I've only driven one 928, and that's my 84. I've owned a 951 and a Boxster, as well as a few other sports/touring cars. I have to say (sadly) that my 928 has the worst brakes of any of them. It requires pretty significant pedal pressure and just doesn't feel like what I expected. My 951 had the brakes that were larger than what was on the 944, and the stopping was unbelievably abrupt, with little pedal pressure.

It makes me wonder if something isn't dialed in. I don't feel air in the lines, but maybe I have terrible pads, or the boost isn't working right?

My question....are the brakes just not really that good on an 84, or is it likely that I have a problem?

 

You have a problem

 

Start with the basic maintenance items like a fluid flush and bleed if the fluid in there now is more than a year old.

Next will be your choices in pads and to a certain extent the rotors. Others will jump in on pad choices. Make sure you have resurfaced or new rotors on there and you'll have a good start.

While you are there, it's undoubtedly time to replace any original brake hoses. Original is a good option. Many like me have changed to Goodridge replacement hoses. Regardless of which hose you choose, you will improve the reliability of the hoses. The inner plastic tube in the original hoses has gotten tired with age and exposure. The tube cracks and may fracture, causing some interesting brake symptoms like locked calipers and eventually loss of brakes. Many find that the Goodridge lines are "tighter" and offer a firmer pedal. Since most of us do the whole refurb project at once, it's tough to say that the hoses are alone responsible for the firmer pedal feel.

Anyway, some careful steps and pad selection will do a lot for you. Short of upgrading to the equivalent of your 951 brakes (same as the 928 S4), these will do a lot for the way the brakes feel.

 

If the <86 928's use the same brakes as my 944S I can say they're not quite as good as the >86 brakes (which are the same as the 951? Hm...). The 944S slows down nicely but does require a fair bit of pressure. The 928S3 stops like you tossed an anchor out the back, despite being a heavier car.

 

+1

 

ok....thanks for the info.

i'm guessing i have a leak in the booster. it coincides with serious no start problems. can a leaking booster drain enough vacuum to cause problems with fuel regulator, spark advance, etc?

 

That's a big vacuum line from the intake straight to the booster. I would say yes.

 

Hey John,
I know that you know your stuff. The last time I heard the car running, I could swear I heard air on the drivers side and at first I thought it was leaking at the intake manifold....sort of dead center of that side. Is the line that runs from the manifold to the booster originating in that area?

This has been a full blown drama. Symptoms changing. Breaking things while fixing other things. All good fun!

 

The entire system is in a very different condition from when it left the factory.

New flexible hoses and complete flush/bleed is fine advice. I would add caliper refurb - replacing all piston seals and dust seals, and cleaning/replacing pistons as necessary. You can’t really compare brake performance across cars as if it’s an inherent thing - theircondition will be very different unless all recently refurbed and rebuilt. Oh and new pads and discs of course!

 

I have an '83, virtually identical to your '84. To pile on in answer to your original question, it definitely sounds like you have some issues. You haven't told us much history, but if the car has not had a top end refresh, that may be a good place to start. That involves replacing all the plastic vacuum lines and rubber connectors under the hood, at a minimum, and also inspecting/testing and fixing any other possible sources of vacuum leaks (of which a partial list under the hood includes injector seals, intake boots, intake seals, the fuel pressure regulators, and the big O-ring at the AFM). You'll also want to test the vacuum system, which is all run off of the venturi at the top of the intake manifold.

First thing to do to test the vacuum system is purchase a Mityvac or similar vacuum testing pump and start testing. Please see Dr. Bob's most excellent description of the vacuum system here describing the THREE different vacuum-related systems: https://rennlist.com/forums/928-foru...l#post15974757.

To test the vacuum system, you'll want to isolate each of those three systems, in turn, and use your Mityvac to pull some amount of vacuum on them and see if they hold vacuum. If not, then you figure out where the leak is. This test will also give you an indication of the magnitude of the leak (how fast the pressure returns to atmospheric).

Once you have an airtight intake and vacuum system, see what problems remain and start tackling those.

