Gunter

Are the brakes still vacuum assist on a 993 with a Servo unit somewhere? Streather’s book shows a vacuum Servo on page 275-276. But then goes on to talk about a hydraulic brake booster and a hydraulic pressure accumulator on page 277.

So, is the power assist hydraulic or vacuum? If vacuum, where is the Servo unit?
If hydraulic, where is the pump and what actuates the pump? The moment the key is turned or when the pedal is pressed actuating a switch on the pedal?
Is hydraulic power application a different unit or is it the ABS unit the power source? Scavenging pump is where? Accumulator is where?

Page 282 in Streather’s book shows a picture of an ABS hydraulic unit. Is this the simple 3-channel version?
Page 287 shows an ABS-ABD unit with the heading: Differences between 3-channel ABS and 4-channel ABS-ABD. The picture itself says: Dual rear brake circuit bias regulators.

This is the same unit as in my ’95 C2. What do I have? Just ABS or ABS-ABD?
My unit is Bosch 0 265 217 001 Porsche 993 355 755 50.

A simple explanation of how the brakes work and what components are involved is very much appreciated.

Thanks.

Stealth 993

C2's have Vacuum assist.
C4's & TT have a brake pump.
The pump is in the front end near the gas tank.

When you start the car, the red brake light stays on until the pump gets the system up to pressure.

ABD was a option, you will have the code for it under your hood. ABD just applied the rear brakes to stop wheel spin. Only works up to 40mph. If you do have it, you will have the 2 ABD lights on the oil gauge.

Gunter

Thanks, I'll check under the hood for the ABD code and see if there are 2 ABD lights in the oil gauge.

The usual location for the vacuum Servo is in line with the Master brake cylinder on older 911's and most cars.
I looked in the trunk around the area where one might find it but there is nothing visible.

Where is the MC and vacuum Servo located in the 993?

JM993

Under the car near the steering rack.

Cheers,
Joe

Bill Verburg

the vacuum booster and m/c are mounted transversely just behind the steering rack



M220 was ABD/LSD option, you can tell it's there from the option list or just look at the ABS pump, if there are 2 rear lines w/ p/v then it's M220 equipped. Besides ABD/LSD M220 includes 4 channel ABS, others have 3 channel
ABS pump, right side of trunk when facing rear, rear lines are the rear most off the outboard side, p/v are the round cannisters

Gunter

Thank you, Bill, no wonder I couldn't find the MC and vac Servo.

Here is a picture of my pump and it looks the same as yours.
What is: lines with p/v? Pressure valves?

Does the ABS pump run with ignition ON, or only with engine running?
Does the pump run all the time?
When running, does the ABS pump develope a certain pressure ready to be applied when needed?
Is that pressure regulated by the accumulator?
Is the scavenging pump an internal function of the ABS pump?
Or a separate unit?

Bill Verburg

yes, you have 2x pressure reducing valves on the rear circuit, Yes you have M220 w/ 4 channel ABS, ABD & LSD

It's got power but isn't doing anything untill the speed sensors on the wheels report in, You can cycle it w/ a "Hammer"



yes & no, it has power while the key is in run but isn't doing anything

I assume so, but don't really know. All the diagnostics including bleeding w ABS5/ABD are doe w/ a "Hammer", it does all the works and reports the status of the system. All you can do is change components.

Gunter

So, with ABS/ABD I cannot flush/bleed the brakes without this Bosch Hammer?

No refreshing brake fluid or bleeding with either a power bleeder or the conventional pumping the pedal?

e9stibi

You can but you will not be able to get all the old fluid out without a Hammer. Some people told me that as a band aid, you can turn on and off the ignition while bleeding which moves the pump a little bit. I am not sure how much old fluid will be left in the system but you should get the vast majority that is in the lines, calipers and the reservoir.

Stealth 993

You can, & you are suppose to "release" the pressure before you bleed them. If you turn the car on, the pump will turn on, & move the old fluid.

If you have the Durametric software, you can cycle the ABS pump with it.

I've done both my C4 & C4S, really it's the same bleeding as any other 993.

Gunter

For me, the nearest service facility is 4 hours driving, one way, so, my aim is for DIY.

I am mostly conerned about damaging the ABS pump by changing brake fluid the conventional way either with a pressure bleeder or pumping the pedal.

Since I don't have the equipment to cycle the pump and am not familiar with the procedure I'll stick with just a power bleeder and the convential way.

With the power bleeder connected to the reservoir, I turn just the ignition ON/OFF a few times during the bleeding process?

Engine doesn't have to run?

mcipseric

What brake system did the RS come with?

Stealth 993

Not sure I would turn the pump on with the bleeders open, & pressure put into the system (pressure bleeder). I think it may gush fluid out. But who who knows it might be worth a try?

I change it the normal way, then turn the car on, let the brake light go out, then shut off the car & bleed again.

Gunter

Don't know but should be in Streather's book.

Mine is a '95 C2 with ABS/ABD/LSD code 220

Bill Verburg

You can bleed it the conventional way but you don't get all the fluid out unless you use a "Hammer" or Durametric or such. If the last time it was changed fully is unknown it's worth it to go to a shop w/ the tools