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Registered User
David's Avatar
Join Date: Apr 2002
Location: Houston (Clearlake), TX
Posts: 9,224
Brake booster problem?

The brakes on my '79 930 have been weak as long as I've had it. Years ago I rebuilt the calipers, flushed the fluid and installed new Ferodo pads. They still sucked. A couple weeks ago I flushed the fluid and installed Porterfield R4S pads and they still don't stop very well. The pedal isn't really mushy but it goes down pretty far before it grabs and you have to stand on it to stop at speed.

I unplugged the vacuum line and it didn't seem to make any difference in the brake feel so I'm wondering if the booster is bad. Any suggestions on how to test the booster?

I also looked up the booster part number on Pelican and they don't list one. Any idea where to get one?

2014 Cayman S
2011 Cayenne Turbo
1979 930 (475 rwhp at 0.95 bar)
Old 03-27-2018, 09:50 AM
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Registered User
Join Date: Sep 2002
Location: Westchester County, NY
Posts: 1,915
Classic test for vacuum boosters is: car off, pump the brakes... hold down pedal while starting car... if pedal sinks further, booster is ok. I imagine it should be the same for our 930s.
1986 930, 1969 Mach1, 2003 540 Sport, 2016 R1200RS
Old 03-27-2018, 10:03 AM
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Registered User
T77911S's Avatar
Join Date: Oct 2006
Location: MYR S.C.
Posts: 13,535
you can also remove the vacuum hose on the side of the intake. pull a vacuum on it and see if it holds.
this is a GREAT test because it also tests the hose or vacuum leaks

you can buy a hand pump for about $30-40 at HF.

I always thought the brake pedal got hard when the booster did not work.

I was not happy with my 930 brakes when I got my car. I thought my 77s was much better.
brake fade was bad, poor stopping power.

low pedal could be air in the lines which means bleeding. -start here,. I would do a gravity bleed.
open the nipples and just let them drain.
you could also have pistons that are sticking in the calipers,
bad master cylinder

I had issues with the calipers. in the end I ended up putting grease on the inside seals so the pistons would all pop out at the same time.
if you remove a caliper, put a 2x4 in place where the rotor goes. apply compressed air, does not take much if they work properly, and all 4 should pop out at the same time.

I used cool carbon this time.
86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
88 BMW 325is 200K+ SOLD
05 BMW 330CI 130K:: [__] RUNNING: [__] NOT RUNNING:
08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
Old 03-27-2018, 10:54 AM
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Turbo Powered Snow Blower
Join Date: Mar 2016
Location: Rocklin, CA .... Galveston, TX
Posts: 3,811
A soft pedal is usually air in the system or a bad master.
It's seldom a bad booster. Unless there is something different on the 930 I'm not familiar with.
I restored the brakes on my car last winter and it took three rounds of bleeding to get all the air pockets out. Right front was the one that had the largest stuck air pocket. I even drove the car a few hundred miles before re-bleeding them. The brakes felt ok, but firm engagement was low. Probably 3-4" of pedal travel. After I got all the air out there is about 1" of travel.


Last edited by 1979-930; 03-27-2018 at 02:18 PM..
Old 03-27-2018, 01:19 PM
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