Strange braking issue - increased pedal travel
08-10-2011, 11:31 AM
#1
Nordschleife Master
Thread Starter
Strange braking issue - increased pedal travel
Ok did a search and didn't come up with anything that seemed to apply.
Issue is this: after taking a hard turn, OR, after driving in reverse and braking, the next time I hit the brakes (driving forward) the pedal travel is increased.
In other words, normally, the pedal travels, say, 1/2" before the pedal gets stiff and the brakes apply. If I back out of my driveway, or out of a parking space, then the next time I hit the brakes (like at the end of my street or the end of the parking lot aisle) the pedal travels 1" before the pedal gets hard and they start to apply. Subsequent stops are fine - pedal travel is back to 1/2", until I drive in reverse and hit the brakes again. Same deal goes for taking a hard turn. Crank a hard turn with some speed (like on an AX course) and the next time I hit the pedal the travel is increased on the pedal before they apply. This is 100% repeatable, every time.
Originally this seemed like a classic case of bad wheel bearings. So I swapped out the bearings for brand new ones, both front and rear (and I *did* find a bad passenger side rear bearing) thinking that would fix it. But the problem is still there. Bearings are torqued properly. I swapped front hubs (with a complete 2nd set of brand new bearings), that made no difference either.
Talked to a buddy of mine (mechanic) and he mentioned the master might be bad. Something about a valve or something that is supposed to prevent backflow might be bad? I admit that while I'm pretty mechanically inclined, I've never really dug deep into the intricacies of Porsche braking systems... but I've never heard of such a valve. He said it was to prevent flow from backing up through the master (?)
I bounced the situation off my dad (former Master Service Tech for GM) and he's also never heard of any kind of "backflow" valve and was more inclined to blame the proportioning valve since my symptoms appear to be caused by the rear brakes (as the rear brakes are usually the first to be applied... so it's like I "sink" through the rears, and the pedal gets hard when the fronts apply.)
The master, as far as I know, *is* original to the car. So it's got a little over 150k on it... and for the last 25k miles and 7-8 years at least, it's been used pretty regularly on the track and AX course. So I might just consider it "good maintenance" and replace the master anyways. But if the prop valve is the issue, that's another $100 or so on top of a master. Not sure how often these go bad... so far all I found with searches here was that the O-ring starts to leak. Mine isn't leaking (at least, externally.)
So... what say you, Rennlist brain trust?
Issue is this: after taking a hard turn, OR, after driving in reverse and braking, the next time I hit the brakes (driving forward) the pedal travel is increased.
In other words, normally, the pedal travels, say, 1/2" before the pedal gets stiff and the brakes apply. If I back out of my driveway, or out of a parking space, then the next time I hit the brakes (like at the end of my street or the end of the parking lot aisle) the pedal travels 1" before the pedal gets hard and they start to apply. Subsequent stops are fine - pedal travel is back to 1/2", until I drive in reverse and hit the brakes again. Same deal goes for taking a hard turn. Crank a hard turn with some speed (like on an AX course) and the next time I hit the pedal the travel is increased on the pedal before they apply. This is 100% repeatable, every time.
Originally this seemed like a classic case of bad wheel bearings. So I swapped out the bearings for brand new ones, both front and rear (and I *did* find a bad passenger side rear bearing) thinking that would fix it. But the problem is still there. Bearings are torqued properly. I swapped front hubs (with a complete 2nd set of brand new bearings), that made no difference either.
Talked to a buddy of mine (mechanic) and he mentioned the master might be bad. Something about a valve or something that is supposed to prevent backflow might be bad? I admit that while I'm pretty mechanically inclined, I've never really dug deep into the intricacies of Porsche braking systems... but I've never heard of such a valve. He said it was to prevent flow from backing up through the master (?)
I bounced the situation off my dad (former Master Service Tech for GM) and he's also never heard of any kind of "backflow" valve and was more inclined to blame the proportioning valve since my symptoms appear to be caused by the rear brakes (as the rear brakes are usually the first to be applied... so it's like I "sink" through the rears, and the pedal gets hard when the fronts apply.)
The master, as far as I know, *is* original to the car. So it's got a little over 150k on it... and for the last 25k miles and 7-8 years at least, it's been used pretty regularly on the track and AX course. So I might just consider it "good maintenance" and replace the master anyways. But if the prop valve is the issue, that's another $100 or so on top of a master. Not sure how often these go bad... so far all I found with searches here was that the O-ring starts to leak. Mine isn't leaking (at least, externally.)
