docmirror

See post 4. I'm not sold on power bleeders. They can only generate about 10PSI. Go from the old method, and start at the MC then the front like the OP.

Here's a weird one. I recall doing a Fiat long ago that was also very hard. We used a vibrating sander on the side of the MC to shake the MC rapidly while cracking the line.

Nicole

Hmmm...Was or wasn't I ever pushing all the way? I am not sure. I know that many times I was conscious about not pushing all the way, because I was worried to damage anything inside the old and later inside the new MC. So I think it's safe to say that most of the time I did not push all the way down.

I did not think that would make much of a difference from a bleeding perspective.

heliflyer

Hi Nicole,
You MUST push the pedal all the way to the floor and hold it there until the bleed nipple is closed. If you don't do this, then any small bubbles left in the system will not be expelled, they will be drawn backwards into the system as you let the pedal rise, in the same way that you will draw air into the system if you let the pedal rise before the bleed nipple has been closed.

Docs idea of a vibrating sander is also a good one - sometimes very small bubbles can become "sticky" and difficult to move - new fluid just seems to pass them by, so using something that will help to shake them free would be a good thing - maybe even try using it on the MC and all four calipers.

Hope you get this sorted soon.

Nicole

Thanks, Jay! I never let the pedal go while the bleeder nipple was open, but I was reluctant to slam it down.

Will do better from now on and report back.

smudaaar

Pump the pedal up and build pressure. Then crack your bleeder while holding pressure on the pedal. As soon as the bleeder is cracked your pedal should be sucked to the floor. Also its easier to bleed your brake system if your MC is primed before it is installed. Send me a pm if u have any more problems. I had a hell of a time getting my brakes to work also... Sorry if I type like a 2yr old but I'm on my crackberry!

jon928se

I should have been more specific about "conventional wisdom" - was thinking more along the lines of furthest from the MC gets bled first.

Examining the 87+ 928 MC innards There seems to be a check valve between front and rear circuits (not sure what it does) and also a check valve that I think closes (not sure as I can't simulate the installed position) between the rear circuit piston and the void where the bleed nipple is. It also seems to me that the system is not truly dual circuit - it relies on hydraulic pressure in the front circuit part of the MC to push the rear circuit piston, thus if the seal to the front circuit fails you have no brakes at all. There doesn't seem to be enough travel in the larger bore front circuit for the piston itself to push the rear circuit piston.

Sorry this is kinda OT but you never know if it helps the overall understanding. May be I should post some pics as I have the MC on the bench

Nicole

Can you explain the "priming" process in more detail?

smudaaar

You need to fill the MC with brake fluid b4 you install it. If you don't you will need to suck all the air that was contained in the MC thru your lines.

Mrmerlin

Nicole try bleeding the brakes like this........ with a full reservoir start at the master cylinder, then LF, RF, LR, RR............... dont put the power bleeder on the reservoir as it will pressurize the rear of the seals once they go past the feed ports by pushing the pedal
Also apply anti seize to all of the bleeders this will prevent air from being drawn back into the threads, the bleeding catch can should also be a tube thats submerged into the old brake fluid.
The other thought was what bleeder on each caliperto open first ?? Follow the suggestion from up top of this thread, do the outer bleeder 3 times then the inner\

Pedal operator : pump the pedal slowly 3 times, then hold down, when you feel the pressure is released push moderately hard all the way to the floor, and hold it there till you hear............

Bleeder operator .. cracks the bleeder and lets the fluid out, dont keep bleeder open for more than 2 seconds..... closes the bleeder and Then says PUMP

Pedal operator : begins the pumping process again, pump slowly as you dont want to suck air into the master, pump slowly 3 times then hold the pedal down you say.. HOLD DOWN... so the bleeder operator then can open the bleeder
Once the bleeder is opened and closed then you will hear..... PUMP

Before you do any bleeding make sure the MC push rod is adjusted correctly, you can feel the rod pushing through the booster and then contacting the MC .
There should be 10mm of play at the pedal

JHowell37

Bench bleeding isn't really necessary. That's why you have a bleed screw on the master cylinder.

F451

x2. I just installed all new brake components - mc, lines, painted calipers, rotors, pads, so my system was completely dry. My pedal pressure is good now. Did not bench bleed the mc.

I started the bleed process with the mc and worked my way out from there. Inner left front, outer left front, inner right front, outer right front, inner left rear, outer left rear, inner right rear, outer right rear.

Used a power bleeder. Did a first bleeding to get fluid in the system, didn't expect to have perfect pressure, and did not. Did it a second time, by myself, and it came out fine.

And I agree with the pushing the pedal down all the way if you're using that method, which I think is the tried and true method.

