GT Master Cylinder size

[West Coast GT]

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]

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]

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]

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.

[Dennis Texas]

I checked today if you have a rebuildable core it can rebuilt for 29 plus shipping. If it can't be rebuilt you pay a small inspection fee and they will ship it back.

[BDD]

Who would rebuild the master cylinder? I think that Autozone told me that but then said that they had no way to actually send my part to someone who would rebuild it. I did get a rebuild kit that is supposed to be for a '72 GT there though. Not expensive either.

I did several searches in the recent past for all posts and threads started by blancojp for Geo master cylinder, Metro master cylinder, Geo Metro master cylinder, Metro, Geo, etc. and no threads turned up about a Geo master cylinder bolting right on and working with a stock Opel booster or with a stock brake system. There were references to other Geo parts like an alernator, etc.
I did notice that in some searches most of the things that turned up were actually posted in the last couple of weeks.

Maybe some older posts don't turn up in a search? There are many broken links for pictures, etc, in other threads. Do you know the name of the thread? The ones about Honda and BMW parts seem promising too.

One thing about searching is that you have to read and read and read and read before you dig up the blanket statement that a certain part will just bolt right on, no problem and only if you read and read and read and read some other threads you might also find mention that you have to bang out the inner fender or might not be able to use your headlights, etc.

To me, 80% or 90% of the purpose of this website is so that anybody who wants to own one of these cars can readily get useful information that other people have learned so you don't have to find it out by trial and error, or by buying the wrong parts. It would also be more useful if threads and posts got more accurate titles about the subject. Too often they might get titles that don't help when using the search function.

[jeff denton]

I'm not convinced the booster is really needed. Especially when using a larger master cylinder??
I left the booster out of my system when I moved the pedals back, and incorporated the stock master cylinder into the pedal assembly. Not that I use brakes much, but when I do, there isn't a lot of effort needed to lock up the wheels.

[oldopelguy]

You can and should rebuild your stock Opel GT master cylinder. I say this because no one here is going to accept any responsiility for you installing anything else and doing it wrong, and if you aren't comfortable with some trial and error then anything but stock is just not for you. There are no alternatives other than the stock GT master cylinder that do not require at least some amount of fabrication skills and modification, nothing else was designed for the GT and will work directly.

To rebuild the Opel master cylinder you need a quality internal snap ring pliers and a brake cylinder hone. Sears sells both under their Craftsman line, and the hone should come with instructions. You will need to remove the master cylinder from the booster and while applying a little pressure to the cneter of the piston from the input end with something (I usually use a 1/4" drive ratchet extension) you will have to remove the internal snap ring from the cylinder and carefully release the spring tension to pop all the parts back out. After a good cleaning with brake parts cleaner you will then need to follow the directions on the cylinder hone and resurface the inner wall of the master cylinder.

Once it is smooth you then replace all the soft parts on the assembly you removed with new ones from the kit you purchased, lube them up with brake fluid, and reassemble the master cylinder. Bench bleed the master cylinder to fill it with fluid and make sure it's all working properly and reinstall.

If that's all beyond your available skills, time, or space, or you need the booster rebuilt as well, swing into a book store and purchase a copy of Hemmings Motor News. Inside there you will find a dozen companies that specialize in rebuilding the master cylinder, the brake booster, or both. I would recommend one in NH I have used twice before, but I don't have the information handy.




If you do want to continue research on modifying your master cylinder you need to understand basic hydraulics. You are applying pressure with your foot on the end of a cylinder in order to raise the pressure in it and move a little fluid to the wheel cylinders to force them to apply brakes. In order to apply those brakes you need both a certain, fixed, amount of pressure and a certain, fixed, amount of volume. If the stock master cylinder is 13/16" diameter, and moves 3/4" to actuate the brakes you can calculate the volume, and the pressure only matters in relative terms.

Say you replace the master cylinder with one that has a 3/4" internal diameter. Now, because of the reduced surface area on the piston you are pushing on, you need to apply less force to get the same pressure, or force per unit area. You also, though, because of that same reduced size, need to move the cylinder further to move the same amount of fluid out to the wheels, say an inch instead of the previous 3/4" (I'm not going to do the math, but you should.) That means your pedal will be further towards the floor before the brakes actuate.

Several companies in the70's and even now used master cylinders made by the same company that made the Opel master cylinders. You can physically bolt several of them to the booster, but none that I know of have the right kind of fluid reservoir because the application in the nose of the GT is unique. That means you either modify the reservoir on the new master cylinder or adapt an Opel one to it, neither of which is super easy. The easiest way to find out what you're up against is to search somewhere like partsamerica.com for various master cylinders, look at their pictures, then compare them to the Opel one in person. I would start with BMWs, 2002 and 3-series, and then move on to Volvo and VW. You also need to worry about brake line locations and residual pressure valves, but those can be adapted later if need-be. Try to find something with disc brakes on one end and drum on the other, and you should be OK.

