IndyMatt

Nicole, I have a similar issue after replacing the brakes hoses. I have bled and bled the lines and super blue and no air comes out of the bleeder valves. I have been using the power bleeder and have an auto. I will try the old pushing the brake pedal for ever!

I have seen several mentions of bleeding the the master cylinder, how is this done? Is this simply done as part of the regular bleeding procedures of loosing the bleeder valves on the calipers?

no doubt

Step one: start your 928s engine
Step two: drive your 928 at least 4 miles per hour so that the ABS pump briefly kicks on
Step three: leave your 928 running during the entire process so as to maintain pressure, but ebrake on (Park for Automatics)
Step 4: Connect your Motive power bleeder to your reservoir
Step 5: Press your brake pedal multiple times
Step 6: Bleed all 4 corners (plus clutch if you have a 5 speed)
Step 7: Bleed your master cylinder if you have a 1986.5 or newer 928

IndyMatt

Well I can do that until I get my TB/WP job done, but will give it a try.

WallyP

The ABS pump runs in only two situations:
- A BRIEF test application the first time that you drive the car after starting.
- When the ABS is limiting pressure to the brake calipers.

The ABS pump does NOT build pressure in the system. Its only function is to pump fluid back to the master cylinder to allow fluid pressure to be reduced to one or more calipers to unlock a skidding wheel. It ONLY operates when the ABS detects a skidding wheel and the driver is applying more pedal pressure than is required to lock the wheel.

I do not believe that it is possible to make the pump run during bleeding. (You could directly power the pump, but that might not be a good idea, and would be a bit of trouble.)

Charley B

Good stuff.

worf928

There is a bleeder valve on the master cylinder. It is difficult to see, but it's there.
What I do is: if I'm just flushing fluid or replacing pads/rotors I bleed the MC after the calipers.

However, if the system has been opened (e.g. bias valve, line replacements, caliper rebuilds) then I bleed the MC before and after the calipers.

Jim bailey - 928 International

That only applies to the 86.5 and up master cylinders..... early cars have no bleeder on the master.....it is really really hard to see as in invisible ! none existent....

dr bob

Need to get to the bleeding truth on this issue.

My pedal was highest after a pad replacement concurrent with fluid flush. Over time, the pedal has been low/soft as Nicole describes, even after subsequent flushes. So is there a reason why we don't vacuum the system to suck out air bubbles? The bleeder I made has a handy fitting on it that might be perfect for appling vacuum to the reservoir. I guess I'd be a little worried that air might be drawn in around the cups in the MC, since they face foward and do their sealing based on maintaining higher or at least even pressure on the forward side of those cups.

So has anybody tried to vacuum the air bubbles out of the fluid/system via the reservoir cap with the bleeders closed? This is how silicone-filled race systems are degassed, BTW.

-----

Wally, good info on the ABS pump. I have been under the impression that the pump and metering block managed to block the pedal circuit from the offending wheel, and then pump-pulsed fluid pressure as needed to that wheel. What I'm thinkng is that if there is a way to cause the pump to run without blocking any of the ports, we could make sure there's no air trapped in the pump or metering unit. Those are higher in the car than the MC, so bleeding/flushing still seems to leave air in that unit with a resulting soft pedal.


So, vacuum the bubbles out or run the ABS pump with the caliper ports open into catch containers with a seal level of fluid?

worf928

Unless I've missed something the above is true only if the car isn't level? So, if the car's on stands the MC is still higher than the ABS hydraulic unit. However, if you're bleeding one caliper at time with one corner jacked up then once you get to the front wheels the hydraulic unit might be higher than the MC?

WallyP

One bleeding method that I haven't seen discussed lately is reverse bleeding. Take a clean pump oil can, fill with brake fluid and pump it from the caliper bleed screw up to the master cylinder and reservoir. It is sometimes easier to push a bubble up than down.

If we think that the problem is really air in the ABS unit, we might try powering the pump and see what happens. If you try this, remember that the pump normally runs only when the ABS unit is limiting pressure to at least one caliper, so that the passage to the caliper is blocked off, and the pump pushes fluid back to the master cylinder against pedal pressure. No idea what would happen if the pump ran with the valve open...

the flyin' scotsman

I've used this method with great effect Wally but only on race m/cycles............can't see why it wouldn't work on a car.

After all air rises.

As mentioned...........solid brakes always bled with the power bleeder but.........I did replace all 3 ABS relays to resolve a battery draw. The culprit was the relay on the CE panel which failed after only 1 year but because it had been replaced I 'assumed' it was another ABS issue that caused the light to come on..........intermittently.

Given that the relays were 20 YO its good peace of mind; what was the real issue for a hard pedal, unknown.....its all good now though.

Nicole

We've done just that... Ask Bill Ball.

There is a nipple at the master cylinder that you can bleed. It's actually quite simple - I dare say it's easier than bleeding a caliper.

Charley B

Another common method is to bleed each side simultaneously. Start with MC, then inside of both passenger calipers, then outside of both, then inside of both driver side, then outside of both driver side and finish with MC again. A two man job with a power bleeder, three man if pedal pumping is included.

IndyMatt

As far as bleeding the master cylinder:
Is the nipple at the very front of the metal block with the reservoir on it, does it have a black rubber cap on it?