GregBBRD

I thought that the master cylinder repair kit (928 355 913 02) was for an early master cylinder, but it's been so long since I used one, I couldn't remember.
So, I pulled one off of one of my shelves and opened it up.
This confused me more.

The bore sizes in an early master cylinder are 26mm and 23mm.
A late master cylinder has bore sizes of 23mm and 20mm.

Both of the replacement pistons in the repair kit are 23.7mm, with piston seals that are 24,8mm. There is a separate piston seal which is 27.8mm and another which is 24.8mm.

I don't see that any of the parts are going to be useful in a late master cylinder...
And can't figure out how the pistons are going to work in an early master cylinder.

I guess I'm going to have to walk over to my Box-O-Old Master cylinders and take both an early and a late apart, to figure out what this kit fits/does.

FredR

Some interesting data there. I was a bit surprised to see that there are two bore sizes- I figured that given there is a separate bias valve that will redistribute the braking force as Porsche so desire there would be a constant diameter bore but maybe the bias valve is more of a triming device?

The other "surprise" was that the later MC's have a smaller bore when logic suggests [given they have more powerful brakes] that the bores might be larger but then maybe they have more powerful servos.

Bottom line if you cannot fathom out these kits what chance anyone else?

With these type of seals the psiton is not going to be particularly tight in the bores and presumably relies on compression of the seals between the piston and the bore to function correctly. Does not take a doctorate in mechanical engineering to fathom out that if the piston diameter is greater than the bore then it is not going to work! Clearly there is an information gap somewhere along the line whatever it may be.

Regarding my original premise- clearly there is nothing particularly special about the master cylinder- whether they are a common spec or a one off with dimensions like no other remains to be seen but the bore size must be considered critical to the succes of the installation. For sure it would be interesting to know how those bore dimensions change model to model- the 993 is lighter than the 928 S4/GTS models hence differences in MC spec. .

Adk46

I innocently inquired of the assembled experts at Frenzy a few years ago about the long travel of my brake pedal. I did the easy thing - new flexible lines to the wheels to fix possible 'aortic dissection' - but that didn't help. The next step would have been to replace the master cylinder, but I've procrastinated. I was unaware of the issues you're raising, so I think I will procrastinate some more. Thanks!

GregBBRD

The "early" Ate master cylinder has a "23" and a "26" cast into the side of it.
Having disassembled one, last night, I now know that those numbers do not indicate the internal size of the bore, which is 24mm all the way through (except in the very front, which is 28mm.)
Perhaps those numbers are casting numbers indicating how small or how big the bore can be guaranteed to be made and still have perfect walls?

At any rate, the 928 355 913 02 repair kit definitely is the perfect fitment for the "early" ATE master cylinders....every part, in that kit, is inside that early master and correct.
So, if you've got a '78 to '83 original Ate master cylinder that has a perfect bore, you can buy that kit from Porsche and rebuild your own master cylinder, and save a couple hundred dollars.,

I'll see if I have time, this evening, to disassemble a "late" master cylinder and see if that kit is appropriate for use there, also. (I'm doubtful.)

FredR

More interesting info- many thanks for taking the interest, time and effort. The only snippet of info I managed to find is that much to my surprise master cylinders used in ABS systems do have some slight differences although what they are and why I have yet to resolve. Maybe it has something to do with comments in other threads about having to bleed the MC in dry installations. Never understood this as I assumed they would be self venting back to the reservoir but maybe not if there are valves of some kind in the inlet ports which appears might be the case.

It will certainly be interesting to learn if there are any differences in the bore of the later model MC's compared to the earlier ATE items..

Mark Anderson

As an added data point we have removed masters with the following casting numbers
23-19
23-20
23-23
17-23
28-23
24-23

ramcram

Try getting a RHD mc! My last 2 mc overhauls were both pitted, common with cast iron and a water absorbent liquid.
So both had to be resleeved and I think, by memory, they use SS sleeves? About AU$300 each for he whole job and they work fine.