As to the booster, all it does is amplify the force your foot exerts on the master cylinder. The GT and 1.9 Kadetts use a roughly 2:1 booster, and the 1.1 Kadetts with disc brakes use a roughly 1.6:1 booster, which means for every pound of force you apply with your foot the booster is roughly doubling that force on the master cylinder. Let's say you use a master cylinder with a smaller bore than stock, as above that requires less force to achieve the same fluid pressure. With a small enough bore, and a strong enough foot, youmay find you don't need the booster at all. (I don't use them myself, I swap to a 5/8" bore master cylinder and eliminate the booster all together, but since I use a pair of them with a balance/bias bar between them and have to make my own brackets it's not something I'm going to recommend to someone else for liability reasons.) All the booster has to do is fit in a location between te ebrake pedal and master cylinder and hook up to the input shaft.

If you're considering replacing the master cylinder, getting the booster that matches the master cylinder makes good sense as you will know they bolt together. As to mounting it up, you need a way to bolt it into place, usually that's either a pair or quartet of bolts that you may or may not have to drill or file the holes in the mounting bracket for. The only other mechanical connection is the push rod from the pedal, and all you need there is to make sure you got the one for the car that the booster came from and cut it somewhere, then cut the Opel GT one somewhere, then have the two of them welded together. You may need to pay attention to where the pedal is and do some careful cutting, and you want the welding to be professional for safety. Attention to detail is rewarded, and sometimes you get lucky on length or such and can thread one shaft to screw into the other so no cutting will be required, but that's a case-by-case thing.

[BDD]

Good info. There is no liability issue. But if someone says a part is a reasonable replacement and "bolts right on" they should then also go on to say....but you'll also have to modify this and this. Anything else is just incomplete.

[Dennis Texas]

Who would rebuild the master cylinder?

ok sorry I did not see you had asked and me like the butt that I am with details I didn't mention who...sorry
heres the email I got after several communications with them this was just within the last week so information should be accurate.


Dear Dennis,

We only list the one master cylinder rebuild kit for the 1972 Opel GT, and
that's the Dorman part number TM35670, which is designated to models with
power front disc brakes.

To order the A-1 Cardone part number 111541, it would be necessary for you to
send in your current master cylinder for rebuilding. The price would be the
price specified in our online catalog - $28.79, plus shipping - but there
would be no core charge. Cardone will inspect the part to determine if it can
be rebuilt - about 20% of the parts sent to them cannot be rebuilt, but the
rest are rebuildable - and then will rebuild the part and we'll then return it
to you. Should the part not be able to be rebuilt, a small fee for the
inspection will be deducted and the part returned.

If you're going to order the return and rebuild done, you should order that by
calling us at the customer service number listed below, rather than ordering
it online. Processing the return and rebuild takes some special paperwork on
our end, which is best accomplished by contacting us by phone.

Hope that answers your quest8ions. If we can be of further assistance, just
reply to this email or send us another.

Thank you,

Ralph
RockAuto Customer Service

phone: (608) 661-1376
fax: (608) 836-5694
toll-free: 1-866-ROCKAUTO (1-866-762-5288)
Auto Parts Fast at RockAuto


I think that Autozone told me that but then said that they had no way to actually send my part to someone who would rebuild it. I did get a rebuild kit that is supposed to be for a '72 GT there though. Not expensive either.

Yes thought I had found a deal on a 72 MC rebuild kit to until someone mentioned the diameter of the actual size. The kit I had ordered was 13/16 when I pulled a few of mine out that is to small so I cancelled the order and thats how I go to this last email.



Maybe some older posts don't turn up in a search? There are many broken links for pictures, etc, in other threads. Do you know the name of the thread? The ones about Honda and BMW parts seem promising too.

I'm not going to change mine out for another type until it has to be, due to no way to rebuild or get replacement. I like things simple and repairable. To me you can always change it out and it's nice to know there are options. Opel GT source has a kit listed to rebuild yours seemed a bit pricy to me but sometimes you have to pay unless you can find another source or method.

One thing about searching is that you have to read and read and read and read before you dig up the blanket statement that a certain part will just bolt right on, no problem and only if you read and read and read and read some other threads you might also find mention that you have to bang out the inner fender or might not be able to use your headlights, etc.

To me, 80% or 90% of the purpose of this website is so that anybody who wants to own one of these cars can readily get useful information that other people have learned so you don't have to find it out by trial and error, or by buying the wrong parts. It would also be more useful if threads and posts got more accurate titles about the subject. Too often they might get titles that don't help when using the search function.

I'm not going to comment about the search engine except I have similar issues with it. It has helped me a lot and when not I ask people, very helpful and you get a lot of opinions, you just have to pursue and find what works for you.

Hope that helps

[BDD]

Dennis:

The price you quote to rebuild the master cylinder is not bad. Hopefully your's is good enough to rebuild. Getting it relined later (stainless steel or brass?)would probably not be a bad idea. I just had mine rebuilt using the kit that Autozone sells. The kit was $20.99.

I've actually been finding recently that Rockauto and some of the other places on-line also do NOT list the appropriate Civic or BMW master cylinders as being in stock either. That surprised me.

