Me with my brakes again lol

My brake pedal feel is very inconsistent. I installed SS brake lines and bled the system. They were still a bit spongy so after a few days I bled again. It seemed to help, but at the track this weekend, the pedal would alternate between nice and firm, and a bit spongy. It never went to the floor or anything like that, but it was very annoying.

Here are a few odd things I noticed:

- there were certain places on the track where the pedal felt too soft *every* time, and certain places where it was fine every time. Other places, it varied from lap to lap. I thought of pad knockback...I do have all new wheel bearings, but I am a little unsure of the tightness of the fronts. My mechanic said I had them too tight, and he backed them off. That seemed to make the wheel too loose, so before the track, I tightned them up a bit. It didn't seem to help the brakes at all...but maybe there is another source of knockback?

- I found that sitting with the engine off, if I pulled the handbrake as tightly as I could, the pedal would actually firm up. If I pressed the pedal and started the engine, it would sink a lot, then after driving to the pit it would be ok again. How could the handbrake have any effect on the pedal feel?

I do have all the parts for a caliper reseal but I am wondering if I should replace the master cylinder too at the same time. Is there any way to diagnose it for certain?

 

It sounds like you have a caliper puck that's sticking badly and when bleeding, the puck is not fully extended. Then, when you actually use the brakes, air bubbles were not bled from that caliper gets in play and the pedal softens. I'd recommend rebuilding the calipers and then re-bleed the system using a motive flow bleeder. I'm pretty sure your problems will be history.

 

Could also be the master cyl.

 

Thanks guys. mytrplseven, what is a caliper puck?

 

He's referring to one of the four pistons in the caliper.

Here's another thought. My limited experience with [non-Porsche] ABS-equipped cars has been that after bleeding, and bleeding, and bleeding, pedal feel usually improved after standing on the brakes, in an attempt to lock the wheels, thereby activating the ABS pump. And to mitigate component wear and tear, this can be accomplished on a slippery surface [wet, dirt, gravel].

Would be curious if other ABS-equipped 944/968 owners have had similar experiences.

 

Funny you say that...I forgot to mention this, but a while back I had to disconnect all the lines from the ABS pump to get some body work done on the battery tray. Afterwawrds, at first I couldn't get any fluid to bleed out of the passenger side caliper, but after a few pumps of the pedal it came back to normal. (I use a Motive pressure bleeder). I drove thousands of miles since then but only just started to get back on the track a few weeks ago. This weekend, the track was wet and I felt the ABS for the first time in a long while...it didn't seem to help the pedal feel, but maybe I will get more air out now that I've activated it...

 

Certainly couldn't hurt to "burp" the system a few more times and then bleed some more, and some more...

 

My pedal is better on my abs equipped S2 after autocross events as i usually hit the abs in a spot or two.

 

Interesting...

 

For what it's worth, the ABS system on later Porsches (996s for example) must be "cycled" to bleed out the air that is trapped in the ABS module. It can be done with a Durametric or a Porsche dealer's factory computers during the bleeding process. But because most people don't have these expensive diagnostic computers, many just activate the ABS as mentioned above to try to push air out of the ABS and then bleed the system again.
There is likely a way to cycle or bleed out the ABS circuit on the 944 models though without the OBD computers.

 

I'll try bleeding again soon...but I was planning the caliper rebuilt anyway so I'll definitely do that. I bit the bullet and ordered a new MC just in case, but I'll to see how the other things work out before I install it.

 

Are you using race type brake pads? Mine will feel soft until they heat up properly. On cold days this effect can come and go. I think an ABS cycle also helps to push any trapped air through the system.

 

EBC Yellowstuffs. But it seems to me the pedal is more often solid when the car is cold...it seems to get softer after driving for a while. But it's not an exact pattern, it's a bit random as far as I can tell.

 

I don't think those pads run in the same heat range as the PFC 08s I'm using.