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I have adjusted the master rod to the limit the wheels are not fully locking up, the front brakes still have a slight drag. This link confirms how little movement is required to close the pressure release holes maybe 3-mm.
Removed the calipers to fully check everything and found this!
When I converted to the big brakes I just slipped them in after the spindle replacement never experienced a problem.
I need to verify what rotors I have and if the pads are correct for this setup. It seems the pads are not correctly seating and a grove is worn in. What is everyone using or is this normal for an upgrade. Notice that some dampener pins are bent and one pulled off. I’m assuming this is my slight drag and ready to order new parts but not sure if even these pads are correct for this application, I could only read a few numbers off of the pad was 24340.
Assuming (which may be incorrect) that the green circled numbers indicate the manufactured date (week.year) that would correlate with Roger's statement.
Please note that I did not disagree with what Roger said...I just pointed out that my prior statement about having this part was true.
Times 4 pieces....
Of interest, these master cylinders have an "ATE Classic" sticker on the box, which in itself is interesting....did they make a special run, or did they put a sticker on these, because inventory was getting very low? (I suspect the later.)
I have/need/use a tremendous amount of 928 parts, which we use on a daily basis, just to be able to repair the 928's which show up, in my shop. Our "in shop" policy is really simple...if a part is missing or broken (and is available), we will always install that part. This, obviously, requires a huge inventory, of pieces.
Does that make me a hoarder?
Not hardly....I never refuse to install a part on a client's car, because it is the "last one". Nor do I charge a premium to a client who's car is in my shop, for that part.
That being said, if I get a feeling that something that certain pieces (which I depend on) are getting low on inventory or may disappear, I will increase my inventory...sometimes radically.
I very occasionally call 928 International to try to find a part that is NLA, but it is very rare for us to not have a mechanical part, that is available from Porsche or other sources, in inventory.
86
Unfortunately I’m back to locking brakes the fronts were dragging after the #3-rebuild. I did find several issues with the S4 mounting bracket’s but had them milled and everything bolts up and rotates add new pads and addressed the dampeners.
Now after the new pads all the brakes are locking up. I removed the pedal rod, unbolted the master and moved to make sure no preload was in it. What I observe is the pistons are not returning unless I open one bleed screw for the front and one for the rear. The front caliper bleed screw opened then both fronts work same with the rears one bleeder releases the rears. I have left a call with the rebuilder not sure what is causing this but ready to ask for a refund, Is there anything I’m missing? Might be time to start pricing out a replacement.
Well all I can offer is my way of doing it. I overhauled my brakes 2 years ago after my euro sitting for a long time. I bled the system a lot. I rebuilt the calipers with new rubber seal and rubber dust boots, new pads, new clips, cleaned everything, the right lube I think, NEW SS lines from the hard lines and a NEW MC and cleaned out reservoir. Basically almost all new cuz you never know what a weak link might be.
I believe the dimples should be in a certain spot. The rubber o-ring and dust boot in the calipers are important, the way you seal the reservoir into MC is important.
If you can afford and find a new MC that would be the way to go. Didn't they make a new batch?
Troubleshooting
Spoke to the rebuilder they offered to look at it again or refund my money basically the bench testing shows it good. The first two repair attempts the first time the spring was installed backwards second time Main retaining C clip popped off opening up the master spilling out the piston. . After the third repair master is now locking up all the brakes, opening one bleeder valve on the front or rear calipers releasing the pressure, brakes free up. SS hoses are good caliper’s have been bleed , new pads installed.
The Abs is deactivated, removed and checked proportioning valve located on Abs for blockages. It just seems the fluid pressure is not releasing back into the reservoir. Before I order a 86 rebuild assuming this is the issue is there anything I’m missing?
In the 86 supplement information manual it addresses a change in the Master for the 4-piston brakes. Described as a stepped tandem with two central valves used for 4-piston brakes. The newer S-4 Master 86.5 and above vin 861000, “mine is 870766” seems to be 928-355-011-25 but is not listed in the main manual as compatible.
The S-4 Master seems to be compatible due to the remote proportioning valve location but have not turned up any research of the differences besides the actual internal valve. If the supplement manual addresses this change,” no part number is shown”, is it not just referring to the new S-4 Master?
Is it possible to use the S-4 861000 and above master 928-255-011-25 if not why, SEEMS LIKE CONFLICTING INFORMATION. I’ve run out of things to research and test and assumptions are the rebuilt is good then I’m at a loss how to proceed. If I order an 86 master is seems like it’s requiring a different one due to the S-4 conversion, I would like to replace it with the correct part that was available the year 86.5 with the S-4 brakes.
Per brake rebuild or I should have In-N-Out fluid with bench testing, I actually hooked it up while installed and here are the results. Looks like it’s working as designed. Any feedback on this or direction to take before they start replacing and ordering parts?
