrong part.

[opeler69]

what year is the Honda master cylinder from?

[blancojp]

Originally Posted by opeler69 what year is the Honda master cylinder from?

'90s Honda civic without ABS

[opelwasp]

Originally Posted by blancojp 1) The first thing to do is to cut the nipple on the brake booster internal rod. Secure the rod with a needle nose pliers and using a dremel tool, cut the tip off which should shorten the internal rod about 1/4".

I'm not too keen on grinding in the brake booster. I think I'll just make a 1/4" spacer instead.

[blancojp]

Originally Posted by opelwasp I'm not too keen on grinding in the brake booster. I think I'll just make a 1/4" spacer instead.

You can do it any which way as long as the M/C does not have preload and it sits securely on the booster.