The brake job from hell... bleeding problems, please help!

Surge · 12-17-2006, 11:08 AM

Hello all,

I just went through the brake job from hell that included a broken lug nut and many other fun distractions... anyways, I'll spare you my frustrations, but your advice would be greatly appreciated.

Basically, I replaced all pads and rotors. I noticed that on the front driver's side caliper one of the piston seals was slightly torn but it didn't look like it was completely destroyed. I decided to hope for the best but it is definitely possible that this is the culprit.

So when I finish installing everything, I start bleeding, and of course since all of the calipers had been removed the pedal goes right to the floor. Of course I didn't know about not fully depressing the pedal because of master cylinder gunk, so that might be the problem too.

I proceeded to bleed the system using the brake pedals. When you pump the brakes, you can feel pressure building. If you stop pumping (with the pedal depressed) and wait 5 seconds, you can feel the pressure fighting against your foot, then I release the pedal and immediately depress it again and the pedal goes right to the floor again (ie. pressure was there until you release the pedal and then pressure was lost).

So I bled and bled and bled and bled... all clear fluid comes out of the calipers and no bubbles as far as I can tell. I must have done each caliper about 10-15 times.

To me it seems obvious that there is a pressure leak somewhere. Does anyone know for sure if a broken caliper seal will for sure cause this problem? Should I order a rebuilt master cylinder as well just in case I damaged that by depressing the pedal all the way?

Thank you all for your thoughts.

JTL · JTL's Garage

My past problem wasn't as severe. I bled the brakes a couple of times but still the pedal got soft pretty quickly. The calipers were good, hoses were good and there was nothing visibly wrong. My buddy found a leaky bleeder screw in the left caliper. A ridge had built on the screw on the outer edge of the point, which let air through.

Check your bleeder screws. They are a cheap replacement.

jt
-84 targa

Pingo · Pingo's Garage

If there is a leak the should be some fluid...

Which car do you have?

Surge · 12-17-2006, 11:29 AM

Thanks JTL, that's a great idea.

78 911SC. But the thing is it's a hydraulic system so it can leak air and not fluid.

juanbenae · juanbenae's Garage

did you have both calipers off at the same time? were they on the bench together at any point? i ask because if you put the right one on the left side and visa-versa the bleeder screw ends up on the bottom and you can never get all the air out of the system.

just a thought.

Surge · 12-17-2006, 11:34 AM

Yes, all calipers have been attached in the correct direction. Thanks k911sc.

Groesbeck Hurricane · 12-17-2006, 11:36 AM

I'm wondering if you don't need to re-build the piston (caliper). Pelican sells a nice kit. Read up on the procedure before attempting. You can hurt yourself when the pistons fly out of the cylinders.

Ron.G · 12-17-2006, 11:42 AM

If you hold presure on the pedal and it sinks to the floor I would think your Master cylinder seal went south from extending it to far during the bleed.
Thats what I did.

avi8torny · 12-17-2006, 12:25 PM

How bout clamping the vent line from the resevoir. Any fluid leaking at the footwell or the front of the car?

porsche930dude · porsche930dude's Garage

is there a bleeder on the master cylender?

dfink · 12-17-2006, 03:08 PM

I am betting on master cylinder. As long as you have pressure on it the cups seal and it holds. Let off pressure and no more hold. To test pump up the brakes and then slowly reduce the pressure on the pedal if the pedal starts to go down bad master cylinder.

jason2guy · 12-17-2006, 03:08 PM

if the fluid level is NOT going down and you still can not build up pressure, you probably screwed up the old master cylinder.
just did this myself last weekend.
this happens when you push the pedal to the floor and force the piston inside the MC to travel in an otherwise unused portion of the bore and usually if its old, there is debris and dirt that ends up ruining the seals on the piston

Green 912 · 12-18-2006, 08:11 AM

Being able to pump up pressure and maintain it but have the pedal go to the floor after releasing points to air in the system not a bad MC. A bad MC would let the pedal drop after you pumped it up and held it. No loss of fluid points away from a bad MC as well. Get a Motive Pressure bleeder if you don't already have one. Put a quart of your fluid of choice in it, follow the directions and flush the brakes out.

