kyle Posted May 18, 2007 Share Posted May 18, 2007 -Apologies from zcar.com attendees noticing the crosspost, but I'm casting a wide net Went to Thunderhill yesterday.... I had brake problems pretty much all day. I replaced an inner seal on one caliper, whose leaks I'd been blaming on low pedal, before heading out. Bled the thing thoroughly, but still had a pedal that engaged lower than I liked. Well, I did what I could, then took it out. Pedal was low but usable for the first session. Decided to bleed the fronts again just to be safe... no air. Then decided to bleed the master, and *I think* I got a bit of air. Called it good, back out for the next session. Pedal was 100% rock strong and awesome for half the session, then degraded to needing a pump-up before extreme braking events (not a big deal, and very predictable). Bled again, and again, did the rear master just for kicks, and none of that made any difference. It feels very much like the rears are engaging but not the fronts, until I pump up. Then the thing stops really really well for something with non-vented rotors and 2"x3" pads. All I can figure is it's the master. Fluid's not leaking out, pads are in okay shape (due for replacement), fluid is ATE 200, Hawk HPS pads in front, OEM shoes in rear, brakes were previously excellent for prior track days. What happens is this: Press the pedal once - I can feel the rears dragging, but not much else. Pump the pedal once, then press - fronts engage a bit, lower than usual, but pedal is firm, stopping is okay, straight and true, but not 100% Pump the pedal twice, then press - solid pedal, great stopping. Then, if I let go of the brakes and reapply (like stop n go driving), I have to pump up again. The pumping is only good for one stop, if that makes any sense - no residual crutch factor. What it feels like is the fronts part of the master is pushing 40% of the fluid it should. Pads still have plenty of meat on them... though now that I think about it, I have those fade-stop-brake-cooler heatsinks (which work fabulously) sammiched between the pads and pistons. Maybe I have something improperly assembled? Hmmm... Quote Link to comment Share on other sites More sharing options...
GrommitZ Posted May 18, 2007 Share Posted May 18, 2007 If you pedal sinks under normal driving conditions, you probably have a cylinder leak. If you haven't overhauled all your cylinders in a while, rust can get lodged between the seals, especially under extreme conditions. The master cylinder can be difficult to bleed. Open and close the bleed valve quick and far enough to get the air out. My pedal tighten considerably after finding the right touch to bleeding the master. That said, I had a similar problem as yours. ThunderHill is hard on brakes. Have you bleed your brakes after your track day? I'm guessing you boiled your fluid even with all your modifications. Brake ducts and heat sinks are next on my list. Quote Link to comment Share on other sites More sharing options...
kyle Posted May 18, 2007 Author Share Posted May 18, 2007 Funny thing is, the pedal doesn't drop under load. It stays high and firm, even with working the brakes HARD. If it slowly dropped, I'd have an easier time diagnosing it: Fluid leaks - bad wheel cylinder/no fluid leak - bad master. This required pump-up is trippin me out. Well, it's easy enough to yank the wheels and check for fluid leaks.... I'll re-bleed everything again too. The nice part was that once I got used to needing to pump the brakes up, it stopped fine, so I was able to keep on trackin. Quote Link to comment Share on other sites More sharing options...
johnc Posted May 18, 2007 Share Posted May 18, 2007 Air in the lines, caliper, or MC. Quote Link to comment Share on other sites More sharing options...
kyle Posted May 18, 2007 Author Share Posted May 18, 2007 Well poop. Suppose it's better that way, just frustrating that I spent so much time bleeding at the track and didn't get the air out. On the other hand, I need to get the car up for a thorough post-offroad postmortem anyway. Quote Link to comment Share on other sites More sharing options...
GrommitZ Posted May 18, 2007 Share Posted May 18, 2007 Funny thing is, the pedal doesn't drop under load. It stays high and firm, even with working the brakes HARD. If it slowly dropped, I'd have an easier time diagnosing it: Fluid leaks - bad wheel cylinder/no fluid leak - bad master. This required pump-up is trippin me out. Well, it's easy enough to yank the wheels and check for fluid leaks.... I'll re-bleed everything again too. The nice part was that once I got used to needing to pump the brakes up, it stopped fine, so I was able to keep on trackin. I still think it's air in the master. I had a very similar problem. Brakes were good in the morning, but the pedal started sinking in later sessions to the point where the brake warning light came on a couple of times. Like you, pumping once or twice before the turn brought them back up. I bedding the brakes in thoroughly and rebleed them several times before the night before. I was bedding them in to hard, heating the fluid, and thus needing to be bleed. The day after, I carefully bleen the master and got the pedal height much higher and firmer than I ever did before. It required the right touch for bleeding the master. I did not get to retest that setup though. Hope this helps. Quote Link to comment Share on other sites More sharing options...
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