Landseer

Its basic, but are you sure the bleed is adequate?
I'm typically circling the car 3 to 4 times on a manual bleed.
All of a sudden they go from quite spongy to really firm.
Just be sure before you go deep into mods.

CFM928

Thanks for all the replies, following closely, very similar to my '80S.

I also replaced pads, bled 2X, sanded / de-glazed rotors. Confirmed no fluid in Booster. (It is the 10" booster, brakes worked well back in the day slowing down from the main straight at Lime Rock.) MC was rebuilt prior to car sitting.

Believe it is vacuum related. Warmer weather will get me under the hood soon.

Thread from 9.2017 before parking for winter:

https://rennlist.com/forums/928-foru...r-rebuild.html

Wisconsin Joe

Well, that depends on who you believe.

I had always thought the Comp Group package was only seats, steering wheel & spoilers.

Yet, I was reading the 82 Road & Track review of the car (it's in the "Gold Portfolio" book that Nicole sold). It says that the Comp Group has the cosmetic stuff, plus Sport Shocks. No other suspension stuff mentioned. No mention of brakes. But it clearly says "Sport Shocks."

I would tend to trust this, although I freely admit that they could have made a mistake.

hacker-pschorr

I have some period publications that say wildly different things about the comp package. My 81 has a separate option code for the sport suspension & I've seen other cars with every "known" tale tail sign of that package without the sport suspension option code listed.

It's very possible they were testing a comp car and were told it has sport suspension & they simply merged it together.... I have that issue somewhere too. Or maybe they all had the sport shocks and the "Sport" option on my car is the springs....it does ride like a dump truck compared to my other cars.

I have a "Porsche Authorized" book from the 80's which claims the Comp package was Porsche's way of sneaking in the EuroS to the US market, complete with engine, LSD, brakes......

I dunno...Porsche kept abysmal records of this stuff too, just look at how many COA's come back with completely false information.


Back to the topic at hand, the OP's issue is not lack of S brakes like previously suggested........

dbone996

79 928 weak brakes
Okay, so I did the vacuum tests I was asked to do. I read 21 with booster connected and 21 without (hose end plugged ) but when the brake is applied it falls and holds at 11. Also when the brake is applied I can hear a steady faint hiss from where the brake booster rod travels through the firewall and the Engine Idle changes slightly. Test performed with engine warm.

Another test I did was was to start engine, apply brake then turn off engine. The brake pedal would push back immediately. So do I have a valve failing inside the booster so its not holding a vacuum when pressed?
I tested the check valve and it is good. I have the new booster to install when I hear back from this forum. Thanks

Mrmerlin

put in a new booster

Wachterot

Been lurking on Rennlist for quite a while (incredible knowledge and support!) and have a problem that's a mystery (at least to me!). I recently added a spectacular 1989 S4 with low mileage - about 23k. It is a beautiful automatic and compliments the 5 speed S4 I have now. I am reasonably inclined mechanically - I have classic minis that actually run - and am stumped by what I think is brake booster failure...

Today I drove about 50 miles in a mix of highway and traffic. On the way home, just entering a four way stop, the brake pedal went to the floor. (I admit I did notice the pedal today was just a bit north of the accelerator in height, but as its new to me, I was still considering whether that was a correct normal or not.) Reprising my autocross skills, I danced the car through traffic...people were not happy, I was relieved....

I limped the car home with a combination of downshifting and e-brake slowing procedures while avoiding traffic.

Once home, I tried pumping the pedal, examined the Master (full), looked for leaks, detached stuff and restarted the car a number of times with the pedal still going on to the floor...I note there is very minimal braking when the pedal is "on the firewall". (Documentation shows that all fluids were all replaced at about 18k...but some years ago....)

So I've been investigating R&I booster next steps and now some five hours later I get into the car and now the pedal is rock hard at normal level. I start the car and the pedal remains up....whisky tango!

So now I'm deciding should I just replace both the booster and MC - or since its now an "intermittent" issue, take it to my friendly P car service shop that has a Classic annex and a 928 guru and see what they think - assuming I remain "brakeable" and go for it...

Thoughts very welcome as to the core issue...the fact its "recharged" now is really puzzling me....

dbone996

Booster is probably fine, I would consider replacing the MC and flushing all the fluid out with a good bleed.

Mrmerlin

I would pull the MC out inspect the booster for fluid leaking into it,
if its dry then leave it be.

Order ,
a new ATE master cylinder,
a new brake pressure proportioning valve, this connects to the ABS unit.
some Dow Corning 111,
a liter of ATE gold fluid,
a set of SS flex lines,
a can of brake cleaner,
and some simple green.

pull the tank off the old MC and clean it with the brake cleaner ,
then the simple green and hot water, use a toothbrush on the tank.
Final rinse the tank with a few oz of alcohol; then blow dry with a hair drier,
this removes all the alcohol and water.

The DC111 will be used on the MC base gasket seal,
and smeared on the inlet ports where the tank grommets seat,
then on the tank grommets,
this will prevent water from damaging your new MC inlet ports.

install the MC , make sure the MC base gasket sealing area is scratch free on your booster.

once the MC is in then put the tank in.
Install the flex lines. at each brake.
NOTE spray some PB blaster on every B nut for the brake lines , including the ABS unit.
NOTE prior to bleeding make sure you have broken loose then snugged each B nut at the ABS unit,
and swapped in the new proportioning valve.

Bleeding instructions.

fill the MC with fluid, then crack the MC B nuts one at a time work from the booster to the tip.

This is a gravity bleed , or if you have a Motive hand pump tool that fits the tank add about 10PSI to the tank.

Open the B nuts one at a time till you see no more air bubbles.
once you get to the tip then open the bleeder port.

With this done,
get a helper then foot bleed the MC.
Start at the booster and work to the tip,
one B nut at a time,
press the pedal down then make sure the air is gone.

NOTE use lots of rags under the MC and ABS unit to catch the expelled fluid

Refill the tank, make sure the tank level stays at 1/3 or higher during the bleeding process.

Move to the ABS unit then do the foot bleed again,
this can take a few times till the air bubbles come out ,work slowly.

NOTE you will find air in the ABS unit, and you should be able to get it out.
take your time.

Then once the the ABS unit is done move on to the wheels.
Bleeding sequence is
LF,RF,LR,RR.
NOTE This is opposite what most bleeding procedures are,
but its outlined in the WSM to be done this way.

NOTE it can take a few rounds of the car to get the air out of the calipers.
Most important is to keep the tank level at 1/3 or higher, and you will use just about 1 liter of fluid

928SingaporeS2

Dear 928Gods. did the author of this thread managed to get the Hissing fixed?

GerritD

Hi ,
I have the same problem with my Euro 928S of 1980. So I have standard brake calipers at the rear and large S calipers at the front.
When braking suddenly, the car doesn't stop immediately by locking the wheels...especially at high speeds over 50 Mpgh
So I replaced my MC since it was leaking fluid inside my 10 inch brake booster.
With the new MC I do not experience any improvement.
So next thing I will do is replace my brakepads and sensors since the brakepads are about 12 years old. Could this be my problem.?
I recently bought TRW front brakepads and TEXTAR rear brakepads....are these OK for normal driving ? Hopefully these new brakepads will solve my problem.

I also tested my Brakebooster : with engine off, press 3 times on brakepedal untill it is firm, then with foot on brakepedal, start engine. Brakepedal goes down about 1 inch