I did a top end refresh on mine a few years ago, and also needed to replace several of the vacuum diaphragms inside/around the dash (these open and close the vents, among other things). There are a bunch of writeups on how to access the HVAC control in the dash and then test each vacuum line there to debug any vacuum leaks. More recently I had the injectors out for professional cleaning (along with a bunch of other stuff, including replacing all the flex fuel lines, and of course the injector seals and connecting hoses). If you're not familiar with the injector seals, there are some pics here: https://rennlist.com/forums/928-foru...l#post15611957. While you may not need to get into any of that just yet, keep that in mind for a project after you fix the bugs -- if you still have original flex fuel lines, changing them out is mandatory and doing an injector refresh as part of that job is a good idea.

Hope this helps.

 

rjtw, i have been pulling vacuum all over the place. and cleaning grounds.

i bought the car a year ago and drove it from idaho to dallas. it did ok. i parked it for a year. in the woods. it's amazing how much a car can age in one year around cedar trees. anyway, i'm sorting out multiple issues, and the priority is a "won't run" problem. i was able to get it to crank a few times and took it around the block and it felt like almost no power assist at times. on occasion i could feel the boost. it didn't occur to me until a couple days ago that the brake issue could be related to a vacuum issue which could also be causing my apparent fuel issue...

thanks for the link!!!

 

There's a saying "relays-relays-relays!" about what to check first when there's an electrical problem on our cars, and really for any European cars of similar vintage with Bosch electronics. I'm going to add "AFM O-ring" as the first place to start when you have poor running and symptoms of vacuum leaks on an L-jet car. It's a big *** O-ring and if the rubber is even slightly old,shrunken or dried then you get a massive air leak. For under $10 I'm going to say replace it as a matter of course. I replace mine every year or two just as a precaution! Also buy a tube of Dow Corning 111 Molykote to lube up the ring. You'll also want that to coat every rubber connector and other rubber component as you do the top end refresh.

 

i swapped the injector relay for a new one. jumpered fuel pump relay. tested at the rail with "thumb over the test port" method. i could feel pretty good pressure when the key was turned.

i actually started at the fuel strainer....squeaky clean. i blew backwards through the fuel filter and i didn't feel any significant resistance and it didn't blow and silt out.

pulled vacuum on the fuel system components individually....all good.

i have a "partially" removed smog pump.....the belt is off, and a vacuum line to whatever the vacuum actuated valve that's near the coolant take is disconnected and the line that was connected to it is blocked off. There is a hose that comes from that valve that is hanging open....I have no clue what to do with it.

i opened up the mass air flow sensor and bent the wipers so that the contact doesn't ride in the same track that has been created by wear. i did notice cuts in all of the wires about 1 inch from the plug.....only the insulation was cut, so i wrapped tape on them.

plugs were totally black....i didn't replace them, i just brushed them, soaked them, and sprayed contact cleaner inside to clean them up. it didn't make any difference. and, i assume the black came on when the fuel supply system went south....

 

You can eliminate the possibility of a brake booster leak affecting starting by removing and plugging the hose to the booster. Car start OK? If so you've quickly answered that question.

A simple brake booster leak test is to start the engine and test the brake pedal. Wait a few minutes. Turn the engine off. Wait a few minutes. Then test the pedal. The first two presses should still have vacuum assist. By the third and fourth pedal press, the pedal feel will be harder as vacuum is depleted. The vacuum should remain for a long time if you don't touch the pedal. Hours or days. If it's gone in less than ten minutes for the sake of this discussion, the leak needs to be fixed.

 

hang on a second....let me process that....the hose to the booster, which creates vacuum in the booster....remove that from the booster, and plug the hose?

seems that would leave an open hole in the booster, and no vacuum in the lines going to the dampers/regulators????


I'm not doubting your expertise....just trying to understand. my initial thought was to connect the hose from manifold to booster....to the small line that goes to the dampers/regulators. bypassing booster.....


i cant really test the brake vacuum assist by that method because i can't get the car to start....