Squishy brakes from the calipers?

[SpeedracerIndy]

I had all four calipers off last night for a good cleaning (and some paint). I noticed that the front dust guards were a little worn. One of them had a tear in it. They were still attached to the piston, but looked like there wasn't much life left in them. Could this cause squishy brakes? I have been having squishy brakes for a while. Sometimes I have to pump them up to get a firm pedal. I have a new master cylinder, and have bled the brakes about five times since the new MC. I am using casterol DOT 4+ brake fluid, and am positive I have gotten all the air out by using a pressure bleeder (all five times!).

I wish I had gotten a couple of rebuild kits since I had the calipers off already.

[Chuck Moreland]

The torn boot isn't causing the squish. Boots are there to keep water and dirt out of the piston / seal. That's all.

Squishy brakes are usually due to air in the lines. You bled them 5 times. Did you get air bubbles each time? That would indicate a leak somewhere in the system. Properly bled brakes should stay bled. Why did you keep bleeding them 5 times?

[SpeedracerIndy]

I bled them five times to try to get the air/squishiness out. I'm stumped now. The pedal has never been very firm. I am comparing it to my dads 88 Carrera. The pedal on his is rock hard. If I pump my brakes with the car off until the vacuum boost is gone, they feel right. I don't think that there is a leak anywhere because I have never had to add any fluid. Is there anywhere the brake system could be holding air? I have tapped all the calipers while I bled to try to dislodge any air bubbles. The only thing I can think of would be the master cylinder.

[vash]

my money is on a bad MC. did you re-bleed the lines the old fashioned way after using the power bleeder? you know the poor assistant at the pedal. how much pressure are you pumping the power bleeder up to? i also tapped the calipers with the wooden handle of my wire brush everytime i bled the system to free any air bubble dingleberries.

[ubiquity0]

CD- I have the same problem. After replacing pads & rotors I got a soft pedal that I first thought was due to the pads not being 'bedded'. Anyway I reset the calliper pistons, replaced the MC & still no improvement. I'm sure there must be air in there somewhere, but Power bleeder (13psi?), manual method (w/ speed bleeders) haven't fixed it. Not sure what to try now.

[vash]

you know, i was having a hard time bleeding the system when i changed my MC and rebuilt the calipers. someone told me to use 20 psi on my motive. i did it, and my pedal started to firm up right away. i was getting these tiny bubbles i didnt see before. hope this helps. ubi, did you look everywhere for leaks.

[Superman]

Sometimes you need to either "bench bleed" the MC prior to installation, or at least go though some gyrations with the MC in the car. Gyrations include jacking up one end of the car so taht the MC tips downward, bleed, then jack and tip the other way, bleed. All the time, tapping gently on the MC so as to dislodge air bubbles. You'll need a helper to do this, I think, or at least you will if you use a pedal-pumping bleed method instead of pressure or vacuum bleeding. Those are nice methods, and very very convenient if you don't have a helper and it works properly. But for stubborn systems, the brake-pedal-pump method is the standard, reliable method.

[anh911]

When you change the master cylinder you have to bleed it before you bleed the system (manually not power bleeders) You'll never get all the air out of a new/rebuilt M/C unless you do. There are 2 ways to do it. one is messy one isn't but it's harder. You can also bench bleed (ok, i guess that makes 3 ways)
Disconnect the lines at the m/c and either attach adapter nipples and hoses (they come in m/c bleeding kits) or you can use old lines if you have them, just cut with a few inches of line past the fitting and add a hose. run the hoses into a container of clean fuid that is above the m/c or back into the res. then SLOWLY, by hand press the brake pedal to the floor and back up. BEWARE: check the fluid every time you do it as it pumps a lot more fluid than bleeding at the calipers and you wil run dry if not careful. You will be amazed at how much air comes out. Then attach the lines and re-bleed the system. This should do it.

[RoninLB]

what Super said.
and I always squeeze the pads in place.
I trim the lining edge for entry.
when I need a whack to fit them it's snugg enough, imo.

