ITA RX7 master cylinder

[dickita15]

I need to order a new master cylinder for my car. The new chassis is a 79 with a 79 booster but everything is being swapped over from my 82. I think I remember that you need to use the master from a 12a disc brake rear car. Is that right? Also I cannot remember what they called that model.
thanks

[lateapex911]

In 1981, they introduced the limited slip rear with rear discs, and called it the GSL. So, I would imagine ordering any 81-85 GSL MC would do the trick.

[dyoungre]

I went to swap mine, many moons ago, and found an interference issue, not to mention the thread change from 10 mm fine to coarse.

Mazda moved the clutch master cylinder over by 1/2" in 1981 to allow for a larger diameter brake booster. If you try to swap the assembly, the booster won't fit unless you relocate the clutch master. From what I remember, the booster / brake master cylinders were not interchangeable, so I stuck with my '79 style booster and master, and then used the brake bias from the disk car.

[dickita15]

I know the Booster is different for the 79 so I am keeping the one from the chassis. All the rest of the brake parts I have taken from the 82 so everything is fine thread. I believe the 81-85 MC will bolt up to the 79 booster. Am I wrong?

[dyoungre]

The last time I had the two assemblies apart and next to each other was 1998; I thought that we tried mating the 1979 booster to the 1983 master cylinder but couldn't. Who knows, though, 1998 was a lot of CO ago ...

[tlyttle43]

One other thing you have to watch out for when swapping brake parts between early and late cars is the length of the piston rod in the brake booster. I did a swap of MCs from coarse to fine thread on a '79 (small booster) car [Stupid reason - I drilled a hole in the floor, directly through the steel line, and I only had fine thread replacement tubing]. After I put in the fine thread MC and steel lines, I absolutely could not get the brakes to work.

The problem was that the rod in the booster was not adjusted properly. I don't remember now what the exact problem was, I think it was holding the piston past the hole to the reservoir and thus not allowing the MC to fill (or maybe it was not allowing a full stroke), but in any case, the brakes simply wouln't work. Check the Mazda manual and it gives you a spec for the pushrod heights for the different boosters. It's threaded and adjustable, but adjusting it is a hassle.

Tom Lyttle

[dyoungre]

Tom,
That's a good point. I found a few fine thread -> course thread adapters, but I had to scour the region for all mom-n-pop auto parts stores. But in terms of the rod, did one version have a clevis attachment, and the other thread in?

Dick, I know that www.blackdragon.com is not quite the end all / be all of technical information, BUT they do show a picture of both boosters and master cylinders, and it shows the different orientation of the mounting bolts. One is side to side, the other is top to bottom. I have NO idea why they changed.

[dickita15]

Thanks for all the good advice so far. This is what I have learned so far. I bought a rebuilt MC from NAPA for an 82 gsl as Jake suggested. I do have the advantage of having quite a few used parts lying around to compare. All the MCs I have, including the one that was on the 79 bolt up at 6 & 12 and all the boosters have studs at 6, 12, 3 & 9 so they would accept either bolt pattern. When I mated the late MC with the early booster it appeared that the push rod was ½” too long and there was about 3/8” of adjustment left. After examining the late booster I found that the tip of the push rod (the part that goes into the MC) was shorter by about 3/8”. I installed the latter push rod tip into the 79 booster push rod and was able to get an acceptable adjustment.
I am still plumbing up car so while it looks right now I have not tested anything yet. I am removing the factory bias valve as I use a wildwood dial bias adjuster. I don’t like the fact that I am making more than one change in the system at a time but will report back if I have any more problems.
This BB is a great resource, thank you for all the help.

[CaptainWho]

You guys are just lucky you actually have an RX-7 master cylinder. When we had to replace the one in the #91 blue car last year, we discovered that it was not any master cylinder that Mazda ever shipped in an RX-7. We don't know what it was. Based on some web searching, it looks like it might have come from a REPU. Now we've got a stock '79-'80 unit but our installer had to rework some of the lines around it, to undo what a previous owner had done to install that orphan that was in it.

[tlyttle43]

Dave -

The rod I'm talking about is the little one that comes out of front of the booster and which actuates the MC. It is a threaded piece with knurled base for grabbing. The problem with adjusting them is that it's a fairly fine thread and Mazda apparently uses loc-tite on the threads to keep it in place, making turning the adjustable rod relative to the booster's internal rod a major hassle (my fix included two vice grips and lots of cursing!)

Another point that I haven't seen anyone mention is that I don't think it is possible to fit a later booster on the '79 car. I also tried that, and the bolt holes were okay, but when I tried to bolt it up, the body of the booster ran into the big body seam behind the booster and wouldn't tighten all the way down. So as far as I know, there is no way to use anything but a 79 booster on a 79 car.

Tom Lyttle