993: possible bad ABS modulator?

[xerxes5]

Has anyone else seen this problem on a 993? I've got pedal sag. The brakes function fine, but the pedal slowly sinks on first use. One pump on the pedal will bring them back to normal. One mechanic suggested it was a valve in the abs system that was leaking, and that the whole pump/modulator would need to be replaced.

Yesterday, I bled the system, and discovered that the brakes were running 9 year old fluid (gee, could that have caused the problem?!!)... still had lacquer on the rubber nipple covers, which doesn't say much for my mechanic.

If it is the ABS pump, can it be changed out without a Bosch hammer to bleed the system?

Any help would be appreciated.

Thanks,

-Mark

[asphaltgambler]

ABS does not come into play until a panic stop. It is only 'there' (at the sidelines so to speak) to modulate / bleed-off brake line pressure to minimize wheel lock-up.

Your problem is the brake master cylinder is leaking internally when you apply the brakes. You need to only replace the brake master cylinder. Not huge deal.

[john walker's workshop]

the master cylinder would fix the problem for sure in a car without the ABS pump, but unfortunately, it's often the pump itself that is the cause. i've done a few master cylinders on them that didn't make any difference. the pumps tend to get filled with a granular substance, from the old brake fluid, which you find after removing the pump and blowing it out with air. the reservoir can have the same stuff in the bottom. sometimes they work after cleaning and sometimes they don't. big bux.

[bb]

My wife's 95' 993 did the same thing, you hit the brakes and it is good...then the pedal starts going to the floor slowly. It was a bad ABS unit, $2300.00 when it was all said and done. My mechanic put a master cylinder first because he thought that was what it was. He ended up taking it off and not charging me for it because it was the ABS unit.

Oscar

[Kmatts]

i've heard of the ABS units going bad on the 1996 models.....friend of mine here in Seattle had the same exact problem as Oscar. and roughly the same price. no issues on my 95 yet....135k miles.

[Emission]

Welcome to the board!

In all honesty, you should ask on the 964/993/996 board for many more responses... technically, none of us (911's to 1989) will have ABS-equipped cars..

[}{arlequin]

can the abs be taken out completely and replaced w/ big reds?

[KobaltBlau]

Yeah, fabcar makes a dual master cylinder kit for 964s and 993s, this would not require a caliper change but would require different lines. I don't think it would be a smart solution for a street car though, imagine if the next owner crashed the car and they or their insurance company tried to come back and sue you because the car didn't have ABS when it was supposed to. You'd have to have the buyer sign a disclosure agreement, which would scare away many street car buyers.

[CBRacerX]

Mark,
What year is your 993? Did you use a power bleeder when changing the fluid?
Chris

[edward993]

Hi Mark,

FWIW, in my 5 years owning one and wrenching it, I have not personally ever heard of the problem. Try the Rennlist board and archives ...very active 993 group there.

Edward

[ChrisBennet]

The ABS unit failed on my 944S2 but my friend (a Bosch Master Mech) was able to repair it or clean it out and get it working again.
-Chris