Brakes question

[Johnny C]

When I depress the brake pedal, it seems like there are two settings; light pressure which slows the car fine but slowly, then at a certain point I have to really jam my foot down which means I don't have a lot of control, gentle braking and emerggency stop almost, there's no gradual but constant increase in braking.

9M looked at it and unblocked a blocked vacuum tube but it's made no difference.

Does it sound like a servo problem?

It's like my old 993 with the engine turned off, you could do so much light braking but then needed to really push down. With the engine on there was a smooth increase as you depressed the pedal.

 

[Johnny C]

Hi Maurice
Not sure what you mean - I looked at the tech article and it seemed to say the brakes would stick, like when the car's sat around for a week in the rain, and you need a few good brakings to clear the rust. Maybe I'm wrong, correct me if so! They don't stick, just past a certain point (about 2 inches depression) you need to jam the pedal to get any extra response.

I think it'll have to go back to 9M.

 

[nfearn]

This is not a "deep" diagnosis but I would be inclined to support Maurice's initial thoughts concerning caliper plate lift. This is the condition where the stainless steel plates in the caliper which are there to prevent the steel backplates of the pads wearing away the alloy of the calipers "lift" because of corrosion between the plates and the calipers themselves.

This results in the pads being clamped between the two stainless steel plates, restricting their ability to clamp the disk.

A common side effect of this phenomenon is that the disks go rusty (or less worn) on the outer part of their diameter.

The problem needs to be checked out by a garage that knows what they're doing as soon as possible.

 

[colt]

question?
Did 9M agree with you that they thought the brakes were not right? With their experience ,they should within a mile or two comment on the normality of your car? Agree caliper plate lift can make the brakes a bit wooden and unresponsive,other problems with hydraulics and servo are rare, but then again the vacuum system at the engine has a number of pipes and one way valves which can fail.............. would have thought that would create a idle problem with the engine as well though....
Cj

 

[clyde]

Doesn't seem to sound like an issue with the caliper plates to me. As colt says they can make the brakes feel a bit dead but generally they are still progressive. If anything the brakes will be inclined to stick on as the pads are stopped from retracting properly.

Could it be something in the master cylinder stopping it from operating smoothly? Maybe a score in the cylinder that stops it being depressed further until a bit more force is added at the pedal?

As an aside but possibly linked, I would suggest that two inches of movement in the brake pedal seems a bit on the high side Johnny, maybe time to replace the fluid? Might help with the problem as well.

 

[les richards]

In a similar vein, what causes the brakes to judder at the final point - i.e., not when first applied - but as the car comes to a stop or slowing, say to enter a roundabout?

No wheel vibrations under normal driving so I don't think it's wheel balance. Car drives straight, requiring no correction at all. The brakes give no cause for concern that they don't work, just judder in the final phase.

Are the front wheel bearings adjustable on the 993 as they were on my IB and 944? Reason I ask is that many, many moons ago I had a similar problem with an Anglia "" yes, that's how long ago "" when the bearings had been over tightened and there was no discernable play. As they got hot a similar judder to that I'm experiencing set in. That said, I think the 993 does it from cold but does get worse after a few miles.

 

[colt]

That sounds like slightly warped or corroded discs and/or caliper plate lift causing the brake pads to bind. This diagnosis assumes that the suspension bushes and ball joints are in good order. Many times (even with genuine Porsche discs) i have had to skim the discs true on the car (with a pro-cut brake lathe) which corrects the run out of the whole rotating mass. incidentally The higher pressure of the heavy breaking reduces the apparency of the judder at the steering wheel or brake pedal, when the run out is small(the pads are literally squeezing the runout away). Can recall at least one occasion when two new sets of discs(genuine) were fitted in a short space of time(for slight judder) to the front of a 993 only to finally cure the slight judder upon light braking with an "on car" skim of the said second set of Porsche discs.
Let us know what you find.
Cj