IceWater

Hi everyone.

I recently posted about a problem I'm having with my 968 where I get several inches of free travel at the top of my brake pedal throw, like there's air in the lines. Replaced the master cylinder, replaced or turned all rotors, installed stainless steel lines, bled the system many times with a motive pressure bleeder, and engaged the ABS extensively on a dirt road. Still have the problem.

Several people have suggested that it could be a problem with the booster, but based on my admittedly limited understanding of how they work, I don't see how it could cause this problem. The pedal is connected directly to the master cylinder pushrod, right? The booster works in parallel with it, not in series. Right again? So how could a problem with the booster ever allow the pedal to move freely?

I just want to know if its worth trying to hunt down a vacuum leak in the rain before giving up and bringing her to a professional wrench.

Thanks,
Tom

mmmbeer

Adjust your brake pedal.

Eirik Kvello-Aune
<a href="http://www.diateam.no/porsche" target="_blank">www.diateam.no/porsche</a>

IceWater

I think the pedal is fine. Its resting on the internal stop in the cylinder. Plus, I do get a very small amount of brake force in the squishy area.

Thanks, though.

Tom

mmmbeer

The next thing that comes to mind are the pistons inside the calipers. I dont think the booster should give you these symptoms since it is just an asisting force. But you can never be sure.

Eirik Kvello-Aune
<a href="http://www.diateam.no/porsche" target="_blank">www.diateam.no/porsche</a>

IceWater

The pistons felt normal while I was working on the calipers. Difficult to move by hand, but not stuck at all. Don't believe I'm losing any fluid, either.

Thanks again, though.

Tom

83na944

The vacuum booster helps move the piston, so a lack of vacumm would make it more difficult to brake.

If you have no leaks and you're sure there is no air in the system, it soulds like the pushrod may not be adjusted correctly.

thomschoon

You may need to bleed the brakes a bit differently with your Motive. You could still be having problems as the Motive can pressurize the system but still leave air trapped. Have you tried pumping the brakes while the Motive is connected?? Just pump it slowly and this should force any air out of the MC. You could also force the pistons to bottom in the calipers by forcing your brake pads back and have the bleed valves open, pressurize the system, check that you are getting clear liquid out of the tube connected to each bleed valve. I had one where there was some trapped air in the caliper and the only way it came out was by pushing the pistons all the way in to force the air into the brake line so it would be forced out by the Motive. I know this sounds like a pain but it appears you have replaced a lot of parts and booster shouldn't cause this type problem.

IceWater

Thanks,

I did pump the brakes a little while bleeding. I might try to jury rig a way to reverse-bleed the system using the motive.

Adjusting the pushrod only changes the hieght of the pedal, so I don't think that's the problem.

I may take off the MC to measure its pushrod and then the depth of the booster pushrod to make sure there's no gap between them. I've been sent the wrong parts before...

Thanks again,
Tom

keith

Are your rotors and pads worn? This will account for a lot of pedal travel. Especially if you have larger rotor/pad thicknesses.

jim968

Water, does your pedal temporarily firm up / come up if you pump rapidly & hold it while driving? This'd be a pretty sure sign of air still in the system. OTOH, if that doesn't make it firmer / higher, air probably isn't your problem.

The only other ideas I can come up with (and I'm stretching here) are:

1) Wrong master cylinder (I was sold a wrong clutch MC once; 968 isn't the same as 944; dunno about brake MC).

2) Adjustment of link rod between pedal and MC; I've never looked on brake system, but there's an adjustmsent on the clutch link rod. The dimension is 968 specific, and fairly critical to proper pedal height & feel.

3) Something (note highly technical term &lt;g&gt amiss with the ABS system. Are you getting the pump activation test below 5 mph when you first drive off?

Jim, ""It's not how you pick your nose, but where you put the booger...."

Untier

try pulling on the hand brake then step on the pedal?

Untier

no, different system.
try bad hoses, expanding.

billybones

This may or not help. couple of weeks back a guy at work was having freeplay on his pedal on a car he just purchased. It ended up being wore out wheel bearings. Enough movement of the rotor in turns to move the caliper piston out of position and a need to pump the brakes to catch up. Was inconsistent though.

PS, You need to make sure that the MC is getting all the way back to allow the fluid to make up what is now gone on the brake pads. IF that makes any sense?

Legoland951

Isn't the rod connecting from the brake booster to the pedal adjustable? If you turn the rod counterclock wise it would extend and pick up slack in pedal travel I think.

IceWater

OK, here we go...

Keith, pads are new, rear rotors are new, front rotors are turned.

Untier, just put in stainless steel flex lines.

Jim, I get a slight amount of firming up with rapid pumping. There were two differnt MC's for the 968, and I may have the wrong one. That's what I hope to learn by measuring the pushrod offsets.

Jim and Legoland, the booster pushrod has an internal stop, so its resting position is fixed. Ajdusting the rod on the pedal only moves the pedal up and down. The ABS does the self test thing, and they seem to work fine on loose surfaces.

Billybones, the problem doesn't get any better if I use the brakes while the car is up in the air, so I don't think its a runnout problem.

Thanks a bunch for the suggestions everyone.

Tom