So... what say you, Rennlist brain trust?
08-10-2011, 01:16 PM
#2
Drifting
When you replaced the bearings, you got them tight enough, right?? A wobbling bearing will cause the rotor to wobble and push the pistons back in their bores.
Since you're not experiencing this in normal, straight-line driving, I'd tend to think the MC or prop valve aren't the issue. Neither knows the difference if you're turning, going backward, or driving straight.
If the MC had a valve to prevent backflow, the brakes would never release.....
The repeatability of it still makes me think your front wheel bearings aren't tight enough. Do you feel any play when grasping in the 6 & 12 o'clock positions and wiggling?
Since you're not experiencing this in normal, straight-line driving, I'd tend to think the MC or prop valve aren't the issue. Neither knows the difference if you're turning, going backward, or driving straight.
If the MC had a valve to prevent backflow, the brakes would never release.....
The repeatability of it still makes me think your front wheel bearings aren't tight enough. Do you feel any play when grasping in the 6 & 12 o'clock positions and wiggling?
08-10-2011, 01:44 PM
#3
Nordschleife Master
Thread Starter
The repeatability of it still makes me think your front wheel bearings aren't tight enough
I tightened them using the method described in this video (thank God for YouTube and the guy that put this video together!)
08-10-2011, 03:35 PM
#4
Nordschleife Master
Thread Starter
Well if it's still a bearing issue, I wonder if perhaps the bearings just aren't seated all the way?
When I had the front bearings done, I had the shop install new bearings in a spare set of hubs I had at the same time as they did the bearings on the hubs on the car (to make it easier to do rotor swaps.) So I wonder if perhaps whoever did it didn't seat them all the way, or seated them crooked or something?
I've had it apart about a dozen times now but I'll take another look tonight when I get home. Maybe I'll try banging on the races a little with a seal/race driver to make sure they're in all the way and square and give it another shot.
When I had the front bearings done, I had the shop install new bearings in a spare set of hubs I had at the same time as they did the bearings on the hubs on the car (to make it easier to do rotor swaps.) So I wonder if perhaps whoever did it didn't seat them all the way, or seated them crooked or something?
I've had it apart about a dozen times now but I'll take another look tonight when I get home. Maybe I'll try banging on the races a little with a seal/race driver to make sure they're in all the way and square and give it another shot.
08-11-2011, 12:22 AM
#5
Have you checked your pad backing plates to make sure they are not loose? I had an issue with pads on my old 944. It does sound like a wheel bearing issue but if you say they are tight I believe you. Are you sure your spindles are ok? Im thinking the hubs may be fine but spindles may not be. Just trying to come up with additional ideas.s
08-11-2011, 09:08 AM
#6
Drifting
If the races weren't seated all the way I think you'd feel the bearing getting progressively looser. As you mention, though, it's probably worth pulling them to check. As Brian suggested, it may be a good time to also check the spindles.
When checking bearing tightness I always put the wheel on for a little more leverage. Plus you're amplifying the bearing movement through the longer lever arm, making this a more sensitive test.
Are you using stock hubs? My first set of Racer's Edge hubs had a machining defect that allowed the bearings to progressively loosen, causing a long initial pedal.
Even when my bearings are a little tight, I still occasionally get an initial long pedal on certain turns where the suspension is heavily loaded and there's a dip or bump in the track. I'm aware of it and give the brakes a left foot tap before the next turn. I figure it's better to do that then overtighten the bearing and cause premature wear and excess heat. However, your long pedal on every turn indicates a problem.
When checking bearing tightness I always put the wheel on for a little more leverage. Plus you're amplifying the bearing movement through the longer lever arm, making this a more sensitive test.
Are you using stock hubs? My first set of Racer's Edge hubs had a machining defect that allowed the bearings to progressively loosen, causing a long initial pedal.
Even when my bearings are a little tight, I still occasionally get an initial long pedal on certain turns where the suspension is heavily loaded and there's a dip or bump in the track. I'm aware of it and give the brakes a left foot tap before the next turn. I figure it's better to do that then overtighten the bearing and cause premature wear and excess heat. However, your long pedal on every turn indicates a problem.
08-11-2011, 10:30 AM
#7
Nordschleife Master
Thread Starter
Thx for the suggestions. So checking it with the wheel on, there is a very very slight clunk when grabbed at 6 and 12 and attempting to tilt the wheel in/out. Visually it's not perceptible but you CAN feel it. I took the wheel and hub cap off, loosened the pinch bolt, and was able to turn the retaining nut a little (maybe 10 degrees.) The last time I put this together I tightened it more than it should have been. So it loosened up.