[RoninLB]

Originally posted by anh911
you have to bleed it before you bleed the system (manually not power bleeders) You'll never get all the air out of a new/rebuilt M/C unless you do.

ditto..
absolutely..
but I've never seen hard info on that belief. I thought it was a 1960's garage mentality ?

[Superman]

My good friend Ron raises an excellent point here. When seals are put in calipers, and even when new pads are installed, the pads can sometimes ride a mm or so away from the rotor, for 500 miles or so. When this happens, you'll have quite a bit of pedal travel. More than you like. With a flashlight, see if you can see light between the pad and rotor. You should not see light.

Also, consider your symptoms very carefully. Pedal travel, then a firm pedal, means you've got the pad problem. A spongy pedal means you have an air-in-the-system problem. Probably air in the MC.

Ron knows what he's talking about, and apparently so does anh911.

[anh911]

Thanks, Superman. There's something else I forgot to mention. Some 911 calipers have a device to prevent the pistons from withdrawing too far back into the cylinders when the brake pedal is released. I have seen these fail on older cars where the pistons just go too far back when the brakes are released and consequently you get too much space between pad and rotor like you said. Most m/c designs have valves in them to prevent this and to be honest I forget exactly when/if porsche changed it. But like S said,

"Also, consider your symptoms very carefully. Pedal travel, then a firm pedal, means you've got the pad problem. A spongy pedal means you have an air-in-the-system problem."

Pedal travel then firm indicates the pads are too far from the rotors.

The calipers in question are those with the pins in the center of the piston bores and corresponding receiver in the center of the pistons. To my knowledge there's no fix if they are bad.

[anh911]

Geez, stop me now, even I'm getting sick of seeing my posts...

I forgot, rubber brake lines can rupture internally and not leak externally for a while. That also causes spongy feel.

And the red lines show what I was talking about on the calipers. There's a friction device inside the piston that rides on the pin.



Thank Wayne, i borrowed the pic from the Tech article section. In which BTW, there is much good info, check it out.

[ubiquity0]

I removed the pads & 'reset' the pistons so that a small hammer was needed to tap the pads back in, as I thought that may have been my problem early on. It wasn't though.

Itlooks like its going to be a brake bleeding bonanza of a weekend. Will try the power bleeder @ 20 psi & then Superman's see-saw technique with the manual pedal-pumping & see where that gets me.

I also discovered a small leak at 1 hard line @ the connection to the MC (very small as in the slightest sigh of wetness after pumping the brakes for quite a bit, but there nonetheless). I never bench-bled the MC when I installed as I understood, maybe wrongly, that a pressure bleeder would be more than sufficient.

[vash]

i didnt bench bleed mine. i didnt want the mess. is it possible to submerge the entire MC into a tupperware (check with wife) container of brake fluid and then slowly and carefully pump it to prebleed it? then wipe it all down and install it. ubiquity, i discovered a tiny leak in the same line, mine at the banjo fitting. it wasnt much of a leak but as you described, a wetness. my pedal firmed up soon after that leak was eliminated.

cliff

[coolbreeze]

mine was soft after a caliper rebuild, i asked the same thing all full of doubt. a cooler head prevailed and told me to drive it and heat up the brakes a bit. with in 10 miles they were back. i did know i was bled though.

[SpeedracerIndy]

Well from the sounds of it, I think I still have air in my MC. The rubber brake lines are all new, and the MC is new. I *tried* to bench bleed it, but got frustrated because it was so messy and gave up. I HATE brake fluid. It's so messy, dangerous, and hard to clean up. I ended up just installing it and used a pressure bleeder at 15 psi. I thought that would get the air out. My pedal doesn't firm up after driving or pumping. I'm pretty sure I don't have any leaks in the system because I've never had to add any fluid or have never seen any wet lines. Thanks for the help.

[HawgRyder]

Anh911....the gimmick in the piston is a retract device, it pulls the piston back into the hole slightly when the pressure is released.
When I rebuilt my calipers it took me a while to figure out what the pin and spring was and what it was doing.
Bob

[anh911]

Unfixed, the tupperware trick will work, if your wife don't catch you!

HawgRyder, my understanding of how it works is that it limits the immediate piston travel after pedal release but I could have gotten that wrong. (it would be the first time since '82 though ha ha)