Tails

I have had success with "leg-draulic" bleeding, pressure bleeding and vacuum bleeding, but with the vacuum bleeders some time I have had to put a thin smear of petroleum gel on the bleeder thread to get a seal to stop air leaking pass threads and out the open bleed screw aperture.

A leg-draulic helper is not always available and the danger here is internal corrosion inside the cylinder of the MC causing leak by.

I prefer the vacuum bleed as I can actually watch the out put into the graduated bleed container and it usually takes me two bleeds of the system to get the best brake pedel feel, that is if I don't forget to top up the master cylinder between wheel cylinders.

Bill and Nicole I followed along with your trial and tribulations saga in trying to get a firm feel back into Nicole's car. You have prbably formed an opinion that the problem could be air or a semi bypass in the actuation cylinder, however to locate and rectify the problem is the rub.

Tails 1990 928S4 Auto

Bill Ball

Good. Tell me what it is and then tell Greg Brown. Nicole's pedal firmness is good but not great.

FredR

Bill,

Your description of Nicole's brakes matches with how I would describe mine despite having stainless lines fitted and Hawk Plus pads fitted fitted to the GTS calipers. Whereas the stopping power is more than a match for my 265 front tire section grip [the tarmac here is rock hard to withstand the V high temps- thus less grip], I have always felt the pedal was somewhat softer than I expected. I watched the main dealers mechanics doing the bleeding and indeed I was amazed that two of them did the bleeding somewhat archaically - this thread tends to suggest why that may be the case.

I feel there is still a small amount of air still left in there but I have not bothered to try and do something about it.

If someone comes up with any "special method" will be happy to try it as I am due a brake fluid change. I was thinking of using my Mityvac kit but not after reading this!

Maybe there is something to the earlier suggestion about the condition of the seals coming from the server rod piston permitting some un-controlled air leakage- I have not studied this so no opinion but there has to be a reason why you "928 smart" chaps were not able to solve the problem 100%.

Regards

Fred R

finally!

If you dont have a helper try this trick,

Get a broom or something similar and put one end on the brake pedal and the other on the upper seat.
1. Use the electric control to move the seat forward which then presses the brake pedal.

2. Go to the brake caliper you want to bleed, open bleed nozzle and then close it.

3. Move seat back so brake is released

4. Repeat 1 -3 as necessary

auzivision

I replaced the master cylinder about a year and a half ago, but I don’t recall the bleeding the MC. Does it have a bleed nipple as well? I couldn’t find one nor did I find it mentioned in the WSM. Am I missing something here?

Another guy helped me with the MC install and bleed job. I did most of the wrenching except for when it came to the bleed work... then I was the pumper so to speak. I couldn’t see what he was doing, but I think it was I apply pressure while he open and close the nipple repeatedly.

dr bob

There's an instruction in the box with some aftermarket-sales ATE master cylinders about bench-bleeding the master before installation. They provide some fitting adapters and short hoses. Screw the adapters into the ports, hoses into the reservoir, MC in the vise level, pump with a screwdriver. The bleed nipple on the end of the MC gets a hose and is opened too. In many MC installs, the end of the forward chamber is a dead space especially when the front end of the MC is slightly higher than the rear. In the case wher the MC is sloped up like that, there's no way to purge air from that spot in the forward chamber with the MC mounted in the car, even using the forwars bleed nipple. Also, the pushrod on the booster and pedal in the car doesn't push the forward pistin all the way into the end of the cylinder, so it doesn't push an air bubble out. Note that this applies to NEW master cylinders; older ones that may be contaminated or have corrosion in the ends will often be damaged if the pistons are pushed into the corroded areas of the bores.


I'm still of the mind that there is a way to pull a hard vacuum on the system, and let it fill from the caliper ends. My only concern is that a hard vacuum will draw air past the rear cups from the booster end due to the shape of the cups. So far the pedal is OK on my car, but it has been firmer in the past. It will likely be a guinea pig sometime soon when the hoses get replaced as part of the upcoming brake job. Pads are here, new rotors are zinced and painted (wheel silver), just need to decide on hose type and buy them. Then the bleeding starts... I have a pressure bleeder to try frst, but may do the full vacuum bleed on it as an experiment. Might happen soon, but more likely later this year due to project schedule. Film at 11.

Bill Ball

3a. Pull broom handle out of hole in seat back upholstery and reposition.

Just joshing you, but it would probably happen to me if I tried it.

BC

When I say "there must be" - I mean i can assure you there is, but I may not know what it is.

Like: "There must be a way to get to the moon"

GlenL

I've fought this on several cars. My 928 has this now as I just replaced the brake lines.

The most effective and painless approach has been to live with it while bleeding the brakes occasionally. It'll come eventually as the bubbles collect enough to be forced out. My picture is that there are a lot of small bubbles clinging to the tubes while larger bubbles are getting forced along and out.

I always use a helper to do a "classic" bleed. Since I change wheels often for track use, and have 4 pit crew members, (aka "children") it's not really a hassle for me.

BC

But that said, I've done two dry systems with new masters, soft lines, and calipers. Well, rebuilt calipers. And it did need multi-event bleed, but I got the pedal firm.

hacker-pschorr

Just do what my co-worker did with his mini-van - he was by himself, no pressure device

1. Up on jack-stands
2. Remove all four wheels
3. Open all four bleeders.
4. Push brake pedal to the floor
5. Close all four bleeders
6. top off fluid
7. Install wheels

Yes he sprayed brake fluid all over his garage.

Needless to say none of us opt to ride with him on lunch trips anymore.

Erik N

I had a car once that needed a "quick stroke" when manually bleeding the system. There was a bubble that would not get ejected unless agitated by the faster flow. I think it lived in the ABS manifold.

jthwan22

I had the same problem with my SRT 4. The air got trap in the ABS pump. I did not want to spend $1000 for a factory scan tool to activate the ABS pump. I went to a gravel road and made many stops making sure tha ABS is pumping. After that, I bleed it again and it is now nice and firm.

When people replace the MC, the whole system usually don't go dry. I think the key is not to let air into the ABS pump.

auzivision

Han's came by last night and we bled the brakes the old fashion way and got a few more bubbles out. The pedal is firmer. Thanks Hans for helping me keep my beer supply fresh!

I only wish I had ABS right now. While I replaced both front side harnesses with ‘rebuilt’ ones, I still have a couple disconnects under the hood to address.

Rebuilt harnesses means used known good (thanks to Mark Benton), but with disintegrated insulation. I cleaned and wrapped them with silicone vulcanizing tape:






Now all I need are two new wear sensors (so I’ll replace all four) and a solution for under the hood connectors:







Then I’ll try the ABS pump trick. For now, it’s always my left rear that locks first. It’s amazing how that butt wants to come around even when traveling in a straight line under extreme braking. Slight lift on brake pressure usually does the trick.

Once it corrected so quickly it almost swung me around the ozer way. Of course this happened in front of a cop with radar. He didn’t seem to notice I scrubbed at least 30 MPH to get down to within 10 of the limit before he came around the bend, nor did he notice the black marks, white smoke, and screeching noise at less than ¼ mile.