[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.