So for a test, I cranked the hell out of them with a pair of pliers. They're definitely TOO tight now. Put the wheel back on, and that same barely-perceptible clunk is still there, though I suppose I could argue it's lessened a bit. Honestly it's hard to tell, the movement is so small to begin with.
I'm chasing after a small oil leak (a long story for another time) so the car is still in the air right now, didn't get the chance to drive it. I'll take it for a ride tonight or tomorrow (just a short drive around the block) and see if the pedal issue goes away with the super tight bearings. If this doesn't cure it then I am kinda out of ideas and am debating just throwing a new master in it, it's the only thing that hasn't been rebuilt so far in this braking system.
So for a test, I cranked the hell out of them with a pair of pliers. They're definitely TOO tight now. Put the wheel back on, and that same barely-perceptible clunk is still there, though I suppose I could argue it's lessened a bit. Honestly it's hard to tell, the movement is so small to begin with.
I'm chasing after a small oil leak (a long story for another time) so the car is still in the air right now, didn't get the chance to drive it. I'll take it for a ride tonight or tomorrow (just a short drive around the block) and see if the pedal issue goes away with the super tight bearings. If this doesn't cure it then I am kinda out of ideas and am debating just throwing a new master in it, it's the only thing that hasn't been rebuilt so far in this braking system.
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08-11-2011, 10:40 AM
#8
Nordschleife Master
Thread Starter
Brian - as for pads, you mean the actual backing plate on the pad itself? Yeah that's fine - it's done it across 3 sets of pads so far (OE Porsche pads in there now, PF97s for track/AX, and Hawk HP-S's I used to run on the street before ordering the set of Porsche pads.)
I'll take a closer look at the spindles tonight and see if I can spot anything that doesn't look right.
I'll take a closer look at the spindles tonight and see if I can spot anything that doesn't look right.
08-11-2011, 01:07 PM
#9
Drifting
With the pinch nut cranked that tight you shouldn't feel any movement in the bearing. You must have something else wrong, likely in the spindle. A friend who raced a 944S had a spindle get worn and gave him similar issues.
08-11-2011, 01:29 PM
#10
Nordschleife Master
Thread Starter
EDIT:
Ok did some searching, based on spindles and spindle failures, etc.
I find it highly coincidental that BOTH spindles have worn out, in the exact same fashion, and to the exact same degree. However, this problem first started back in May, right after I had my car prepped for DE. I took the car in to an indy for an alignment and while it was in, I asked him to swap out the front wheel bearings since I was running out of time (and also - I tried punching out the old races, didn't have much luck, and didn't want to risk damaging the hubs.) I had a spare set of hubs with me and had him swap in new bearings in both sets of hubs.
Both hubs now exhibit this behavior. When I took the car apart last night, I noticed I was able to tighten the bearings some more... and the last time I put it together, I put it together pretty tight - tighter than it should be. So they loosened. So my current theory is that the races aren't seated fully in the hubs. I figure it's much more likely that the same technician installed both sets of bearings at the same time, and would have somehow done it wrong (not seated them all the way) then to have a coincidental equal wearing of the spindle such that both front wheels have some motion in them. I think the problem is with the hub/bearing somewhere.
Last edited by User 52121; 08-11-2011 at 02:54 PM.
08-11-2011, 10:29 PM
#11
If you still have movement after tightening the bearing too much then you have another problem. Cracked hub maybe, cracked races in the hubs from being too lost maybe. I would check both. You may also have an issue where the camber adjustment is. Those are known to wear and end up loose cause its a terrible design. Check your camber adjustment bolt. Its one of the 2 bolts that holds the hub to the shock. Just some additional ideas.
08-11-2011, 10:31 PM
#12
Also all these suggestions are assuming it is your front brakes that are getting pushed back. It may be the rears that are the problem but the fact that it does it while turning leads me to think front end vs rear.
08-12-2011, 12:33 AM
#13
Nordschleife Master
Thread Starter
Well so I didn't drive it tonight... but I did (a) rule out the wheel bearings and (b) discover another problem.
My dad came over to give me a hand, we took a look, and figured that something had to be up with the bearings. So we first made sure the bearings were seated all the way (which they were) so then we knocked the bearings out of the hub completely to inspect them... and couldn't see anything wrong.
Inspected the spindle closely. Pushed and pulled, no cracks, no movement. Then for grins I grabbed the strut body and pushed/pulled... and found the source of the barely perceptible clunk. Strut bearings. No visible movement but you can feel it. It's the same clunk I could feel when tugging on the wheel.
