Brake bleeding problem

[Norway 911T]

This has me completely baffled. I am restoring a 1972 911T. I have installed a complete new system. Everything part from the fluid container down to new calipers is new.
I am using the good old fashioned method for bleeding using tube to container and pumping the peddle. I have cleared to back brakes OK but the front seem to be getting more air in as I bleed them. I'm not talking tiny bubbles but huge slugs of air coming through the tube. I have pumped litres of fluid through to no avail. Their are no leaks anywhere so how can this air get in? Could it be from the front brake feed line? I even went out and bought another new master cylinder thinking there was a fault there. Anyone else experienced this problem? It has me so frustrated.

Derek.

Norway.

[twistoffat]

I presume you have a helper pumping the pedal while you are working the bleeder valves and topping up the reservoir. If the reservoir isnīt kept topped up with new fluid air can get sucked in that way

[Oldmother]

I know it sounds trivial and I don't mean to insult, but make sure the front calipers are on the correct side, with the bleeder nipples on top. And as the previous poster said, keep the MC topped off with fluid.

[Chuck.H]

did you bench bleed the master cylinder first? I've had problems with the air just moving back and forth around the MC if you don't bleed it when it's separated from the system...

HTH,
Chuck.H
'89 TurboLookTarga, 336k miles

[Rebel911]

Originally Posted by Oldmother

I know it sounds trivial and I don't mean to insult, but make sure the front calipers are on the correct side, with the bleeder nipples on top. And as the previous poster said, keep the MC topped off with fluid.

I screwed that up once. easy to do, putting them on the wrong sides.

[dave 911]

The Motive Power pressure bleeder system makes bleeding so much easier -- I'd recommend the $50 or so investment, it was one of my better purchases.

[Targalid]

Derek,
I had a similar issue recently. The air would not leave the master cylinder after I had worked on the brakes. Just by chance, I filled the reservoir, had the front of the car up in the air to re-bleed the front brakes and suddenly the reservoir fluid level went from the top to the bottom and I had a firm brake pedal. Then I filled the reservoir to the top mark. There must have been a large air bubble that finally worked its way out. Look at the angles at the master cylinder and maybe you will see how to help the problem by adjusting the angle of the master cylinder. Possibly you can loosen a brake line at the master cylinder to allow the air to exit. Good luck! .

[Norway 911T]

Thanks all for the tips so far. I spent most of the day trawling through past posts in the forum and picked up most of the replies I've had so far. I had the front brake calipers on correct with the nipples up but the back I had with the nipples down. The car is level. I have help with bleeding the brakes and keep the master cylinder topped up also. I've tried a vacuum type bleeding tool also. The problem is not that I cannot get the air out. It the amount of air continually coming out. I'll fix the rear calipers and go through the hole system again and check I have it all correct. I also picked up the tip of not pressing the peddle down to far but dare not tell the wife she's doing it wrong :-). I'll put a piece of wood under it instead.

[mnez]

I had an instance in one of my other cars where loosening the bleeder screw too far apparantly allowed air to be sucked past the threads and come down the tube as bubbles. Closing the screw a little eliminated the air bubbles. Opening a bit more caused the bubbles to reappear. I don't really understand it, just my observation.

[Flat Six]

Originally Posted by mnez

I had an instance in one of my other cars where loosening the bleeder screw too far apparantly allowed air to be sucked past the threads and come down the tube as bubbles. Closing the screw a little eliminated the air bubbles. Opening a bit more caused the bubbles to reappear. I don't really understand it, just my observation.

Or using a bit of grease or petroleum jelly around the threads minimizes the re-introduction of air at the nipple . . .

[schumicat]

what about bench bleeding the master?

Originally Posted by Norway 911T

Thanks all for the tips so far. I spent most of the day trawling through past posts in the forum and picked up most of the replies I've had so far. I had the front brake calipers on correct with the nipples up but the back I had with the nipples down. The car is level. I have help with bleeding the brakes and keep the master cylinder topped up also. I've tried a vacuum type bleeding tool also. The problem is not that I cannot get the air out. It the amount of air continually coming out. I'll fix the rear calipers and go through the hole system again and check I have it all correct. I also picked up the tip of not pressing the peddle down to far but dare not tell the wife she's doing it wrong :-). I'll put a piece of wood under it instead.

