papatuki

928. 1984 auto. About 70% of the time i have to lightly one pump the brake pedal to get it firm. Rest of the time its fine. Gets quite soft if light pressure is held on the pedal. No fluid loss..Bled multiple times. Any advice?
Wait. Jumped the gun. Finally found some useful info in my forum search. Think i'll bleed again.

Mrmerlin

from your info I would suggest to get a
new ATE booster
a new ATE master cylinder
and 4 flexible SS brake lines,
and a liter ATE gold brake fluid
If your rotors are rusted then replace them all 4 with fresh Zimmerman Z coat rotors,
and Hawk or EBC redstuff pads
all in your about 900.00 for a new set of brakes that wont ever be a question for you.

Or keep wasting time with a few bleed cycles and floaty pedal... NOT

NOTE I rebuild enough of these cars now that I wont waste my time or my customers money screwing with questionable original brake components.
If you have the time to do so,
then go ahead.
The info I provide is so you can fix the brakes and have them work perfectly for the next 15 years or more

rjtw

Just out of curiosity, are the ATE and Zimmerman components OEM or aftermarket? And these are specifically developed for the 928, drop in replacements? Just curious as I don't think I've seen this discussed on the list before, or at least missed it. My '83 is doing fine but one day when the time comes for a new booster or master cylinder, I'd like to save this information.

Mrmerlin

ATE still makes original spec master cylinders and new boosters,
along with the fluid to work in these components.

The Zimmerman rotors are drop in and they will last longer compared to uncoated rotors ,
as the inner vanes are also coated with zinc the anti corrosion coating.
NOTE most 928s will spend a lot of time parked since most are not used for DD service,
and some will be exposed to the elements the Z coated rotors will resist this damage for a longer period.
That said the swept area will rust if its exposed to moisture just like other rotors will

rjtw

Thank you, Stan!!!

dzaprev

Hey , did you free up your calipers pins. Sounds like you could have sticking caliper or pin

dr bob

Let us know what works for you. Your symptoms (and Stan's recommendations) seem to show up more and more these days.

I went through a similar episode a bunch of years ago, and spent a bunch of a day vacuum bleeding and reverse bleeding and more, before I finally managed to get the pedal to "rest" near the top and have the nice firm feel that the car came with. I suspect that I had air trapped in the ABS pump, something that isn't applicable to your car. But the system doesn't know location, it just knows there's air where there should be liquid.

papatuki

Thank you Stan. I replaced the rotors with Zimmerman OEM and Mintex Street pads and SS brake lines. Using ATE Super Blue fluid. All of this about 3000 miles ago trying to solve this very problem. Always garaged. Looks like the booster and master cylinder are next. I do find parts hunting tedious though.
Dave.

BRB-83-911SC

http://www.928srus.com/
http://www.928intl.com/
https://www.928gt.com/

Any one of these 3 suppliers can get you just about anything you could ever need for your 928. It's best to use a 928 specific parts supplier to ensure you get the right parts for whatever project you have underway - they are all 928 enthusiasts who know these cars. Also, it is best to phone or e-mail as opposed to depend on the website for parts availability. Dialogue with these suppliers can also result in options you hadn't considered, and advise on what you do and don't need as part of the project.

papatuki

Thanks. I'll keep ya'all posted.
Dave

Mrmerlin

The funny thing is when you install brand new brake parts,
they usually work perfectly,
so you dont have to do much tail chasing before the brakes work great

Otto Mechanic

Far be it from me to even suggest an improvement or clarification to Stan's recommendation, but I personally favor the Hawk Performance HP+ pads. I've had very good to excellent results on the street and even on the track. They may make some noise, but only my wife seems able to hear it.

SS brake lines will help if you have heat problems. I use Goodridge, they're DOT approved.

A power bleeder helps getting air out and I recommend the Motive.

I use ATE fluid exclusively.

PS: Zimmerman (as Stan suggests) slotted front rotors. I don't favor drilled since they'll crack and come apart sort of violently if you don't watch them like a hawk. If you're racing, or very vigilant, they're fine, but you can't get by with just installing them and forgetting. The slotted rotors work very well even with extreme use and they don't detonate on you without warning.

Otto Mechanic

Dave, where did you find Super Blue if you don't mind me asking? I haven't been able to get it for a few years now.

dr bob

Discussion is swerving away from the hydraulic problem the OP is chasing. Stay focused on that if you can please.

The "Gets quite soft if light pressure is held on the pedal" has me curious. The booster has maybe three good stops worth of vacuum in the can after engine stop. Is the soft pedal after that stored energy is gone, Try pushing the pedal everal times until it's no longer getting any assist, and try the "light pressure" again to see if the "gets quite soft" continues. If it does, the problem is almost undoubtedly the MC.

Otto Mechanic

It would help to know if the OP is experiencing the problem at all times in all circumstances. Does the problem occur when the brakes are cold? Is it continuous? That suggests air in the lines to me, leading to my suggestion of the Motive power bleeder. Maybe bleeding the hydraulics "yet again" is banal, but it may well be more effective and cheaper than replacing the master cylinder? Let's also consider that replacing the master will involve bleeding the entire system yet again?

I've found bleeding the hydraulics to be a repetitive and challenging process.

PS: I personally miss ATE Super Blue and would like to find a source for it.