Another trick to do is preset the pistons. After you push the pistons back in the calipers in order to install new pads they can sometimes have too much pull back as they then move foward to meet with the new pad backing. This is more prevalent after a rebuild but can happen on pad replacements as well. What happens is the piston draws back too much between pressure strokes and the next time the pedal is pushed some of the MC stroke is spent making up the larger than normal pad to rotor gap. Remove each pad one by one and pump the pedal enough to move the piston in enough so the pad will not fit. Now move the piston BACK just enough that the pad can be wiggled back in. It is important the the piston be moved BACK to make the space and not pumped forward. Do this to all pads that have pistons one at a time. Many a wrench as flushed himself silly trying to get air out that was not there after a brake rebuild.

mnmike · 12-18-2006, 11:35 AM

i remember when i bought my car, it had two of the same calipers on the front (one bleed screw was pointing up, the other wheels pointing down..)

lateapex911 · 12-18-2006, 11:49 AM

You know, whether it IS, or ISN'T the MC is, to me, irrelevant.

They are famous for this behaivior. I track my car from time to time, and the last thing I want is to get a little fade, havea slightly long pedal, and then neer have good brakes again.

Seems to me that not replacing such a known issue is shortsighted, and potentially risky down the road, esp as the part is so inexpensive, and the replacement pretty straightforward.

Ounce of prevention, and all that. (I replaced mine when I redid my brakes, and I have seen them go regularly..old cars..)

Green 912 · 12-18-2006, 04:01 PM

To me, and this all just an opinion, Prophylactic Replacement is one thing, diagnoses of an existing problem is another. The MC being bad or not is relevant in this case IMO. It might not hurt but it will likely not corect the existing problem based on the reported symptoms and could, if not pre primed right, hamper the flushing the air of the system in and of itself. I am no expert but I remember MC failure to be sinking pedal and or leaking and some other things that relate to dual systems, not soft first stroke but pumps up and holds. The latter points to air or excessive pull back.

On track cars the type, age, moisture content of the fluid and the heat loading of the calipers is far more an issue than the MC. The MC does not see the high heat that the calipers do. In fact MCs often last well in track cars as the fluid is changed often, the moisture does not have a chance to build up and condense and the corrosion inhibitors stay active. In unchanged fluids they wear out after extended service time. Street cars often see many years on the same fluid and it goes bad and then the corrosion starts and damages the internal wetted parts.

lateapex911 · 12-18-2006, 04:23 PM

Quote:

Originally posted by Green 912

On track cars the type, age, moisture content of the fluid and the heat loading of the calipers is far more an issue than the MC. The MC does not see the high heat that the calipers do. In fact MCs often last well in track cars as the fluid is changed often, the moisture does not have a chance to build up and condense and the corrosion inhibitors stay active. In unchanged fluids they wear out after extended service time. Street cars often see many years on the same fluid and it goes bad and then the corrosion starts and damages the internal wetted parts.

Right....to my thinking, 28 years of who knows what condensing/ corroding in it is reason enough to just yank it...regardless if it is the issue or not.

Agreed, it might NOT be the culprit, but jeez...it's kind of an important part in my book, LOL.

Surge · 12-18-2006, 10:04 PM

Well guys, I really appreciate all of your advice. I got more fluid today and a 1 man bleeding bottle. Again, I followed the manual bleeding procedures and there were absolutely NO air bubbles whatsoever. The fluid was clear and clean. I did this for all 4 calipers again. I've bled hydraulic systems many times before and I really think that this one is bled. Has anyone had experiences where they've had no air bubbles whatsoever and the brakes still were not bled?

Since I finally had some help today, I had my housemate hold the pressure (after pumping) and I could still grab the wheel studs and move the rotors! There is little to no pressure being applied even when you pump up pressure and hold it.

This makes me think that you guys may be on to something with the piston setting trick. I will give that a shot and hope for the best.

Finally one last question. I think I will replace the master cylinder in the name of preventative maintenance. Green912 was talking about pre-priming the master cylinder. Guess what? Bentley doesn't say anything about doing that... so now what do I have to do to not screw that up?

This is ridiculous! I've bled brakes and clutch systems so many times in my life... what the hell?!

Thanks guys!

coolngroovy · 12-18-2006, 10:15 PM

I've got the same problem, but i know my M/S is caput!
Santa, are you listening?

dfink · 12-19-2006, 05:27 AM

When you pump it up and the pedal is hard can you turn all 4 rotors or just 2. Master cylinders generally have two sections one may be holding and the other not. I don't know if there is enough piston travel to pop the piston out without the pad in the car. I would be carefull however about how far to move the piston out with no pad. I am sure those who have done this will chime in here about that issue.