I'm in the same boat here. Restoring my 1986.5 928S after a decade+ sitting in a barn.
I rebuilt the brakes with new seals and dust boots (not new pistons or clips). New rotors and pads. I replaced original rubber brake lines with steel braided, and since the flare nuts were so rusted, I replaced most of the hard brake lines. I got a reconditioned brake booster from 928Intl and had my MC rebuilt at White Post.
Car works great for the first 10-15 minutes with rear brake discs cool. I think that as the the fronts start warning up, something happens and the whole system drags and all four brakes come on hard with all discs hot. Everything frees up when cool. I've ensured the proper distance from booster to pedal. I've shaved down the brake pads to ensure there's enough play with the existing clips in the calipers.
ABS light would come on once I start driving, and I removed relay and fuse to isolate problem - no change.
I reached out to the MC rebuilder this week for next steps. I'll be following this post closely!
I'm in the same boat here. Restoring my 1986.5 928S after a decade+ sitting in a barn.
I rebuilt the brakes with new seals and dust boots (not new pistons or clips). New rotors and pads. I replaced original rubber brake lines with steel braided, and since the flare nuts were so rusted, I replaced most of the hard brake lines. I got a reconditioned brake booster from 928Intl and had my MC rebuilt at White Post.
Car works great for the first 10-15 minutes with rear brake discs cool. I think that as the the fronts start warning up, something happens and the whole system drags and all four brakes come on hard with all discs hot. Everything frees up when cool. I've ensured the proper distance from booster to pedal. I've shaved down the brake pads to ensure there's enough play with the existing clips in the calipers.
ABS light would come on once I start driving, and I removed relay and fuse to isolate problem - no change.
I reached out to the MC rebuilder this week for next steps. I'll be following this post closely!
Kevin
@Tampa 928s What's the free play in the MC push rod? I'm asking becasue I've had a similar experience with an OT car that was never releasing the brakes enough to allow the MC to return to the rest position and it only happened when warm so it took me awhile to figure out. Caliper can only return fluid when the MC piston is fully retracted and in it's rest position. I believe the 928 has an adjustible push-rod on the brake pedal so ensure you have free play and the MC is fully at resting position.
If it is adjusted OK, I'd start looking into how the MC interfaces with the Vacuum boooster and ensure there is nothing keeping the MC piston from returning to the Zero or at rest position.
I wouldn't worry about the S vs S4 MC at the moment. I upgraded from S to S4 suspension and kept the ealy MC and the brakes feel great.
Last edited by icsamerica; 02-21-2023 at 02:22 PM.
The rod adjustment is 158mm +/- 2mm, I unloaded the rod to make sure it was not preloaded and even removed the Master nuts to release the pressures, still no resolution.
The rod adjustment is 158mm +/- 2mm, I unloaded the rod to make sure it was not preloaded and even removed the Master nuts to release the pressures, still no resolution.
That's what I would have done , loosen the nuts on the master and see if it resolves.
Do the same with the line at the master... Conform it resolves when you crack the line at the master and if so then just get a new master and be done with it.
I'm not sure you can see this on the 928 mc but if it's working properly you should get a little squirt out of the bore fill hole right before the Piston seal covers it. That would be a good indicator if there were some sort of blockage preventing fluid pushback. Just an idea and perhaps a second way to confirm a problem
Last edited by icsamerica; 02-21-2023 at 04:58 PM.
Assuming (which may be incorrect) that the green circled numbers indicate the manufactured date (week.year) that would correlate with Roger's statement.
Well, you got me to go look. and take a picture.
Certainly not a big deal, actually fairly typical.
But someone's clearly wrong., since Porsche sent this to me 6 entire years, after Roger said they were NLA. Master cylinder picking tag
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
Today I removed the master cylinder and found that there's about 1 mm of push between it and the brake booster (see pic). This means that the MC is not getting fully into rest position. So likely with a bit of heat expansion the problem occurs. But the pin on the brake booster protrudes just a tiny amount - the same amount that my old brake booster's pin protrudes that's not in the car. So it's not the brake pedal extending the push rod (plus, I checked the 158 mm spec.). The folks who did the MC rebuild say they use the same snap ring, so that distance couldn't be shorter.
So I can:
Shave down the booster pin. I don't know how I'll get in there to do so and I don't like grinding something down that's expensive. Plus it's nice and rounded now.
Drill out the master cylinder receiver. Same concerns as above.
Make a gasket between the two with 2mm rubber.
Your ideas??
When I did all this the first time getting the 13 mm inner bolt between the Bb and the MC was easy because the engine was out. This is NOT easy with the engine in.
01-15-2023, 04:35 PM
Update