With all this, getting your Opel master cylinder rebuilt this way is not a bad deal at all and it keeps your car stock if you want it that way.

[Dennis Texas]

Dennis:

The price you quote to rebuild the master cylinder is not bad. Hopefully your's is good enough to rebuild. Getting it relined later (stainless steel or brass?)would probably not be a bad idea. I just had mine rebuilt using the kit that Autozone sells. The kit was $20.99.

I've actually been finding recently that Rockauto and some of the other places on-line also do NOT list the appropriate Civic or BMW master cylinders as being in stock either. That surprised me.

With all this, getting your Opel master cylinder rebuilt this way is not a bad deal at all and it keeps your car stock if you want it that way.

I have more to rebuild I just needed two right now, I'm sending them 2 rebuildable cores and asked if they would make me up a kit for the rest they (person I spoke with) didn't sound too keen on that idea but he had no idea since it is jobbed out. From what I have found, it is a good price to have it rebuilt for me, of course shipping will add to the figure. What size was your kit from autozone? the piston size? Did it work and feel good? That is good information and price since no shipping would be involved. Thanks The explanation they gave me on the size (since opel GT are power disc as they say it only fits) is that they use different venders and sometimes do to no working knowledge they just have various kits. It did not sound like a reasonable explanation but I am sending the MC's in this weekend for rebuild. As far as being stock items, it appears there are too few rebuildable cores out there to have enough stockpiled for sale.

Thanks

[blancojp]

Good info. There is no liability issue. But if someone says a part is a reasonable replacement and "bolts right on" they should then also go on to say....but you'll also have to modify this and this. Anything else is just incomplete.

I have posted over time close to 8 different braking systems I have installed on my cars and/or customer cars. The post with the Geo Master cylinder stated that "only two holes needed to be drilled since the mounting holes on the Opel Booster were further inboard." The same statement applies to the Honda master cylinder and some of the other I have used. Besides, the Geo unit is tiny, about 5" in length so there is no interference problem with the reservoir or anything else. It also uses two forward ports which are 10mm and one rear port which is also 10mm so the existing tube nuts can be used.

I have installed the Geo assembly on vehicles with front 2.5" diameter pistons and 11" rear drum brakes with 1 1/8" wheel cylinders. Never had a single problem with cars under the 4000Lbs range. And if you do a search on the web for Ford Falcon brake upgrades, the Geo mastercylinder/booster combination is the recommended cheap upgrade to a car that is over 3000Lbs.

Everyone says you must use a proportional valve and so on. What everyone misses is that on most of these Jap master cylinders, the unit comes balanced for 60/40 operation due to the placement of the inlet port, in respect to the outlet. So nothing else is needed besides a 10lb residual valve for the rear which with the emergency brake properly adjusted, I don't believe is required and I never used one.

Though I no longer have an Opel with a front mounted master cylinder, if anyone would like to see how it is done just let me know. I can show step by step what needs to be done to mount a Geo master to the Opel booster. If all you need is a stock master cylinder or booster, I have a good qty of new, rebuilt and used units in stock.

[opelwasp]

Though I no longer have an Opel with a front mounted master cylinder, if anyone would like to see how it is done just let me know. I can show step by step what needs to be done to mount a Geo master to the Opel booster. If all you need is a stock master cylinder or booster, I have a good qty of new, rebuilt and used units in stock.

Yes pleas show us. I have a 94 Geo Metro in my driveway right now and am looking to do this exact mod.

[BDD]

Yes, this would be very good to know. These parts should be readily available too for awhile. The one I saw, which Sawdust posted a picture of seemed to be very compact. There are two setups (in general) one up front and one on the firewall. Both would be interesting to know more about. I think the one up front and on a stock booster would be best with less modification. I would also think that the Geo mc and booster could be placed up front too.

[blancojp]

Yes, this would be very good to know. These parts should be readily available too for awhile. The one I saw, which Sawdust posted a picture of seemed to be very compact. There are two setups (in general) one up front and one on the firewall. Both would be interesting to know more about. I think the one up front and on a stock booster would be best with less modification. I would also think that the Geo mc and booster could be placed up front too.

I posted way back both methods but if you want to see the firewall mounted unit, take a look at the first few posts on Stealth. I will post the how to of the installation on a new post, but bare in mind I don't have a car to mount this on at this time.

If mounted as a combination (Geo booster/mastercylinder) up front in the stock location, you will need to make a brake rod adapter. This will set you back a good $1.25 if you have the tools.

[Dennis Texas]

I have more to rebuild I just needed two right now, I'm sending them 2 rebuildable cores

Thanks

I received my parts back today one master cylinder was not able to be rebuilt (charged me 5.00 labor for inspection) but the other they did a bang up job on, cleaned everything and even replaced the grommets for the reservoir to clip on to. Total cost was 28 bucks and shipping was another 11 something (actually that was shipping for both I sent 2 of them) to ship it to them.
Just for information, and it is recent received them back 8/07/08

JFI