Edwardoo928s

Hello friends

I am trying to undo a modification done by a previous owner that drives me nuts.
These pictures are from my 1985 manual 928, I do have the same car with an automatic and it's bone stock.
I know it came originally with single piston calipers, I'm assuming a big brake 4 piston was installed at some point. I want to put smaller wheels on the front like 16' or 17' but these suckers barely fit the 17'.
You can see the rusty mount to put the later calipers on, while still retaining the stock spindle. The rotors are from a 993 911 turbo and the dust shield is way to big.
Ok this set up never really worked well, i have to stand on the brakes and I can't lock up the wheels, (no anti-lock). I bought a new power brake booster and master cylinder from a 1987 thinking the volume wasn't the same as a
factory single piston master. I have yet to install it. Dose it need a larger master cylinder?
The big calipers don't fit on top of the brake pads like I would think, some material is not in contact with the rotor on the very top
I would like a normal 4 piston brake system from an 86 for my 85. How big a deal is it to convert it?








Don't know about this conversion.

The Forgotten On

Those are GTS calipers if you are running a 911 turbo rotor.

You can get 993 C2 or 928 S4 calipers and run a base 993 rotor to get it to fit under 16s no problem. It's just a straight across swap.

IIRC the piston size in 928 master cylinders never changed.

I stuck 993 calipers on all 4 corners of my 81 (rear 993 calipers have 20% larger pistons than S4/GTS) and it had great brake feel with the original sized master.

I think your original master might be worn out (seals are past it) and either needs to be rebuilt or replaced.

Petza914

I have Big Blacks on my 79 convertible with 993TT rotors and the braking is fantastic. I don't think the caliper and rotor setup is your problem and it's either the master or booster or old closed up rubber brake lines if they're still on there.

My 928s run 18" wheels so no clearance issues and I understand you're wanting to downsize the brakes to be able to downsize the wheels. Those calipers on your car are expensive and someone will buy them for good money once you remove them

Edwardoo928s

That's what the box said as I replaced them 2 years ago, 911 993 turbo rotors, got them from Roger. S4 calipers and stock 993 rotors sounds like it could work. Can somebody show me how it would look and or perform.
I also think the pads probably got overheated because I tried to brake them in and the master locked up. I had to pull over and cool them off before I could get home. probably happened 3 or 4 times.
Think it was a street/track pad, I'll go back to street pads only. I did track it once but I'm not going to anymore.
I will replace the power brake booster this month, I have the interior completely out for a new Breman radio and amplifier. The rubber lines have been replaced with new braided lines, also the master is new, so I hope to figure this out soon.

Great advice, I'll see if anyone can sell me S4 brake calipers, not easy to find. These might be for sale if it works better.

Edwardoo928s

Hey Fellas,

Please look and comment on my findings.

I'm having brake problems with this old brake mod. (previous owner)
My 928 is from 1985, it has these 4 piston brake calipers as shown above. Using the side block to move the mount above the normal 2 piston spindle. I've had master cylinder problems like not releasing fast enough and not much help showing the car down.
I make this observation today that the new 85 master I removed and a new S4 master has a different and longer slit. I was betting it would be but can a GTS master be even larger providing more brake fluid to the front.
Pictures
Am I going crazy here I need some help

Edward

PS: still has the 1985 Suspension not S4

Smaller factory 1985

longer S4

Alan 91 C2

Hi Edward,
The braking is a function of clamping pressure to the pads. The slit in the MC, has no role in the problem you have.
The clamping force is determined by the size of the MC piston area, and the combined Wheel cylinder area, per caliper.
Read the following for the math behind braking.

https://www.brakeandfrontend.com/bra...to-stop-a-car/

Edwardoo928s

Hello there,
I see your point, I think I have a solution if for some reason the MC is the same through out all the years (physically) , not including the piston size, I need to find out what is the difference in the piston size in according to the difference years.
It's data that I don't know where to find. Assuming the BB is also some what similar.
I've changed the idle solenoid because of a high idle (1000 rpm), something I've read about on RL. Thinking the vacuum was a problem with braking.
Before, it would stall if you hit the brakes, idle was very low and not steady. MAS was calibrated and seems to be ok.
The LH computer was rebuilt.
I'm in the process of replacing the engine mounts and oil pan gasket right now and changing out the headers to factory S4 type.
Remember, I might have forgotten to say, my 928s is from 1985 and the engine and transmission is from 1987. Many parts included is this swap, which was how I bought it.
I have a factory 1985 engine in my parts car but I'd rather keep it this way, I have way more money in it than it's worth, even as a 5-speed.
I will never sell it.

86.5 is an S4, I plan on replacing the 85 suspension with S4 upper and lower control arms with the spindle of an S4 + MC which I have now. It's not a big headache.
I have OCD with this car, I think most people wouldn't even bother changing everything.
Wife supports me as I do this but a new 911 would be a better car in her estimation . I keep torturing myself.

Tampa 928s

I have an 86 with the same set up I had to mill the adaptor down about 3mm so the pads would ride even with the rotor. I had braking issues there is a post on it ended up with 87 master.