[Norway 911T]

I haven't bench bleed the master cylinder yet. I am on my second new one so far. I have read a lot about this but it seems too me that if this was so critical it would be mentioned in repair manuals such as Haynes etc, and only required if you are struggling to get the last bit of air out. Would bench bleeding stop air getting in as I bleed the system? This is what appears to be happening. I'll try the grease on the nipple tip also. Is it critical to have the feeder lines into the MC vertical to the MC?. Now matter what I do they always seem to be crooked. Can air be drawn back in here if it does not seal correctly?.

[schumicat]

if there's air in the master, you can bleed at the brakes for years and not get the out. but you're welcome to keep trying.

Originally Posted by Norway 911T

I haven't bench bleed the master cylinder yet. I am on my second new one so far. I have read a lot about this but it seems too me that if this was so critical it would be mentioned in repair manuals such as Haynes etc, and only required if you are struggling to get the last bit of air out. Would bench bleeding stop air getting in as I bleed the system? This is what appears to be happening. I'll try the grease on the nipple tip also. Is it critical to have the feeder lines into the MC vertical to the MC?. Now matter what I do they always seem to be crooked. Can air be drawn back in here if it does not seal correctly?.

[MTemp]

I assume you have a dual master cylinder. This is the process I used to find out where the problem was with my 66 swb (single master cylinder), and I'm sure it will help id where the problem is. First get a few bleed nipples the same size as the rear calliper ones (so they will fit in the M/C ports where the brake lines go). Put them in all the outlets from the m/c and bleed the m/c. It should bleed very quickly!.... and the pedal should be rock hard. This confirms your M/C is ok. Fit a bit of plastic sheet under the reservoir cap to avoid too much fluid draining out, then connect one of the lines to the front wheels. Remove the sheet from the reservoir and bleed the front calliper. If rock hard, then that's not the problem, repeat for other front, repeat at three way after the bulkhead, repeat at one rear wheel and then the other. Somewhere in all this you should find out where your problem lies!

I found my problem was that there are two bleed nipples on the rear callipers, and I couldn't see the outer one as I bled the brakes with the wheels on before I fitted the engine. Numpty of the year award according to my 'bleeding' assistant (wife)

Mick

[H-viken]

Leave the hose attached to the open nipple and tape it up so that the end of the hose sits higher than the reservoir (on a broomstick for ex). Then just let it sit with the nipple open for a couple of hours. It will work itself out with the help of gravity.

[Norway 911T]

Thanks Mick. Thats a really good tip to bleed and check the MC without having to remove it. I'll try that along with a full system check and the rest of the tips I have received so far.

[Packet82]

I had the same thing happen last year. It ended up being a bad master cylinder straight out of the box. Hopefully yours is just a large air bubble.

I went through 6 liters of brake fluid with bubble still showing up before I figured out the problem. I have the motive bleeder but found it to be more of a pain than it's worth. speed bleeders are awesome though. Stomp on the pedal, make sure the reservoir keeps topped up, and you're good to go.

[Ian Comerford]

Not sure if this has been mentioned before but I understand that it is possible to damage the internals of the master cylinder by depressing the pedal too far during bleeding. By too far i think this means all the way to the floor or just about. If this is true you need to ensure that you only depress the pedal just over half way during bleeding.

Regards

Ian

[Ronnie's.930]

If you can do as I describe in the linked thread, and you do not have a flawed master cylinder, then all your brake bleeding problems will be over and you will never bleed them the conventional way again (or use speed bleeder nipples):

A Big Shout-Out to Cole !!!

[WEST COAST 02]

Bench bleeding the master is a must first, then pressure bleed the system, not vacuum, the Motive bleeder is very good and is what i use, It's not expensive and does a god job