GT Master Cylinder size

[bq97]

Does anybody know what the bore diameter is for the stock GT master cylinder?

TIA
Brian

[heimue]

3/4" for all of the smaller Opels of that time.

[bq97]

Thanks, I though that they were 3/4" but wasn't sure. Now I'm wondering if a 1/16" larger bore diameter is worth the trouble of switching master cylinders.

[heimue]

A larger bore in the master cylinder does NOT make your brakes work better, but the other way around. A larger bore means that the pressure (pressure = force / area) decreases, you have to press harder on the pedal than before. A larger dia on the brake cylinders however means that with the same pressure as before you have a higher braking force (force = pressure x area). A larger master cylinder bore should normally be done in conjunction with a larger booster, or a tandem booster. If your new master cylinder is 13/16 instead of 3/4, your necessary brake pedal force will increase by around 17%.

Dieter

[namba209]

That's true Dieter, and here's another couple of things to consider. A larger MC bore will also take less travel on the brake pedal to apply or lock up the brakes, and by the same token, a larger wheel brake cylinder will require more travel of the brake pedal to get the same braking effect. The larger MC bore pushes more fluid than the stock bore at the same travel, and the larger wheel brake cylinder will require more fluid to move it the same distance as the stock bore. Confused yet.

[baz]

clear as mud ron
i always like the look an kids faces when they put a bigger M/C on and almost freak out when it does not stop as well and the pedal has locked up and will not move ,or so they think
bigger is not always better ,a longer M/C of the same bore can be the best brake tune up for low bucks as it has the same feel but more fluid moving at the same working pressure

[bq97]

Here is what I’m looking at.

My GT has the stock sized master cylinder, 75 Manta sized rotors and calibers on the front, ¾” wheel cylinders on the back. All the components are new and the booster works fine. This set up is a vast improvement over the standard GT brakes. However, the most braking force is when the pedal is near the end of it’s travel with a single push of the pedal. If I pump the brakes a second time I get better braking force. To me that is an indication there is not enough fluid flow out of the master cylinder for the larger brake components. The system has been bleed repeatedly and there is no air in the lines. The rubber brake hoses have been replaced with new braided steel hoses.

A friend has 78 BMW 320i with a disc/drum setup. The BMW front brake components are the exact same as the ones on my GT. His car is 500+ pounds heavier. The braking distance for the BMW is less than the GTs if I don’t pump the GT brake pedal. If I pump the GT’s brake pedal the stopping distance is about equal. Both cars have the same brake pads.

I know the pedal geometry, tires, and a few other things also play a role in braking force but it would seem to me that since the brake components are the same it might be a good idea to use the same master cylinder as well.

Any thoughts?

[nobody]

The BMW 5 series MC is what you need. Direct bolt up and new is about 55 bucks.

[bq97]

The 5 series has a 15/16' bore diameter and I have only found them with two output ports both for disc brakes. I could Tee the front port and add a residual pressure valve in the rear brake line but the 3 series seems to be setup correctly already. Well other than the ports on the wrong side.

Brian

[nobody]

They have the same thread sizing so use the valve off of your old one and then use a tee from a rear diff and you will stop like you can't imagine.

[N61WP]

I'm still working on my next setup. I'm using a 1" Wilwood M/C (50-50 split, aluminum) with aftermarket booster. I have the larger 3/4" rear cylinders installed and larger front calipers and rotors from a early 80's VW Scirroco.

If I can ever get the time to complete it, I'll give everyone a report on how it works out.

Jc

[bq97]

Originally Posted by nobody They have the same thread sizing so use the valve off of your old one and then use a tee from a rear diff and you will stop like you can't imagine.

Thanks Nobody, I'll try the 5 series Master cylinder and If I don't like it I'll revert back. So I guess I get to spend tomorrow machining a billet of aluminum into a new master cylinder resivior.

[nobody]

The draw back is you get an even brake bias and loose the nose dive and potential spin that the GT is known for. Ya that is a bummer.

[namba209]

Originally Posted by bq97 Here is what I’m looking at. My GT has the stock sized master cylinder, 75 Manta sized rotors and calibers on the front, ¾” wheel cylinders on the back. All the components are new and the booster works fine. This set up is a vast improvement over the standard GT brakes. However, the most braking force is when the pedal is near the end of it’s travel with a single push of the pedal. If I pump the brakes a second time I get better braking force. To me that is an indication there is not enough fluid flow out of the master cylinder for the larger brake components. The system has been bleed repeatedly and there is no air in the lines. The rubber brake hoses have been replaced with new braided steel hoses. A friend has 78 BMW 320i with a disc/drum setup. The BMW front brake components are the exact same as the ones on my GT. His car is 500+ pounds heavier. The braking distance for the BMW is less than the GTs if I don’t pump the GT brake pedal. If I pump the GT’s brake pedal the stopping distance is about equal. Both cars have the same brake pads.

