Pelican Parts
Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   Pelican Parts Forums > Porsche Forums > Porsche Autocross and Track Racing


Reply
 
LinkBack Thread Tools Rate Thread
Author
Thread Post New Thread    Reply
Registered User
 
Join Date: Nov 2002
Location: Cincinnati, OH
Posts: 166
Garage
nearly no brake pedal at start of session

just got back from a weekend of HPDE driving. On my 944 when entering the first turn on the first lap at the beginning of the session my brake pedal will go almost to the floor before I get anything. After that initial depression of the pedal, the pedal is firm for the rest of the session. It did not do it during the morning sessions and started in the afternoon. After two sessions of it happening, on the third i pumped the brake pedal several times in the pits before going out and didn't have a problem. Did it once yesterday in an afternoon session.

What could be causing this?

__________________
Running Cars
06 Boxster S, 85 944, 83 928S
Project Cars
74 914, 74 911, 72 914, 66 911, 65 912
Old 07-27-2014, 07:00 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #1 (permalink)
Registered User
 
michael lang's Avatar
 
Join Date: Feb 2005
Location: adamstown md
Posts: 1,093
Garage
I assume when you were doing your pre-event inspection and were flushing the brake fluid you were very careful to close the bleeder on the brake caliper so no air was getting in. I'm not very well versed in 944s, have you looked at the master cylinder and brake booster? Are you seeing any fluid seeping from either? If you had it only happen one time on the second day in the afternoon session, is that because you flushed the brake fluid between the two days? If you knew you had this problem, why did you go out for your session in the first place?
__________________
Mike

'89 CARRERA
#402
Old 07-27-2014, 11:28 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #2 (permalink)
Registered User
 
Join Date: Nov 2002
Location: Cincinnati, OH
Posts: 166
Garage
First off I am new to the track and learning so I don't know all the proper procedures just yet. I last bled my brakes and put on new pads in May before the the first of three DE events I have done so far this year. I used a power bleeder and yes I properly closed the bleeder valves. I checked fluids before each day and everything looked fine. Maybe I just need to start bleeding them before every event or every track day.

Maybe it was stupid to go out when there was a known issue but like I said, as soon as you depressed the pedal once the brakes were solid throughout the entire session. There was no fade at all. I had an instructor with me the entire time. I told him what was happening. I told him when I last bled my brakes and he said they should be fine. He didn't seem to be concerned since the car was still braking.
__________________
Running Cars
06 Boxster S, 85 944, 83 928S
Project Cars
74 914, 74 911, 72 914, 66 911, 65 912
Old 07-28-2014, 05:46 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #3 (permalink)
Super Moderator
 
cstreit's Avatar
 
Join Date: Feb 2000
Location: Naperville, IL USA
Posts: 13,385
Garage
Sounds to me like you were getting pad knockback as you went out on the track. Knockback is when a loose wheel bearing (usually) allows the pads to get pushed back too far into the caliper. Then it takes a pedal press to move the pads back close to the disc.

In fact, reading your description, I am sure this is it. Check your wheel bearings, one of them is probably loose. Was there a series of bumps or a rumble strip some time prior to that first pedal press?
__________________
Chris
----------------------------------------------

2001 GT3 Cup - "Pepin"
1999 Spec Miata
1996 993 RS Replica
2010 Harley 48
2013 Honda 250 CRF
1971 Norton 750 Commando
Alcon Brake Kits
Old 07-28-2014, 05:57 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #4 (permalink)
Registered User
 
