Brakes - Soft on first press, firm on second press
02-28-2013, 11:18 AM
#1
Burning Brakes
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Brakes - Soft on first press, firm on second press
Just got my car back after a lot of work.
Motor was pulled, trans was pulled, lots of stuff (new brake lines too).
However, as per the title, I have the above condition. Pedal is soft on first press, it works, just longer throw, and soft, and on second depress (double tap, or press twice), it feels good.
In an old thread, Van suggests the wheel bearing could be bad, but I've had my car tech'd a few times, but the last time I drove it was on track, before a failure, which included going off, and then pitting in, and that was it.
I just wanted a check list of things to do/things to check. The great guy who helped me with the car did bleed the heck out of it, he said. He also said it passed Clarks-garage test for brake MC.
1) Put car up, push/pull rear wheels (fronts too, why not) for wheel bearing play check
2) Bleed brakes with vacuum bleeder, why not.
3) Try to visually inspect MC for leaks?
Input needed. Thanks RL!
Motor was pulled, trans was pulled, lots of stuff (new brake lines too).
However, as per the title, I have the above condition. Pedal is soft on first press, it works, just longer throw, and soft, and on second depress (double tap, or press twice), it feels good.
In an old thread, Van suggests the wheel bearing could be bad, but I've had my car tech'd a few times, but the last time I drove it was on track, before a failure, which included going off, and then pitting in, and that was it.
I just wanted a check list of things to do/things to check. The great guy who helped me with the car did bleed the heck out of it, he said. He also said it passed Clarks-garage test for brake MC.
1) Put car up, push/pull rear wheels (fronts too, why not) for wheel bearing play check
2) Bleed brakes with vacuum bleeder, why not.
3) Try to visually inspect MC for leaks?
Input needed. Thanks RL!
No, the rotor doesn't have to be warped for a wheel bearing to be bad. Wheel bearings just wear out, and when they develop play, the cornering forces cause the rotor to move sideways -- this is why we grab the wheel and shake it to check wheel bearings. (Of course, cornering a 3000 lb car at .75 Gs puts a little more force on the wheel than you or I can by shaking it on a jack.) Anyhow, this movement of the entire wheel, hub, and rotor push the pistons back into the caliper.
When you step on the brakes, you're usually going straight-ish, so there's no side deflection on the wheel. The pads come back in contact with the rotor, they squeeze nicely, and, with a second pump of the brake pedal, the brakes feel nice.
If this is the case, IF the pedal is firm on the 2nd pump, then the MC is probably good.
I've had good personal experience with rear wheel bearings failing, and they are hard to diagnose early on. However, this is a good test (once front wheel bearings get loose, it's easy to feel in the steering...)
When you step on the brakes, you're usually going straight-ish, so there's no side deflection on the wheel. The pads come back in contact with the rotor, they squeeze nicely, and, with a second pump of the brake pedal, the brakes feel nice.
If this is the case, IF the pedal is firm on the 2nd pump, then the MC is probably good.
I've had good personal experience with rear wheel bearings failing, and they are hard to diagnose early on. However, this is a good test (once front wheel bearings get loose, it's easy to feel in the steering...)
02-28-2013, 11:25 AM
#2
Rennlist Member
First, make sure all calipers are installed with bleeder nipple at the top. 
Then, with all the brake work that was done, not surprising that some air bubble(s) still remains in the system. Its the air that'll cause the soft initial pedal; once compressed, the pedal feels better.
Probably need to continue bleeding, and just wait for the bubble to appear. Can be tedious. Also, if the car has ABS, I believe it requires special Porsche programming tool to properly bleed out any air of the ABS module.
IMHO, pressure bleeding (like a Motive) would be better than vacuum bleeder. Also, ATE makes Blue & Gold fluids, so that you can confirm the color change during a full system flush.
Then, with all the brake work that was done, not surprising that some air bubble(s) still remains in the system. Its the air that'll cause the soft initial pedal; once compressed, the pedal feels better.