That's the point I was trying to make in my earlier post, and what Dave was referring to. Bigger wheel cylinders, or as in Daves case, the rear disc setup, the MC has to be able to actuate all the pistons evenly and it just won't happen with larger rear pistons and the stock MC. But as Baz pointed out, there has to be a balance of forces. If I remember correctly Dave went through a few MCs before he found one that worked with his setup, so it may be a trial and error period until a match is found to your liking. HTH.

[N61WP]

Originally Posted by nobody The draw back is you get an even brake bias and loose the nose dive and potential spin that the GT is known for. Ya that is a bummer.

But that's what the proportioning valve is supposed to allow me to adjust untill I get the feel just right....right??

Jc

[Killer Texas GT]

The 75 Manta has a bore of 13/16" (20 mm). When I had the 75 big brakes and the Impulse rear disc; the stock GT Master cylinder was more that adequate to stop the GT and in a lot shorter distance than the stock set up. But when I went with the Impulse disc brakes front and rear the GT master and booster would not stop the car and was worse than the orginal stock set up. I opted to use the Impulse booster, master cylinder (7/8" bore) and bias control, and with some minor modifications to adapt it to the GT brake rod, it works so well it will throw you through the windshield when you have a panic stop.

[RallyBob]

I just measured up a GT master cylinder, I came up with 21 mm on the bore diameter, works out to .826".

Bob

[Killer Texas GT]

I stand to be corrected both the GT and 75 mastercylinders do have a bore of 21mm, however 20mm as indicated on the casting numbers of both units. The the difference then is in the length of the bore with the 75 being longer than the GT. Thus more fluid to move the pistons. Would this be correct conclusion?

[maxxheadroom]

So I am going to the junkyard this weekend, I am going to be checking out Honda mc and boosters, this being my primary choice due to the amount of them available, and the small size of the booster. Has anybody gone this route?

[BDD]

The BMW 5 series master cylinder will bolt directly to a stock Opel brake booster and will work with an otherwise stock Opel brake system? I suppose you need one that is NOT for anti-lock brakes.

[West Coast GT]

Originally Posted by BDD The BMW 5 series master cylinder will bolt directly to a stock Opel brake booster and will work with an otherwise stock Opel brake system?

I am confused.
Are you telling us this will work?
Or, are you asking us if this will work?

[blancojp]

Originally Posted by BDD The BMW 5 series master cylinder will bolt directly to a stock Opel brake booster and will work with an otherwise stock Opel brake system? I suppose you need one that is NOT for anti-lock brakes.

Most 3-series and 5-series non-ABS will work with the Opel brake booster. As a matter of fact, the BMW brake booster bolts up to the stock Opel location without modification but with a little contouring of the fender well. The only problem is the lines are on the opposite side so when routing the front lines, you sometimes find interference with the headlight bucket.

The Opel master cylinder comes in two flavors, 19mm and a 20mm. Both will work with the stock system. Most BMW master cylinders have a 17mm front circuit and a 20 or 22mm back circuit. This means you will have more pressure to the front wheels and depending on the M/C used, less pressure on the rear.

Through the years, I have installed BMW, Geo, Honda, Volvo, Ford, Wilwood and a few others on different Opels. The one that works best with the stock system is the Geo non-ABS master cylinder wich does not require a "T" for the front circuit.

[BDD]

The question mark means I was asking if it would work. I was asking for clarification. I've read in other posts that the BMW master cylinders are perhaps 1" wider and do interfere with the headlight rotation, the reservoir doesn't fit and has to be modified (cut down, I believe), the reservoir can't be reversed, etc.

Maybe there are solutions for these issues. Maybe there are matching reservoirs that do fit in the stock location, etc.

Blancojp, you are also saying that the Geo Metro non ABS master cylinder also bolts right on in front of the stock Opel booster? I have seen it mounted on a firewall with a different booster but that involves more modification than I am interested in doing and could make getting to the starter more difficult.

Thanks.

[Dennis Texas]

Originally Posted by heimue 3/4" for all of the smaller Opels of that time.

yikes I just ordered a rebuild kit for a 72 Opel Gt Master Cylinder, it listed it as 13/16"

[blancojp]

Originally Posted by BDD The question mark means I was asking if it would work. I was asking for clarification. I've read in other posts that the BMW master cylinders are perhaps 1" wider and do interfere with the headlight rotation, the reservoir doesn't fit and has to be modified (cut down, I believe), the reservoir can't be reversed, etc. Maybe there are solutions for these issues. Maybe there are matching reservoirs that do fit in the stock location, etc. Blancojp, you are also saying that the Geo Metro non ABS master cylinder also bolts right on in front of the stock Opel booster? I have seen it mounted on a firewall with a different booster but that involves more modification than I am interested in doing and could make getting to the starter more difficult. Thanks.

I posted the method to install the Geo master cylinder and also the Honda master cylinder both on the stock booster and also on the firewall. I have even gone as far as to install the m/c and booster combo from a Geo on the stock location but it does require some cutting and drilling.

If you don't have the will, tools and time to explore options, I would recommend you install the stock unit at the stock location.