M. Schneider

Are the 993s brake master cylinder's, (the ABS 3 channel and the ABS 4 channel version) interchangeable? Size matters, right? Isn't the standard 3 channel M/C vacumm, that and the C4S/TT 4 channel M/C is both larger and hydraulic?

Bill Verburg

All the electro-hydraulic boost cars, C4, C4S, tt, RS, Gt2 have 4 channel ABS and 25.4mm m/c

all others, C2, C2S have vacuum boost and 23.8mm m/c, some of these cars have optional(M220) ABS/ABD/LSD which includes 4 channel ABS

the m/c's are not interchangeable, if you want a 25.4 in a vacuum system the whole electro-hydraulic setup needs to be t/p'ed in

geolab

If you are asking for normal Carrera and Carrera with ABD M224 like you have,
they are not interchangeable only, the MC's are the same.
You have a vacuum brake system with 4 channel ABS

M. Schneider

Thanks Bill & Geolab -

Sounds like there's a vacuum based 4-channel ABS variant; LSD/ABD optioned 2wd cars. and not the electro-hyraulic based 4 channel ABS of the C4S, tt, RS, etc. cars.

M. Schneider

Out of the blue while starting the car one morning the brake pedal travel became excessive, but ulitmately there was brake. I'd alternately use the daily driver that day and would have the 993 towed to the nearby Porsche dealer weeks later.

The course of action was to flush the brake & clutch systems while replacing the brake master
cylinder in the process. I'd order this coupe new and thought it had "the" 4-channel arrangement of the C4S and TT, etc. ... as a result of the LSD/ABD..... surprise, surprise on there being a vacuum based 4-channel ABS variant.. The Porsche factory manuals do not identify a vacuum based 4 channel ABS.

All that said, the brake pedal feel after the repairs has a longer travel charactor than ever before with a one stroke pump shortening the pedal's travel. I'll be returning the coupe to the dealer for further curcuit bleeding.

Go figure.

Bill Verburg

That's correct, the 4 channel ABS can be identified by the 4 separate brake lines that come out of it, the 2 rear lines will each have a p/v on it, the 2 round cylinders are the p/v's for the 2 rear lines

M. Schneider

Yeap, The above ^ picture resembles my 4 ch ABS unit, but of course what threw me was there being a 2wd variation of a 4ch ABS which is vacuum based.

Now,,, if the brake pedal can only regain its positive action as before. Man, the solid brakes the 993 once had! ... In a word the brake pedal action is no longer linear.

geolab

I am sorry but what do you mean ? The pedal stroke is not in a straight line anymore?

M. Schneider

The brake pedal feel is now soft with much travel in relation to what the brakes are actually doing. In the past the Porsche brakes were nothing but immediate almost telepathic. The relationship btw brake pedal effort and the brakes themselves was linear. The brake pedal action currently is no longer linear.

Bill Verburg

There is something wrong then, the stock 23.8 mm m/c is fine even w/ RS brakes which have the largest caliper pistons, 25.4 is just somewhat better

geolab

the only thing I could think about right now is that the MC membrane has tilted.
MC should be bench bled and bleeding the system should be done with bleeder, and not pedal kicking, since the
stroke is so small.

M. Schneider

Thanks Bill & geolab -

Agreed, something is out of place. The Jim Ellis Porsche store shouldn't have let his car out of their shop in this half baked condition. The current pedal action isn't even close to normal .... Live and learn...

Thanks guy's for the details!

Cupcar

Sounds like air in the system to me.