Join Date: Nov 2002
Location: Cincinnati, OH
Posts: 166
Garage
Thanks Chris. What you say makes sense. I will check the wheel bearings and the brakes in general to see if it is doing what you are saying and if there is anything loose. I did spin a couple times and go 4 wheels off so maybe I knocked something loose on one of those excursions. There are no rumble strips or any bumps leading to the pits or out to the track though. The track I was at was Putnam Park.
__________________
Running Cars
06 Boxster S, 85 944, 83 928S
Project Cars
74 914, 74 911, 72 914, 66 911, 65 912
Old 07-28-2014, 07:51 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #5 (permalink)
Registered
 
safe's Avatar
 
Join Date: Feb 2004
Location: Sweden
Posts: 3,656
Garage
Quote:
Originally Posted by 9fourteen View Post
just got back from a weekend of HPDE driving. On my 944 when entering the first turn on the first lap at the beginning of the session my brake pedal will go almost to the floor before I get anything. After that initial depression of the pedal, the pedal is firm for the rest of the session. It did not do it during the morning sessions and started in the afternoon. After two sessions of it happening, on the third i pumped the brake pedal several times in the pits before going out and didn't have a problem. Did it once yesterday in an afternoon session.

What could be causing this?
Happens to me every time.
If you come in to the pits with hot brakes and let them cool down the pistons will pull back into the calipers a little bit.
__________________
Magnus
911 Silver Targa -77, 3.2 -84 with custom ITBs and EFI. Just works!
911T Coupe -69, 3.6, G50, "RSR", track day. Sorting out issues...
924 -79 rat roddy...
931 -79 under total restoration...
Old 07-28-2014, 10:46 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #6 (permalink)
 
Registered User
 
Join Date: Nov 2002
Location: Cincinnati, OH
Posts: 166
Garage
I'm going to pull it all apart this weekend and see what I can find. Only way to find out for sure is to take a look.

I will bleed the brakes as well just to make sure that is not the problem although I don't suspect that to be the issue.
__________________
Running Cars
06 Boxster S, 85 944, 83 928S
Project Cars
74 914, 74 911, 72 914, 66 911, 65 912
Old 07-30-2014, 05:02 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #7 (permalink)
Super Moderator
 
cstreit's Avatar
 
Join Date: Feb 2000
Location: Naperville, IL USA
Posts: 13,385
Garage
...note that he mentioned pumping the brakes a few times in the pits. That means the piston retraction happened after that.... Although Putnam. is pretty smooth.... I still stand by my guess.
__________________
Chris
----------------------------------------------

2001 GT3 Cup - "Pepin"
1999 Spec Miata
1996 993 RS Replica
2010 Harley 48
2013 Honda 250 CRF
1971 Norton 750 Commando
Alcon Brake Kits
Old 07-30-2014, 07:23 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #8 (permalink)
Registered User
 
Join Date: Nov 2002
Location: Cincinnati, OH
Posts: 166
Garage
when i pumped them in the pits before going out it didn't happen which supports what safe said.

There is a dip on the straight after you come out of the pits before you get to turn one. I remember hitting it a few times. It's not much but maybe that would be enough to knock the pad out if there was something loose which would support your theory.

Like I said, a full inspection and service should tell me if there is something loose or broke in there.
__________________
Running Cars
06 Boxster S, 85 944, 83 928S
Project Cars
74 914, 74 911, 72 914, 66 911, 65 912
Old 07-30-2014, 08:44 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #9 (permalink)
Super Moderator
 
cstreit's Avatar
 
Join Date: Feb 2000
Location: Naperville, IL USA
Posts: 13,385
Garage
The fact that they come back is indicative of the loose bearing allowing the disc to wobble and push them back. Are you saying they were low in the pits? ...or low in turn one?
__________________
Chris
----------------------------------------------

2001 GT3 Cup - "Pepin"
1999 Spec Miata
1996 993 RS Replica
2010 Harley 48
2013 Honda 250 CRF
1971 Norton 750 Commando
Alcon Brake Kits
Old 07-31-2014, 09:28 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #10 (permalink)
Registered User
 