I also rapped my calipers with a small wrench as I bled, but it did not seem to release any additional air in my case, but it doesn't hurt to do something like that and can only help.

Good luck and keep us posted. -Ed

mark kibort

Is that only if the MC fails? I lost all fluid pressure in the front due to a severed line, and still had some rear brakes when I lost them in my last race at Laguna Seca. It felt like no brakes at all, but i could feel a slight drag on the rears as i came to a stop in the paddock after the race.

mark kibort

You are really supposed to work from the farthest to the nearest, but I dont think its that important, as ive done just fronts many times and even out of order there too. two person bleding is the only way, as using the power bleeder doesnt always work, as I found out seconds before a race with Tom and me using it. a quick change to the traditional pump method, fixed that .

the pushing and release of the brake pedal should be slow as to not cavitate to produce bubbles. try closing the valve just before the brake pedal pusher, gets all the way to the bottom, especially on the last push.

dr bob

I didn't realize, until I saw Jon's pictures of the MC internals in a separate thread, that the MC piston is actually two pieces. This dramatically affects how you bleed if you are using the two-man method. Note also that this does not apply to diagonal-braking early cars, and does not necessarily apply to later cars when a power bleeder is used. The power bleeder does not rely on pedal presure to make hydraulic pressure in the rear piston circuit (front calipers) before pressuring the front piston to move, so there should be no difference bleeding front calipers or rear calipers first with the power bleeder.

If there is a bleeder on the front of the master cylinder, and that should be the THIRD place you bleed if you are using the two-person method, first if you are using the power bleeder..

So start off with the fronts, RF first, then LF. The order is important here only so you minimize the chance of getting old fluid back from the RF into the LF when done out of order. RHD cars should do this in the reverse order, so you go farthest of the two, then nearest. You are bleeding lines fed by the rear piston in the master cylinder.

Once the fronts are done, then bleed the front of the master cylinder.

Then proceed to the rears, again doing the farthest caliper first, then the closest. For US and other LHD cars, this would be RR then LR, based on the line routing to the rear brakes under the driver's position on the left side of the car. The RR brake line is slightly longer on LHD cars.

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Cars with two-piece pistons in the master cylinder have unique bleeding requirements if you use the two-person method. You MUST remove air from the rear piston circuit first to get the front piston circuit to work correctly. Bench bleeding is strongly recommended if any air has been introduced into the piston areas, perhaps by running the reservoir dry when changing brake hoses or painting calipers off the car. Or when not paying attention while bleeding. Bench bleeding offers an opportunity to make sure the forward piston is actually retracting, something that is more than very important. A stuck forward piston means the rear is doing all the work. A forward piston that's stuck in that forward position can usually be retracted with compressed air to the forward port or the forward bleeder, but this is something that really needs to be done with the MC out of the car and drained. Otherwise you'll have air pushed into the lines, brake fluid everywhere, and brake fluid eats paint faster than you can get it washed off. Bench bleeding also offers you an opportunity to rotate the MC some as you purge it, so that the ports are high and air can pass out of the ports smoothly. Do the rear piston port first, then the front bleeder, then the front piston port. In my collection of things, I have a set of plastic adapter screws that match the threads of the ports, with hose nipples on the ends. A short section of clear hose goes on each, and the other ends are submerged in fluid in the reservoir. The adapter screws came in a rebuilt ATE MC kit from somewhere in my dark past with the aircooled cars, but should be fine on our water-cooled 928's.


I would hazard a guess that Nicole's persistent very low and soft pedal is related to a sticking forward piston in the MC. We may also have the option to vacuum bleed her car, drawing vacuum at the reservoir and bleeding fluid back into the evacuated system from the calipers. I think I have al the pieces needed to do that, if I can get closer to her car some day. Or ship the fittings and pump home with Bill Ball at Sharktoberfest next month...

Jfrahm

I have had some luck with the old motorcycle trick of putting pressure on the system for several hours and then bleeding. This is supposed to make the bubbles smaller so they migrate to a place where they can be forced out.

I use a block of wood on the seat and a scrap extended to the brake pedal, and push the pedal with the seat motors.

You can put your pressure bleeder on there too I think, to keep the pressure up when you lift the pedal. You could try bleeding some more using the normal pressure bleed process after each long-term applied brake session.

So pull the brake light fuse or disconnect the battery, apply the brake, then go to work or sleep, then pop one bleeder, pressure bleed the others, reapply brakes, wait again. Then do another. This may help. It'd take a while but perhaps you could do the inner bleeders the slow way and the outers with the pressure bleeder and get it done in a day or two.

The other, more messy thing I have also had to do is bleed a brake system at every banjo bolt I could find to chase out some pesky bubble of air.

Has anything been done with the ABS unit?
-Joel.