Probably need to continue bleeding, and just wait for the bubble to appear. Can be tedious. Also, if the car has ABS, I believe it requires special Porsche programming tool to properly bleed out any air of the ABS module.
IMHO, pressure bleeding (like a Motive) would be better than vacuum bleeder. Also, ATE makes Blue & Gold fluids, so that you can confirm the color change during a full system flush.
Last edited by aj986s; 03-01-2013 at 08:15 AM. Reason: corrected ATE brake fluid brand
02-28-2013, 11:50 AM
#3
another check. make sure your wheel bearings are tight and there's no wobble. if the hub wobbles, the rotor presses the pads outwards. initial brake press recenters the rotor. second brake press does actual braking.
02-28-2013, 12:22 PM
#4
Burning Brakes
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Car was initially done with a Motive I believe. I had a motive and sold it, I didn't like it. I've had good luck with the vacuum bleeders (good ones), so I will use the list I made above (wheel bearing check, then bleed from both nips in the correct order)
02-28-2013, 02:39 PM
#5
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Regarding the vacuum style bleeder tools, they can work well. But I've had to deal a variety of track cars, and have found that some bleeder nipples on calipers don't seal well on the hose, and I'll start seeing bubbles that are coming from a bad seal on the nipple, versus actual air in the hydraulics. That's why I prefer to push the fluid instead.
02-28-2013, 04:36 PM
#6
Nordschleife Master
I always prefer to do manual brake bleeding for 2 reasons.
1. More Pressure
2. Movement of the Master Cylinder/Caliper Pistons
I have had air pockets simply not get worked out with motive or vac bleeding methods.
1. More Pressure
2. Movement of the Master Cylinder/Caliper Pistons
I have had air pockets simply not get worked out with motive or vac bleeding methods.
02-28-2013, 10:15 PM
#7
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My suspicion is bleeding is the answer. However, for the sake of completeness, I would think that warped rotor(s) could cause the symptom as well. I'm thinking if you had an off, perhaps you came to a quick stop while your brakes were hot and as they cooled, since they were static, they have warped. This would push the pads away from the rotor during driving so that the first time you press the pedal, you're taking up the space, and the next time you press, you actually get the stopping power. You'd probably get some pulsing in the pedal if the rotor were actually warped. But it's worth checking the runout if none of your other efforts pan out.
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03-01-2013, 12:10 AM
#8
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Yeah, the wheel bearing thing will only have an effect under hard cornering loads. Not if you're just sitting still.
Sounds like air in the system to me.
Two techniques I like to do when I have a hard time bleeding the brakes are:
-While the Motive is hooked up and a bleeder is open, tap on the brake lines (hard and flexible) and the caliper with a rubber mallet to try and "dislodge" the bubbles (they might be "stuck" due to surface tension).
-With the Motive hooked up, have an assistant pump the pedal a few times, then hold the pressure on firm while you crack a bleeder - the extra pressure from the pumping can help get stubborn air bubbles to move down the line.
Sounds like air in the system to me.
Two techniques I like to do when I have a hard time bleeding the brakes are:
-While the Motive is hooked up and a bleeder is open, tap on the brake lines (hard and flexible) and the caliper with a rubber mallet to try and "dislodge" the bubbles (they might be "stuck" due to surface tension).
-With the Motive hooked up, have an assistant pump the pedal a few times, then hold the pressure on firm while you crack a bleeder - the extra pressure from the pumping can help get stubborn air bubbles to move down the line.
03-03-2013, 01:24 PM
#10
Drifting
I replaced the rotors, pads and lines and used the motive power bleeder.
Starting with the farthest away first (right rear) and then next farthest (L rear) then passenger front and then drivers front. It was the easiest and most complete bleed procedure I've ever done. I also follow the recommended blue fluid this time and next year going to gold to see when the old fluid has been totally replaced.
Starting with the farthest away first (right rear) and then next farthest (L rear) then passenger front and then drivers front. It was the easiest and most complete bleed procedure I've ever done. I also follow the recommended blue fluid this time and next year going to gold to see when the old fluid has been totally replaced.
03-03-2013, 05:12 PM
#11
Rennlist Member