Join Date: Nov 2002
Location: Cincinnati, OH
Posts: 166
Garage
low in turn one. But if I pumped the pedal several times in the pits before going out they would be good in turn one. It was definitely weird. Pulling from the paddock area into the pits for staging they would seem fine but I was hardly using them. Didn't seem to be a problem until I was getting into them hard. Seems to be more prevalent once they were hot in the later sessions of the day.
__________________
Running Cars
06 Boxster S, 85 944, 83 928S
Project Cars
74 914, 74 911, 72 914, 66 911, 65 912
Old 07-31-2014, 11:40 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #11 (permalink)
Registered User
 
michael lang's Avatar
 
Join Date: Feb 2005
Location: adamstown md
Posts: 1,093
Garage
I'm interested to know what you find out this weekend. I have my suspicions that a caliper is hanging up but without looking at it I can't be sure. I know it's overkill but part of my offseason routine is to rebuild all four brake calipers. I didn't do it this past winter because I haven't driven it since April a year ago but when I'm ready to take it out n track again I'll replace the seals & gaskets and inspect the pistons.
__________________
Mike

'89 CARRERA
#402
Old 08-02-2014, 12:26 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #12 (permalink)
 
Registered User
 
Join Date: Nov 2002
Location: Cincinnati, OH
Posts: 166
Garage
I took at look. Not sure exactly what I am looking for but I pulled and shook everything I could and everything is solid. Nothing loose in the calipers, rotor, suspension, etc. the gap between the pads and the rotors looks good. Everything is cooled down now obviously so maybe once heated it would all be different. Is there anything specific I should be looking for?

I ordered some Motul 600. I have the ATE OEM fluid I it now. I am going to bleed them out and replace the fluid. If it is a heat/boiling issue that should take care of that.
Old 08-03-2014, 11:47 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #13 (permalink)
Registered
 
Join Date: Apr 2011
Location: behind the redwood curtain, (humboldt county) california
Posts: 951
Garage
A few WAGS...

Feel for any looseness in the front wheel bearings, gripping the tire at 6 and 12 to see how much rock you get.

Check the tightness of the brake calipers to the front struts.

Use a feeler gauge to see what the clearance is from pad to disc. If you have the caliper off of the spindle, check for free piston retraction, (watch out for the fluid level in the reservoir). Wouldn't hurt of check the tightness of the disc to hub bolts as well.

Just for fun, check the disc run-out, anymore than a few thou could give you a low pedal, (pad knock back),but I think it would be low all of the time and possibly, if bad enough, give you a pulsing pedal as well.

I am not familiar with the 944 rear brakes. Are they like the 911, with the small shoes inside the disc bell? I doubt Porsche continued the mistake of the ATE integral parking brake, as used on the 914.

We usually look immediately to the front, since that is where most of the braking effort is, but there may be some looseness in the rear wheel bearings or some piston sticking there that cause the excessive travel you are experiencing.

The puzzler is that it is a one shot deal, which, I would think would rule out a bleeding problem.

good luck,
chris
Old 08-03-2014, 02:09 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #14 (permalink)
Registered User
 
Join Date: Nov 2002
Location: Cincinnati, OH
Posts: 166
Garage
just an update on this..this past weekend I went to mid-ohio for a DE. brakes were fine all weekend. I bled them about a month ago and put in Motul 600. When I bled them there was a large air pocket. Not sure if it was in there or I introduced it when I sucked all the fluid out of the reservoir before putting in the new fluid. So, I don't know what it was but the bleeding seem to fix the problem.
__________________
Running Cars
06 Boxster S, 85 944, 83 928S
Project Cars
74 914, 74 911, 72 914, 66 911, 65 912
Old 10-25-2014, 05:37 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #15 (permalink)
Registered User
 
michael lang's Avatar
 
Join Date: Feb 2005
Location: adamstown md
Posts: 1,093
Garage
That's very interesting, thank you for the update. I've never used the Motul 600, I always flip flop back and forth between Super Blue and ATE Gold but I have read that the Motul is very good and has a much higher boiling point for racing. Maybe that was your problem all along, how long had it been before you flushed the brake fluid previously? Which could explain the air pocket you found.

__________________
Mike

'89 CARRERA
#402
Old 10-26-2014, 12:08 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #16 (permalink)
Reply

Thread Tools
Rate This Thread
Rate This Thread:

 


All times are GMT -8. The time now is 08:21 PM.


 
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2020 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page
 

DTO Garage Plus vBulletin Plugins by Drive Thru Online, Inc.