So the wheel bearings are (or... were, before I boogered it up removing it) fine. Going to call a local parts guy and get a bearing tomorrow and put the hubs back together, and talk to a friend to find out what he'll charge me to swap in new strut bearings + realignment.
So I solved the clunk. But still didn't solve the braking issue. When I put it back together tomorrow I'll tighten the daylights out of the bearings and go for a road test to see if the problem persists. If it does, I'm really out of ideas.
Brian - As for the rears - there is NO play in the rears. Tight. Rear bearings are also new - I swapped those in after Blackhawk (borrowed Mark's press... removing and reinstalling was a piece of cake!)
I'll be at BHF tomorrow working tech. We can talk more then.
My dad came over to give me a hand, we took a look, and figured that something had to be up with the bearings. So we first made sure the bearings were seated all the way (which they were) so then we knocked the bearings out of the hub completely to inspect them... and couldn't see anything wrong.
Inspected the spindle closely. Pushed and pulled, no cracks, no movement. Then for grins I grabbed the strut body and pushed/pulled... and found the source of the barely perceptible clunk. Strut bearings. No visible movement but you can feel it. It's the same clunk I could feel when tugging on the wheel.
So the wheel bearings are (or... were, before I boogered it up removing it) fine. Going to call a local parts guy and get a bearing tomorrow and put the hubs back together, and talk to a friend to find out what he'll charge me to swap in new strut bearings + realignment.
So I solved the clunk. But still didn't solve the braking issue. When I put it back together tomorrow I'll tighten the daylights out of the bearings and go for a road test to see if the problem persists. If it does, I'm really out of ideas.
Brian - As for the rears - there is NO play in the rears. Tight. Rear bearings are also new - I swapped those in after Blackhawk (borrowed Mark's press... removing and reinstalling was a piece of cake!)
I'll be at BHF tomorrow working tech. We can talk more then.
08-12-2011, 08:59 AM
#14
Drifting
Didn't realize both were exhibiting the same looseness. As you mention, probably rules out the spindle unless the tech damaged them in some way installing the bearings.
I just replaced my front driver's side bearings last weekend and the outer bearing needed some persuasion to fit onto the spindle. Hopefully the tech didn't hammer the outer bearings on and mess up your spindles.
Did you see any ridges on your spindles? If the inner race bumps up against a ridge it will feel tight as you crank on the spindle nut but won't be completely seated against the outer race.
You mention you have a spare set of hubs. Have you tried swapping them?
Maybe you're just cornering so hard that certfifugal force is pulling the caliper pistons away from the rotor? ;-)
I just replaced my front driver's side bearings last weekend and the outer bearing needed some persuasion to fit onto the spindle. Hopefully the tech didn't hammer the outer bearings on and mess up your spindles.
Did you see any ridges on your spindles? If the inner race bumps up against a ridge it will feel tight as you crank on the spindle nut but won't be completely seated against the outer race.
You mention you have a spare set of hubs. Have you tried swapping them?
Maybe you're just cornering so hard that certfifugal force is pulling the caliper pistons away from the rotor? ;-)
08-12-2011, 02:05 PM
#15
Nordschleife Master
Thread Starter
Didn't realize both were exhibiting the same looseness. As you mention, probably rules out the spindle unless the tech damaged them in some way installing the bearings.
I just replaced my front driver's side bearings last weekend and the outer bearing needed some persuasion to fit onto the spindle. Hopefully the tech didn't hammer the outer bearings on and mess up your spindles.
I just replaced my front driver's side bearings last weekend and the outer bearing needed some persuasion to fit onto the spindle. Hopefully the tech didn't hammer the outer bearings on and mess up your spindles.
Did you see any ridges on your spindles? If the inner race bumps up against a ridge it will feel tight as you crank on the spindle nut but won't be completely seated against the outer race.
You mention you have a spare set of hubs. Have you tried swapping them?
Maybe you're just cornering so hard that certfifugal force is pulling the caliper pistons away from the rotor? ;-)
Gonna take the car for a drive tonight after work. Local euro parts place has new bearings in stock so I'll get the hubs put together, I'll tighten the bearings way way way over tight on purpose, and take it for a ride around the block to see if the brake issue goes away. If it does... then my answer is that I just wasn't tightening the bearings enough. If it doesn't, then Brian gave me a spare master he had kicking around in his trailer. I'll swap it in and see if that solves it. If it does, then I'll buy a new master. If not... then I have NO idea.