Tobias

the brakes on my 993 c4 felt mushy so I decided to bleed them (not w. a power bleeder, but with the help of a friend). Besides getting out some dirty fluid, we also got out some fluid which evidently had air trapped in it (foamy fluid). Right after that the brakes felt nice and stiff again (with ignition off). But after having started the car the brakes felt softer than ever and a whining sound (as from air flowing) came from behind the brake pedal (master cylinder?).

When I push the brakes hard 10-15 times, the warning light for low brake pressure comes on. We also tried to bleed the ABS system by opening the valve, but saw no air coming out. So,:

*Should I bleed again (this time w. a power bleeder!) and try to switch all of the fluid? I've only put in 1 litre now.
*Could it be the ABS system that traps the air, and if so, what is the procedure to bleed it?
*Last (but not least I'm afraid), is it a bad master cylinder? If so, is it possible to have the seals restored, or is it necessary to change the whole cylinder?

I've browsed through old threads about this topic and it seems that a lot points to a bad master cylinder, but I wanted to hear your opinions first.

Patrik S

Tobias,
Ring mig (call me)! 070-8506118
Kan kanske hjälpa dig lite.
/Patrik

Greg Fishman

Tobias,
If you master cylinder is bad just replace it. I don't think they are re-buildable. I am sure Gert at <a href="http://www.carnewal.com" target="_blank">www.carnewal.com</a> could get you a good price.

I would bleed with a pressure bleeder and take it to a dealer or shop with a hammer to bleed the ABS system (unless they will let you borrow it).
If you press the brake and the pedal just sinks gradually, my guess is the MC.
Greg

Robin 993DX

Tobias,

Before you replace your master cylinder you could try to bleed the system first with the ABS system activated. Since this would be a lower cost alternative than replacing the master cylinder.

In order to activate the ABS system you will need to hook the Bosche Hammer unit to the car to activate the ABS unit while the hydraulic system is being completely flushed.

Flying Finn

How much does the master cylinder replacement cost?

Greg Fishman

FF,
MC is around $400. Took about an hour to change and then an hour to bleed the system.

My theory (please correct me if I am way off base), Is that if there is air in the ABS system and you brake hard enough to get into the ABS it will release the air bubbles and you can bleed the caliper to remove it. The Hammer just opens and shuts the valves in the ABS like what would happen when braking. I don't see how not bleeding the ABS system would cause a severly mushy pedal.
Hank's mechanic told me that it could be caused by the MC or the ABS pump. The cheap thing to replace was the MC first since the ABS pump is $1200 or so.

Greg

freeman

Been there, done all, put in new MC, fixed problem. I was in MC denial but lucky I did not get in any accidents,

KC993

Guys,

I thought one should not use the pedal method to bleed C4??? Can one use Motive bleeder on C4(S) and Turbos??? <img src="confused.gif" border="0">

Flying Finn

Thanks Greg,

I was asking because my brake pedal did that couple of times (once few days ago and once yesterday), when I was braking, pedal was soft & went almost to the floor, after that, it was fine.

Anyway, I'm gonna change the brake fluid tomorrow (I have speedtrial & autoX on sat - sun) and see if it's fine after that.

Fred R. C4S

C4, C4S, and Turbo need to be pressure bled with the Bosch Hammer to cycle the electro hydraulic valves in the ABS and Differential control systems. It's explained in the service manual. The same manual that Ray Calvo always refers to. I have to second Ray that if your really serious about knowing and understanding your car, you out to pop for the shop manual. It's a good read for us mechanical engineers.

Greg Fishman

[quote]Originally posted by freeman:
<strong>Been there, done all, put in new MC, fixed problem. I was in MC denial but lucky I did not get in any accidents,</strong><hr></blockquote>

I was in MC denial as well until I went through turn 10 at Road Atlanta with the brake pedal to the floor at 100mph or so.

FF,
My pedal would sometimes go to the floor and other times brake like it should. I got in the habit of pumping the brake pedal with my left foot as I went down the front straight or before a hard brake zone. Inmy case there was no external leaks and I always bleed by brakes with a pressure bleeder and I bought the car with 13K miles so the PO shouldn't have had to bleed more than once or twice at most.

I haven't checked the manual but I wonder if there is a procedure to check the MC.

Greg

Greg Fishman

[quote]Originally posted by KC993:
<strong>Guys,

I thought one should not use the pedal method to bleed C4??? Can one use Motive bleeder on C4(S) and Turbos??? </strong><hr></blockquote>

You are correct. and those can be used on Turbos and C4S's.
Greg

KC993

KC993

[quote]Originally posted by Fred R. C4S:
<strong>C4, C4S, and Turbo need to be pressure bled with the Bosch Hammer to cycle the electro hydraulic valves in the ABS and Differential control systems. It's explained in the service manual. The same manual that Ray Calvo always refers to. I have to second Ray that if your really serious about knowing and understanding your car, you out to pop for the shop manual. It's a good read for us mechanical engineers.</strong><hr></blockquote>

SORRY,

Did not know that once again, I pissed off another ME degree guy.
<img src="graemlins/xyxwave.gif" border="0" alt="[bigbye]" />

Flying Finn

Let's see how it is after the fluid change...
If it doesn't be as it should be, it sounds like I'm in for a MC replacement.